The increase in concrete structures’ durability is a milestone to improve the sustainability of buildings and infrastructures. In order to ensure a prolonged service life, it is necessary to detect the deterioration of materials by means of monitoring systems aimed at evaluating not only the penetration of aggressive substances into concrete but also the corrosion of carbon-steel reinforcement. Therefore, proper data collection makes it possible to plan suitable restoration works which can be carried out with traditional or innovative techniques and materials. This work focuses on building heritage and it highlights the most recent findings for the conservation and restoration of reinforced concrete structures and masonry buildings. The construction sector depletes 40% of the planet’s energy resources ,2. To de2 penetration, which determines cement matrix carbonation and, in presence of moist air, is responsible for reinforcement corrosion [The durability of concrete is strongly influenced by the interactions between concrete itself and the surrounding environment, which is rich in contaminants . Among sorrosion ; chloridorrosion ; heavy morrosion .From “The Law of Fives” by De Sitter , it is wThe purpose of this review is to analyze the latest findings on materials and techniques devoted to ensuring prolonged durability and proper conservation of reinforced-concrete-based heritage.The diagnosis of durability should take place through a methodological path primarily based on the visual analysis of the structure to detect possible visible degradation phenomena and on the acquisition of historical–geographic data. These data are complementary to the results obtained by in situ and laboratory tests to assess the diagnosis and to define the maintenance interventions to be carried out.On one hand, the main destructive tests performed on samples extracted from the structure and analyzed in laboratory include the chemical , physical (mercury intrusion porosimetry), morphological  and elasto-mechanical  characterization of materials.2 and chlorides, whereas potential mapping and resistivity measurements are used to monitor reinforcement corrosion.On the other hand, on-site non-destructive tests, mainly focused on elastic–mechanical characteristics of the concrete, are endoscopy, sclerometry, thermography, pull out, and sonic and ultrasonic investigations. Colorimetric methodologies  are usedHowever, continuously monitoring selected parameters through embedded probes, in order to detect the initiation of degradation as early as possible, can further decrease costs and improve the safety of structures, because it increases the chance of detecting any significant variation of the chosen parameter related to a degradation process. Therefore, numerous probes to be embedded in concrete have been developed, such as electrical resistance probes (ERP) , macro-cIn particular, the free corrosion potential is easy to measure and can be used to control the durability of reinforced concrete structures, since it is sensitive not only to the corrosive state of the reinforcements but also to the water saturation degree of the concrete between the monitored reinforcement and the embedded probe. It is worthy to underline that water is the main factor influencing the durability of structures, since it transports the main aggressive agents and it is the medium in which deterioration reactions take place. A continuous monitoring system for preventive and planned maintenance of structures based on simple measurement of the free corrosion potential has been already proposed  are sensorized non-metallic reinforcement systems for concrete , computemography ,21, and mography  . Electrmography . This abmography ,26,27; hmography . Concernmography , graphenmography , carbon mography ,32, and mography  have beemography ,35,36. Cmography  focused mography , a recenmography . This remography .2  and (ii), by reaction of H2CO3 with calcium compounds 2)), to produce calcium carbonate (CaCO3) and water. The decrease in the portlandite concentration reduces the pH of the pore solution to a value of about 8.3 after complete carbonation of concrete [2 reacted over time, as carbonation is favored in the RH value range from 40 to 90% [Carbonation is a two-step reaction process between: (i) atmospheric COconcrete . However0 to 90% . The car0 to 90% .w/v) solution of phenolphthalein in ethanol, usually sprayed on cylindrical samples cored from concrete structures [, where destructive (based on pore solution extraction/leaching) and non-destructive (with embedded sensors) approaches were deeply illustrated.The most common method for the evaluation of carbonation is destructive and involves a 1%  electrodes [2O [x [Even if potentiometric sensors for pH measurement are commonly used in many areas, their use to monitor the pH of concrete pore solution is somewhat limited . Iridiumectrodes ,51,52,53s [2O [x  sensors Electrode pH probes to monitor pH evolution at different depths were also investigated. For example, a fiber-optic sensor based on a pH-sensitive layer entrapping a pH indicator which changed color in response to the carbonation state of the cementitious matrix was described by Habel and Krebber . Other pIn some cases, specifically when concrete damage is due to reinforcement corrosion, conventional repair techniques may not be enough to guarantee the required service life of the intervention. For instance, this may be related to the inadequacy of concrete cover thickness, high aggressiveness of the environment, or the need to remove large quantities of structurally sound concrete. In such cases, electrochemical techniques can be an advantageous option, since they can interrupt corrosion propagation, leaving the concrete in place, provided it has not been damaged yet.2) to protect steel reinforcement. Re-alkalisation and chloride removal are temporary treatments that rely on the application of a much higher current density (up to few A/m2) for several weeks or a few months to change concrete’s composition and restore its ability to protect the reinforcement.Electrochemical techniques can be permanent or temporary ,64,65,66All techniques rely on the application of a cathodic current to the reinforcement, usually supplied by a metallic anode placed on the concrete surface . The main effect of the current is a reduction in steel potential, and a subsequent reduction in the corrosion rate. This effect is temporary (it is lost if the current is switched off); as long as the current is circulating, the rate of the anodic reaction (oxidation of iron) is depressed and the rate of the cathodic reaction (production of OH-) is increased. If very negative potential values are reached, the cathodic reaction of hydrogen evolution can also occur, and its consequences should be carefully evaluated in the case of high-strength steel bars in prestressed concrete structures ,68.The applied current also changes the properties of the concrete. The already mentioned production of alkalinity increases the pH at the steel–concrete interface, strengthening the protection so that short interruptions of the applied current can be tolerated. Moreover, the movement of anions in opposite directions to the current spreads alkalinity over larger areas of concrete and takes chlorides away from the rebar. These effects are beneficial, but marginal, in the case of cathodic protection; conversely, they are of primary importance for temporary treatments.All electrochemical techniques need monitoring to check that the current is flowing and it is enough to protect the reinforcement  .Traditionally, cathodic protection is applied to structures suffering chloride-induced corrosion, typically slabs of bridges and viaducts or marine structures; however, it can be conveniently applied to carbonated concrete as well. The use of sacrificial anodes is also possible, by inserting discrete elements of zinc into the concrete and connecting them to the rebars  ,71,72.Temporary treatments are used when it is not possible to apply a permanent anode, either for technical or aesthetical requirements. They can be used on complex shapes  or if the texture of the surface cannot be changed . Their effectiveness is more limited in time, and also because the cause of corrosion  continues after the treatment; it is also reduced if corrosion products are abundant at the steel–concrete interface ,74,75.In the last decade, graphene-based cement composites (GBCCs) have been strongly investigated due to their enhanced mechanical properties, often combined with smart properties , fire resistance and freeze-thaw resistance . MoreoveThe family of graphene-based materials (GBMs) is wide and includes several nanomaterials, classified on the basis of three key parameters: number of graphene layers, average lateral size and C/O atomic ratio .A number of different characterization techniques are thus needed to fully characterize GBMs, such as Raman spectroscopy, infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) combined with an extensive investigation by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) . RecentlOne of the main issues with graphene-based materials is the workability of the fresh mortars, which is severely dependent on both the type and amount of the loaded GBMs. Usually, due to the tendency of these nanofillers to absorb water molecules, the flowability of GBM-modified cementitious admixtures is reduced; the extent depends on several different parameters, but mostly on the C/O ratio . Further− ions in cementitious matrix [Analogously, scientists involved in this adventurous field are aware that the mechanical and physical properties of hardened graphene-based cement nanocomposites strictly depend on the selection of type  and dosage  of the loaded graphene-based materials . For this matrix ,87. Du as matrix  showed ts matrix  publisheObviously, safety, costs and life cycle assessment (LCA) should be rigorously evaluated for large-scale applications ,91.The development of new inorganic matrix composite systems with strong mechanical performance has introduced into the construction industry new potential tools to repair and improve both the resistance and durability of existing concrete and masonry structures ,93,94. TFRCM systems consist of an inorganic matrix  reinforced with fabrics in the form of open grids . These systems, which have a thickness of between 10 and 15 mm, are specifically designed to be applied as external reinforcements on existing masonry or concrete structures a–c. As cIn recent years, the scientific community has paid great attention to this new class of composite materials, and some guidelines giving instructions on how to design a strengthening intervention with FRCM have been provided by the American Concrete Institute (ACI 549.4R-13) and Italian CNR (DT215/2018) ,96, whilCRM systems are made of a pre-impregnated composite mesh (FRP) embedded within inorganic mortar  with a compressive strength between 5 and 20 MPa. In these systems, the FRP mesh bears the tensile stresses, while the structural mortar is responsible for the stress transfer between the composite grid and the substrate. The transfer of stresses between the support to be reinforced and the reinforcement mesh is also guaranteed by the presence of connectors, which ensure the structural collaboration between the wall element and the reinforced plaster. The total thickness of CRM systems is usually between 30 mm and 50 mm. The most-used fibers are made of AR glass, carbon or basalt, coated with a thermosetting polymer matrix. These systems have had strong development in the last 5 years and a high diffusion especially into the Italian construction market. Further spread of these systems has also been facilitated by the recent technical guidelines for identification, qualification and acceptance control of CRM issued by the Italian CSLLPP .Although recently introduced, CRM showed promising results in improving the mechanical performance of different masonry structures, such as walls and arches ,118,119.3), small size aggregates (<6 mm), binders , superplasticizers and a low water/cement ratio (w/c < 0.2). Steel fibers (length of 10÷20 mm) are usually added with a dosage of about 2–3% by volume to significantly improve tensile strength and strain capacity [UHPFRCs are advanced cementitious materials with excellent mechanical properties: they can achieve compressive strength greater than 150 MPa, flexural strength higher than 40 MPa and considerable tensile-strain-hardening behavior ,122. Duecapacity ,132,133.capacity ,122, is capacity  and the capacity . HoweverDifferent strategies can be used to avoid or to reduce structure degradation or to repair structures that are already damaged . The useA further improvement in terms of sustainability can derive from the substitution of traditional aggregates used for cementitious mortars with more innovative ones. Indeed, by varying the nature, the morphology and the chemical properties of the aggregates, several benefits can be obtained. As for fibers, aggregates can also be prepared starting from secondary raw materials ,158,159.Silica fume (SF) and natural pozzolans (NP) have been widely used as substitutes for Portland cement (OPC) for concrete manufacturing, in relation to their advantageous properties, including reduced environmental impact, low heat of hydration, low water permeability, high chemical resistance and resistance against the alkali–silica reaction, improved fresh properties, low shrinkage and reduced cost ,168. In The interaction, in terms of the production of C-S-H, between NP and SF and an aqueous suspension of nanolime particles (nCH) produced in the laboratory by means of a patented, eco-friendly and one-step synthetic route based on an ion-exchange process makes it possible to obtain large amounts of product ,175,176.2, 0.79% Fe2O3 and minor amounts of MgO, K2O, CaO and Al2O3, while the NP is composed of 69.17% SiO2, 1.16% TiO2 and minor amounts of Al2O3, K2O, SO3 and Fe2O3. BET analyses reveal surface area values of about 12.5 m2/g and 60 m2/g for SF and NP, respectively. The synthesized nCH appears pure and crystalline, in the form of hexagonal lamellas consisting of primary nanoparticles with dimensions ≤ 10 nm. Moreover, the aqueous suspensions of nCH show a high reactivity, guaranteeing a complete conversion into calcium carbonate, in the form of pure calcite, in a few hours, even in low-relative-humidity conditions [2 is consumed and the C-S-H formation proceeds in time appearing more crystalline, mainly in the C-S-H(I) form. The mixture with NP is characterized by a high reaction rate, especially at low hydration times, with the formation of C-S-H after only 1 day of aging. Nevertheless, the Ca(OH)2 is not completely consumed up to 14 and 60 days, considering nCH/NP ratios of 1:2 and 1:1, respectively. The obtained results are very promising if compared with the literature data, which indicate a progressive reduction in free lime over time, leading to a total consumption at 90 and 365 days when silica fume or natural pozzolan are employed, respectively. Considering BET results, specific surface values of about 121 m2/g and 154 m2/g are obtained, referring to nCH/SF and nCH/NP mixtures with 1:2 ratios, respectively. Finally, SEM images of nCH/SF mixtures after 28 days of hydration show the formation of crumped foils, with a thickness of few nanometers, characterized by the typical fibrous structure. In the nCH/NP mixtures, the formation of highly wrinkled layers covering all particles, constituted by marked crumple and rough-edge surfaces, is observed as well . Furthermore, the acquisition of data makes it possible to provide a large functional database for the development and refinement of the repairing materials and techniques of the future.2 emissions, limited consumption of energy and natural resources, reuse of waste products and use of recyclable materials) capable of guaranteeing a prolonged service life for the building as discussed in a previous review by the authors [Therefore, starting from the knowledge of the building, the choice of traditional or innovative techniques will have to take into account, in addition to all the issues related to the quality of the restoration work, also sustainability, thus preferring the use of low-environmental-impact solutions . Moreover, some innovative techniques and materials for repairing building heritage  are presented.


In the foamed-concrete-backfilled-gas-pipeline project, the fluidity of foamed concrete has a great impact on the construction quality. This research studied the fluidity of foamed concrete through laboratory tests. By changing the water–cement ratio, admixtures, additives, foaming-agent content and other test parameters, foamed concrete with different fluidities was prepared, and the effects of the above parameters on the fluidity of foamed concrete were analyzed. At the same time, the construction equipment was improved in the three steps of transportation, production and pouring. The results show the factors affecting the fluidity of foamed concrete are, in order of importance, foaming-agent content > water–cement ratio > water-reducer content > admixture content. According to the orthogonal-test results, the control scheme meeting the fluidity requirements of the actual engineering project had gains as follows: the water–cement-ratio range from 0.5 to 0.6, the amount of admixture from 35% to 40%, the water-reducer content at 1% and the foaming-agent content from 3% to 3.5% so as to ensure the automatic leveling of foamed concrete under the best flow state. The overall route is shown in The China–Russia East Route is a long-distance natural-gas pipeline with the largest diameter and the highest pressure in China and even the world. Its trunk route starts at Heihe City, Heilongjiang Province; passes through nine provinces, including Heilongjiang, Jilin, Inner Mongolia, Liaoning, Hebei, Tianjin, Shandong, Jiangsu and Shanghai and ends at Baihe Town, Shanghai. The total length of the trunk route is 3334.6 km, the designed transmission capacity 380 × 108 mAccording to the design scheme of the project, three D1422 mm pipelines are laid in the Yangtze River Shield Tunnel, one for the China–Russia East-Route pipeline, one for the Jiangsu coastal pipeline and one for the reserved pipeline shared by both parties. According to the requirements of the Engineering Construction Project Department of the National Pipe Network Group, the tunnel shall be backfilled after the pipeline installation in the tunnel is completed, as shown in The common backfilling methods for gas-pipeline tunnels include water backfilling, air backfilling and foamed-concrete backfilling. Water backfilling is widely used due to its convenient construction and maintenance, but it has a great impact on the tunnel structure. For air backfilling, the tunnel section must be extended to ensure that there is enough inspection space inside. In addition, air backfilling also calls for channels, ventilation facilities, lighting systems, gas detection, etc., and mandatory measures to prevent pipeline expansion or contraction caused by temperature changes, which will increase the project investment. Therefore, the foamed-concrete-self-leveling-backfilling method is adopted in this project.Foamed concrete has advantages such as light weight and thermal insulation. With this method, the pipeline can be prevented from being damaged by external forces and daily maintenance. The pipeline has good restraint and structural seismic performance. It can not only improve the safety of pipeline operation but also facilitate pipeline operation and management. Foamed concrete is a lightweight porous concrete made of cement, the main cementitious material. After introducing bubbles into the slurry made of aggregates, additives, water and other components, it is mixed, poured and cured. At present, it is widely used in the fields of pipeline backfilling, roof insulation, cushioning and energy absorption ,2,3,4,5.Scholars at home and abroad have done a lot of research on foamed concrete. Xin Yucong et al.  have stuTaking the research results of scholars at home and abroad into consideration, it is found that the self-leveling-construction method of foamed concrete has high requirements for fluidity and on-site pouring equipment. To realize the automatic leveling of foamed concrete in construction, it is necessary to optimize the mix proportion of foamed concrete.Niu Yunhui  found inTherefore, this paper first carried out the test of foamed-concrete-fluidity determination, and determined the mix proportion range when the fluidity of foamed concrete reached the maximum. Then, through the investigation of the project and referring to the existing construction technology and methods, the working mode of the construction equipment was optimized, and a new foamed concrete production and backfilling system was designed to ensure that the bulk cement can be transported to the production vehicle in time and in sufficient quantity so as to produce the foamed concrete immediately and use it to cast in place without delay.The new foamed-concrete-production-and-backfilling system was combined with foamed concrete with high fluidity to play a great role in the backfilling construction of natural-gas pipelines in long-distance and confined-space tunnels. Thus the poured foamed concrete can level automatically and construction risks and human disturbance can be reduced so as to make the whole pouring process almost automated.Cement is an important cementitious material of concrete, and the hydration of cement is the basis for the hardening of concrete. Therefore, the performance of cement has a great impact on the performance of concrete. The cement used was ordinary portland cement from Huaxin Cement (Nantong) Co., LTD., Huaxin Cement Co., Ltd., located in Nantong, China, whose physical indicators are shown in GB/T51003-2014 Technical code for Application of Mineral Admixtures [Adding a certain amount of HTFC foamed-concrete admixture into ordinary portland cement can improve the problems of easy pulverization and cracking of foam concrete in the later stage and reduce the hydration of the system at the same time. HTFC foamed-concrete admixtures (hereinafter referred to as “HTFC admixtures”) were selected as admixtures. HTFC admixtures are nano-level inorganic mineral fine powder formed by compounding and evenly mixing with a variety of minerals as the main raw materials. The HTFC manufacturer is Henan Huatai New Material Technology Co., Ltd., which is located in Henan, China.The testing standard is mixtures . HTFC adThe water-reducer agent can significantly improve the fluidity of foamed concrete with the same mix proportion, significantly reduce the amount of water used with the same fluidity and cement dosage, improve the strength and durability of concrete and effectively reduce the cost. Polycarboxylate superplasticizer was selected as the water-reducer agent, The manufacturer of water reducing agent is Laiyang Hongxiang construction admixture factory, which is located in Laiyang City, China, and its indexes are shown in Foaming agent is one of the main additives in the production and preparation of foamed concrete, which has an important impact on the physical and mechanical properties and pouring constructability of foam concrete. In this experiment, HT foamed-concrete-foaming agent was adopted. The manufacturer of foaming agent is Henan Huatai New Material Technology Co., Ltd., which is located in Henan, China. Standard of Water for Concrete (JGJ 63-2006) [The water used for mixing foamed concrete was tap water from Qingdao City, which conforms to the requirements in 63-2006) .Foaming machine: Bl168-8, Hefei Baile Energy Equipment Co., Ltd., Hefei, China; Mixer: HJW60, Wuxi Jianyi Instrument Machinery Co., Ltd., Wuxi, China; Truncated-cone round die: 100 × 70 × 60 large clean slurry testing die with upper and lower caliber and height of 70 mm, 100 mm and 60 mm, respectively. Thetruncated-cone round die is shown in The orthogonal test ,24,25 fuFirst, HTFC admixtures and other cementitious materials, as well as other materials except foaming agent, were evenly mixed, and the water-reducer agent was dissolved completely in water, which then was added to the uniformly stirred cementitious material and stirred for about 3 min. HT composite foaming agent was added while maintaining high-speed stirring for about 60 s.The smooth-glass plate, truncated-cone round die, flat cutter and measuring cup were cleaned and wiped, and then the truncated-cone die was placed on the smooth-glass plate, while the sample measured by measuring cup was poured slowly in the truncated-cone round die. The truncated-cone round die was tapped by fingers outside to fill it completely. The flat cutter was used to scrape along the port plane of the round die. Then slowly the truncated-cone round die was lifted vertically with both hands and kept there for 1 min. Finally, the maximum horizontal diameter was measured with a vernier caliper; thus the fluidity was obtained.4) with four factors and three levels was designed, as shown in There are four factors, including water–cement ratio, HTFC admixtures dosage, water-reducer-agent content and foaming-agent content, and three levels were set for each factor. Therefore, the orthogonal test of L9Transportation trolley system setting: two sets of 20 t large-storage and transportation-tank transportation trolley were configured as the transportation trolley to transport bulk cement. The working mode was reciprocating and non-intermittent transportation. One set was transported after being filled, the other continued to load, and after being transported in place and unloaded, it returned to the bottom of the shaft to continue loading and then continued to transport so as to ensure that the bulk cement is transported to the production trolley timely and sufficiently;(2)Production trolley system setting: the cast-in-situ foam concrete shall be prepared and used at any time, and the retention time shall not exceed 30 min. Therefore, a set of on-site production system of foam concrete was set on the electric track trolley, so that it can move freely in the long-distance tunnel. The system consists of a set of 20 t bulk cement storage tank, a set of 15 t bulk cement storage tank, a closed cement mixer, a cement foaming machine and a tractor. All equipment shall be installed horizontally to meet the needs of production and pouring construction in the confined space;(3)3/h, the single-layer pouring in the pouring block can be completed in a shorter time to ensure that the foam concrete flows smoothly and evenly under the best flow state.Setting of pouring trolley system: the pouring trolley system was set at the front end of the foamed concrete production trolley, and the mixed foam concrete can be immediately pumped to the pouring trolley through the pipeline for pouring. The pumping wooden frame was set at the front end and both sides of the pouring trolley. The height of the wooden frame can be adjusted according to the pouring needs. The pumping pouring pipes were placed equidistant on the pumping wooden frame, and multiple pipes were poured at the same time. In combination with the digital control foam concrete production trolley system with the rear foam concrete conveying capacity up to 60 mIn order to meet the requirement of foamed concrete self-leveling construction technology, this paper further designed a set of digital foamed-concrete-production-and-filling system suitable for long-distance and confined-space foam-concrete backfilling, and the scheme is as follows:The complete set of digital automatic control system is composed of three parts: transportation, production and pouring trolley system. This system has the advantages of fewer errors and large output and is applicable to the featured requirements of the confined space and long-distance self-leveling construction of the project. The digital foamed concrete production backfilling system is shown in Relying on the foamed-concrete-filling project of the Yangtze River Shield Crossing Project of the China–Russia East-Route natural-gas-pipeline project, the self-leveling method applicable to the construction of long-distance and confined-space tunnels filled with foamed concrete is studied. The influence of water–cement ratio, admixtures, additives and foaming-agent content on the fluidity of foamed concrete is analyzed. The results show that the factors affecting the fluidity  of foame3/h, which was suitable for the featured requirements of confined space and long-distance self-leveling filling of the project. Only when the mix proportion of foamed concrete was controlled to make the slurry meet the fluidity requirements and new construction technologies and equipment were used, could the foam concrete achieve better flow effect in the pouring process.After a series of preparatory work, it is found that the foamed-concrete self-leveling-construction method has high requirements not only for the fluidity of foamed concrete but also for the on-site construction equipment. The existing construction technology and equipment could not achieve the desired effect. In order to meet the requirements of foam-concrete self-leveling construction technology, this paper further designed a set of digital foamed-concrete-production-and-filling system suitable for long-distance and confined-space foamed-concrete backfilling. The system adopted digital automatic control and had the advantages of fewer errors and large conveying capacity. The conveying capacity could reach 60 mThe research results were applied to the foamed concrete filling project of the Yangtze River Shield Crossing Project of the China–Russia East-Route natural-gas-pipeline project and achieved good application results in the project construction. On the basis of this project, this paper forms a theory about the foamed-concrete-self-leveling-construction method and construction equipment through systematic tests. This theory is not only applicable to the foamed-concrete-filling project of the Yangtze River Shield Crossing Project of the China–Russia East-Route natural-gas-pipeline project but also generally applicable to the foamed-concrete-filling project of the tunnel with long construction distance and complex internal structure. The novelty of this study lies in that, firstly, foamed concrete with a fluidity of 400 mm innovatively exceeds the maximum fluidity of traditional foam concrete, and a new maximum fluidity of foam concrete is obtained. Secondly, the existing construction equipment and technology can only be backfilled in short-distance tunnels. The layout of the equipment in the three construction processes of production, transportation and pouring is scattered and the operation is complex. Based on this, we innovatively design this equipment in a unified way to form a complete set of automatic foam-concrete production and backfilling system so that it can complete foam concrete backfilling in long-distance tunnels.In a word, foam concrete appeared not late in China, but foam concrete is rarely used in tunnel filling in China. The digital foamed concrete production backfilling system designed in this study and the proposed mix proportion scheme of foam concrete are expected to provide a theoretical basis for the research of domestic advanced construction methods and technologies.


In the case of the chemical interaction of BP with DPA, the reaction product corresponds to DPA chemically functionalized BP sheets having an imino-phosphorane (IP) structure. The electrochemical oxidation of BP sheets chemically functionalized with DPA in the presence of PTA leads to an increase in the weight of P-N bonds as a consequence of the generation of PDPA doped with the PTA heteropolyanions, as shown by FTIR spectroscopy and Raman scattering. This process is evidenced by a shift of the Raman line from 362 cm−1 to 378 cm−1, assigned to the A1g mode. This change was explained by taking into account the compression of the layers containing P atoms, which is induced by PDPA macromolecular chains. The decrease in the intensity of the PL spectra of DPA as well as PDPA, in the presence of BP, indicates that BP acts as a PL quenching agent for these compounds. A preferential orientation of the PDPA doped with the PTA heteropolyanions on the surface of BP sheets is highlighted by the variation of the binding angle of the PDPA on the surface of BP sheets from 44.7° to 39.9°.In this work, new information concerning the optical properties of black phosphorus (BP) sheets chemically/electrochemically functionalized with diphenyl amine (DPA) and its macromolecular compound (poly(diphenylamine) (PDPA)) in the absence/presence of phosphotungstic acid (PTA) is reported. Raman scattering and FTIR spectroscopy studies indicate that the interaction of BP with PTA leads to the elimination of the P The following conclusions can be drawn:−1 and 876 cm−1. The formation of iminophosphorane groups on the BP surface was also reported through the interaction of black phosphorus with 4-benzoic acid azide )


This study investigates the engineering and mechanical properties of basalt fiber-reinforced (FRF) concrete, giving special attention to residual flexural strength and dynamic modal parameters. These properties, which have not been thoroughly investigated elsewhere, are a precursor to structural design applications for dynamic compliant structures . Accordingly, the standard notched flexural tests have been carried out to assess the basalt fiber-reinforced concrete’s residual flexural strength with an additional 0.125%, 0.25%, 0.375%, and 0.5% of volume fraction of basalt fiber. In addition, dynamic modal tests were then conducted to determine the dynamic modulus of elasticity (MOE) and damping of the FRF concrete beams. The results indicate that concrete’s toughness and crack resistance performance are significantly improved with added fiber in basalt fiber reinforced concrete, and the optimum fiber content is 0.25%. It also exhibits the highest increment of compressive strength of 4.48% and a dynamic MOE of 13.83%. New insights reveal that although the residual flexural performance gradually improved with the addition of basalt fiber, the damping ratio had an insignificant change. Fiber-reinforced concrete (FRC) has gained momentum in industrial applications in recent decades, especially for bespoke products and components using a low-to-moderate strength of concrete. With the pioneering publications of steel fiber reinforced in concrete by Romualdi and Batson  and RomuGenerally, the improvement of fiber reinforced concrete from ordinary concrete includes flexural strength, spitting strength, compressive strength, elastic modulus, crack resistance, self-healing ability, abrasion resistance, shrinkage and expansion resistance, and thermal characteristics. Meanwhile, with the development of high-performance materials technology, there are more types of fiber, such as glass polypropylene and carbon fiber, that have been applied in the research field of fiber reinforced concrete. The advanced fiber not only contributes to the improvement of mechanical properties, but is more environmental-friendly and even more inexpensive than the traditional steel fiber ,12,13,14Recently, basalt fiber, as one of advanced fibers applied in the field of fiber reinforced concrete, has drawn attention from concrete engineers due to its excellent work in the improvement of mechanical properties and its contribution to sustainable development . HoweverBasalt is known as a natural, solid, dense, and dark brown to black volcanic igneous rock formed about 100 to 300 km under the Earth’s surface. It can turn the surface into molten lava with a melting temperature ranging from 1500 to 1700 °C .2 and Al2O3 are the two primary components.The condition of basalt is significantly influenced by the temperature of the quenching process, which results in highly complete crystallization. As the two significant minerals, plagiocene and pyroxene account for over 80% of basalts. By examining the chemical makeup, Fiore V et al.  made it 3, the tensile strength is between 2600 to 4840 MPa, and the range of the elastic modulus is 80–115 GPa [Basalt fiber is also regarded as a new highly-recommended reinforcement material in concrete compared to the relatively common types, including carbon, aramid, and glass fibers. As the quenching process is environmentally friendly with high security ,18, such–115 GPa .Although basalt fiber can be relatively more expensive than some other fibers, it is cheaper than carbon fiber and S-glass fiber. Meanwhile, basalt fiber is more stable in a radioactive environment than synthetic and organic fibers. Since the appearance of related research on the composition of basalt fibers in the 1980s, exhaustive studies of the features and durability of concrete reinforced with basalt fiber have been conducted in the past five years which shows the material’s comparatively good resistance to salt, water, corrosion, and severe degradation in an alkaline environment ,33.Moreover, it is globally accepted that chopped basalt fibers can be added to concrete to offset the defects. Therefore, in the past two decades, blending fibers into concrete has been widely applied in some projects ,36,37,38The dosage of basalt fiber and aspect ratio could deeply influence the mechanical behavior of basalt fiber reinforced concrete . Therefo3, respectively. According to the pervious study, the performance improvement with the addition of basalt in normal strength concrete or high strength concrete are similar with each other [By studying the impact of the length and volume fraction of basalt fiber on the compressive and flexural strength test under three different lengths and three different dosages, Iyer et al.  have furch other . This shFollowing the examination by Highet al.  of the uFurthermore, according to Algin’s  study onLimited studies have been conducted on the dynamic properties of basalt fiber reinforced concrete under dynamic loading conditions such as blast loading, impact response, damping ratio, dynamic modulus of elasticity, and dynamic compression characteristics.Several studies have recognised moisture content as a primary element of damping capacity. For example, according to Jordan , the moiMeanwhile, the effect of frequency  on dampiAlthough studies on the impact of adding steel fiber on the dynamic properties of fiber reinforced concrete show that the damping property of wet-cured fiber reinforced concrete is over 50% greater than plain concrete, the dynamic modulus of elasticity stays relatively constant. Furthermore, ageing and drying can significantly decrease the logarithmic decrement on both fiber reinforced concrete and plain concrete . In addi3. Meanwhile, the concrete specimen is also found to behave in a notably ductile manner due to the increased toughness with the strain rate. In addition, basalt fiber has been tested to optimize the micropore, which is also suitable for high-grade concrete.According to Zhang et al. ,the impaAfter exploring the dynamic properties of basalt fiber reinforced concrete with various fiber volume fractions (1~3%) under different temperatures, Ren et al.  have conFurthermore, according to the micro studies of Fu et al. , basalt The amplitude and frequency content of the excitation source  can greaHowever, the dynamic properties of basalt fiber reinforced concrete have not been identified in the literature, and limited studies have been conducted on combining the residual flexural performance and dynamic properties of fiber reinforced concrete. Therefore, this unprecedented study will primarily explore the performance of basalt fiber-reinforced concrete on residual mechanical properties with the variation of fiber dosage and its dynamic modulus of elasticity test. Moreover, the micro-crack in the real construction work can hardly be identified and some advanced non-destructive testing will be expensive. However, the micro-cracks will be sensitive to vibrations, and the natural frequency is a key parameter to describe the vibrations in concrete. In that case, using the model test to get the natural frequency calculating the dynamic modulus of basalt fiber reinforced concrete in real construction, then comparing the results with the dynamic modulus proposed in this study, is beneficial for engineers to justify the location and number of micro cracks. In this context, this study is the world’s first to identify the dynamic properties of basalt fiber reinforced concrete and the potential relationship between the dynamic and mechanical properties by a series of experimental studies. The new dynamic property tests of basalt fiber reinforced concrete are essential to further clarify the performance of basalt fiber reinforced concrete in shock absorption and sound insulation, and is beneficial to the development of new non-destructive testing.The concrete specimens in this experiment are all made with Portland cement, well-graded aggregates, water, and basalt fiber . PortlanThe alkaline-resistant basalt fiber is adopted from Deutsche Basalt Faser GmbH (DBF), in which the basalt is chopped to the desired length under a high temperature. This type of fiber also known as integral fibers, such chopped alkaline-resistant fibers have been tested to show no damage when exposed to an alkaline environment at 40 °C and a pH value of 13.5 for 365 days according to the description of Institute of Construction Materials of the Technical University of Dresden. Furthermore, they also present better adhesion performance in concrete due to their special coating. In addition, as shown in Five concrete mixes have been designed for this study with a water-to-cement (w/c) ratio of 0.55. For all the mixes, the proportions of each concrete mix are 1: 3.1: 2.25 by the mass of cement, fine aggregate, and coarse aggregate, with the details of each concrete mix shown in The coarse aggregate has been prepared under full washing in case of sand’s adhesion on the surface. In addition, both coarse and fine aggregates are dried at the temperature of 105 °C for 48 h.0 (plain concrete), M1 (0.125%), M2 (0.25%), M3 (0.375%), and M4 (0.5%).As demonstrated in a review of previous literature, limited studies on the mechanical properties of basalt fiber-reinforced concrete have shown that the optimal dosage of the basalt fiber roughly ranges within 0.5% of the total concrete volume. Hence, four different dosages are used for each type of basalt fiber reinforcement, ranging from a low dosage to the maximum mixable dosage. In addition, the chopped basalt fiber is added at 0.125%, 0.25%, 0.375%, and 0.5% of content by the total volume of the concrete mix. Finally, the concrete mixes were labelled as MxC/Bx. For example, M1C2 means that the concrete specimen is the second cube containing 0.125% of basalt fiber reinforcement. Similarly, M2B3 shows that the concrete specimen is the third beam containing 0.5% of basalt fiber reinforcement.According to the standard test method for fundamental transverse, longitudinal, and torsional resonant frequencies of concrete specimens (ASTM C215-19), to test the hardened concrete, two sizes of concrete specimen have been prepared for compression testing (Cube: 150 mm × 150 mm × 150 mm). This study employs the modal test and the three-points load bending test (rectangular prism: 150 mm × 150 mm × 600 mm), while three beams and five cubes have been made for each mix to ensure enough sample size and avoid errors. Hence, each concrete specimen is labelled as MSome relevant tests in the laboratory have been conducted, including the slump test (BS EN 12350-2:2019), the compression test (BS EN 12390-3:2019), the residual flexural strength test (BS EN 14651:2005), and the dynamic modal test (ASTM C215-19).In these tests, the components have been placed in the remixing container in the following order: aggregates, cement, basalt fiber, and water. When the cone and base plate are dampened and the excessive moisture with the moist cloth is removed, the concrete matrix is filled to the slump retention at a specific time with three layers compacted with 25 strokes of the compacting rod, respectively. The cone is then raised for 2 to 5 s to avoid the lateral or torsional motion formed on the concrete. By measuring the heights of the cone and the highest point of the slumped test specimen, the slump h can be calculated and recorded, as shown in 3 (cube) and 150 × 150 × 600 mm3 (rectangular prism). Moreover, the 5-mm-wide notch is placed on one side of the cuboid mold for a three-point bending test of a single-side notched beam on the specimens. Finally, the specimens are demoulded after 48 h and cured under the water for at least 28 days.When the mixing of the concrete matrix is done, it is cast in a mold with the sizes of 150 × 150 × 150 mmThe compressive strength of the cube specimens has then been tested under a universal testing machine from Norton Hydraulics (Midland) in Birmingham, UK at the rate of 0.5 MPa/s. In fact, the flexural performance of the beam specimen can be determined by the load-displacement curve obtained by simply testing the supported beam specimens and load-CMOD (Crack Mouth Opening Displacement) curve obtained by the Crack Opening Displacement (COD) gauges.Furthermore, the resonant frequencies of the dynamic properties of specimens can be obtained from the detected driving force with a hammer, which is shown in Lastly, the modal test is normally set ahead of the three-point loading test because it causes no damage to the specimen. In addition, the mass and dimensions of the specimen are measured to calculate the dynamic MOE (modulus of elasticity) from Equation (1) in ASTM C215-19.M = mass of the specimen, kg,n = fundamental transverse frequency, Hz,3T/dt3), m−1 for a prism,C = 0.9464 .0. However, when adding basalt fiber with the volume fraction of 0.125%, 0.25%, 0.375% and 0.5%, the slumps of the concrete matrix has fell to 40 mm, 33 mm, 26 mm, and 18 mm, respectively. Therefore, the addition of fibers in concrete can result in a reduction in the workability of the concrete. In this way, the dispersed basalt fiber in the concrete, which might form a net-shaped structure, makes this phenomenon possible because it has prevented the mixture’s segregation and flow. According to the degree of workability suggested in the standard, the range of slump is 25–50 mm, which means that the workability of concrete is low and the slump between 0–25 mm is very low. It can be seen that the addition of basalt fiber led to a decrease in the workability of concrete, and the level of workability dropped to very low workability with a fiber content of 0.5%. Due to the fiber’s high content and surface area, it may absorb more cement paste to wrap around the fiber itself, making the mixture more viscous while reducing the slump [According to he slump .4, the value of the 0.5% volume fraction appears to be flat on the M1 (0.125%). However, compared to plain concrete, certain improvements can still be achieved.According to 2 are shorter than plain concrete specimens after the compressive loading. In other words, the distribution of cracks is considered more dispersed with the increasing fiber in the concrete specimen. According to Besides, a previous Branston study  shows thfc,t,F the span length shown as L, and the residual flexural tensile strengths referred to as R,1, FR,2, FR,3F, and R,4F listed in j. In this way, R,1F, R,2F, R,3,F and R,4F are determined by the corresponding CMOD values of 0.5 mm, 1.5 mm, 2.5 mm, and 3.5 mm, respectively.WhereThe results of the three specimens of each mix were obtained under the there-point load bending test. Each median result of the three specimens has been taken as a reference for a more concise analysis on flexural performance considering the results of each mix varied in a small proportion, as shown in ct, L) of the plain concrete specimen stays at 3.49 Mpa with the peak load of 9.078 kN. With the increasing volume fraction of basalt fiber from 0.125% to 0.5%, the increment amount ranges from approximately 4.15% to 46.85%. In addition, the displacement at the peak load is shorter and earlier than plain concrete when more fiber volume fractions are added. Therefore, the increment has shown a uniform growth curve, with M4 (0.5%) having the peak load of 13.331 kN and the fastest speed to reach it.The continuous growth of flexural strength can be observed to exist with the increasing dosage of basalt fiber in the concrete specimens. Meanwhile, the LOP . According to the experimental result, the dynamic MOE and the natural frequency present a similar trend when the basalt fiber is added. The natural frequency will increase to 1345.6 Hz when 0.25% of basalt fiber is added to the concrete, showing a dynamic MOE of 31.44 GPa. When considering plain concrete as a benchmark, the increment of dynamic MOE is 13.83% at the peak (0.25%). The dynamic MOE then sees a continuous decline with a slower slope than the rising slope. In comparison, when adding 0.5% of basalt fiber, the dynamic MOE is 29.86 GPa, which is lower than the M2 (0.25%) value. However, compared to plain concrete, it still has an 8.1% increment.2 (0.25%) showing the highest compression and dynamic MOE value. In addition, according to the load-displacement curve of the residual flexural tensile strength, M2 (0.25%) is the mix that has reached its peak load, which, in another method, can represent the relationship between the strength and strain of the concrete specimens.From the results of the modal test, as shown in 2 (0.25%) has the highest compression and dynamic MOE value. In addition, the load-displacement curve of residual flexural tensile strength also showed that M2 (0.25%) is the fastest mix to reach its peak load, which, in another method, could display the relationship between the stress and strain of the concrete specimens.From the result of the modal test  and compResearch  shows thFurthermore, a previous study  also shoBased on the results from a series of experiments, conclusions can be drawn to develop new insights into basalt fiber reinforced concrete for industrial applications.The addition of basalt reduces the workability of concrete significantlyThe compressive strength of basalt fiber concrete does not increase with the addition of fiber, and the suggested fiber content is 0.25%The flexural strength increases sharply with the addition of basalt fiber when keeping the fiber content lower than 0.5%The MOE of basalt fiber concrete is affected by the compressive strength, and there is no significant impact on the damping ratio of concrete with the addition of basalt fiber.The workability of fresh concrete elicits signs of decline with the increasing dosage of basalt fiber, with the slump decreasing from 44 mm to 18 mm from plain concrete to concrete, with a volume fraction of 0.5% for basalt fiber. Among concrete with volume fractions of 0.125%, 0.25%, 0.5%, 0.375%, and 0.5% of basalt fiber in plain concrete, the compression of concrete with 0.25% of basalt fiber dosage has the highest performance (43.98 MPa), which is 4.48% higher than that of plain concrete. Meanwhile, the flexural strength (LOP) has greatly improved, showing a monotonical trend of increase when the volume fraction of basalt fiber is increased from 0.125% to 0.5%. The peak flexural strength is 4.15%, 20.61%, 28.70%, and 46.85%, respectively. Furthermore, the residual flexural tensile strength shows a smooth descending CMOD-load curve after the peak when the basalt fiber content is above 0.25%. In addition, the damping ratio remains around 0.60, which ranges from plain concrete to a volume fraction of 0.5% for basalt fiber. Given the results of the experimental study, these new conclusions can be drawn:This study is the first to demonstrate the essential parameters of mechanical and dynamic properties of basalt fiber concrete through experimental studies. Based on these new insights, engineers can evaluate the performance of their basalt fiber concrete structures, especially in aggressive environments. In addition, the dynamic moduli and damping coefficients reported in this study can be applied for dynamic structural designs and the non-destructive testing of basalt fiber reinforced concrete structures.


Global climate changes cause extreme temperatures and a significant reduction in crop production, leading to food insecurity worldwide. Temperature extremes (including both heat and cold stresses) is one of the most limiting factors in plant growth and development and severely affect plant physiology, biochemical, and molecular processes. Biostimulants like melatonin (MET) have a multifunctional role that acts as a “defense molecule” to safeguard plants against the noxious effects of temperature stress. MET treatment improves plant growth and temperature tolerance by improving several defense mechanisms. Current research also suggests that MET interacts with other molecules, like phytohormones and gaseous molecules, which greatly supports plant adaptation to temperature stress. Genetic engineering via overexpression or CRISPR/Cas system of MET biosynthetic genes uplifts the MET levels in transgenic plants and enhances temperature stress tolerance. This review highlights the critical role of MET in plant production and tolerance against temperature stress. We have documented how MET interacts with other molecules to alleviate temperature stress. MET-mediated molecular breeding would be great potential in helping the adverse effects of temperature stress by creating transgenic plants.  Plants are infrequently affected by certain factors, and temperature stress alone or combined with other stresses may trigger oxidative impairment in plants. Consequently, HS and CS impede plant development as a consequence of cellular injury or even cause cell death by reducing membrane fluidity and antioxidant enzyme activities, biosynthesis of diverse proteins and secondary metabolites, and amendments in hormonal signaling after continuing exposure.14Plants grow in a situation that implements various environmental factors (abiotic and biotic stresses), and discrepancy in any of these factors can hinder several morphological, physiological, and molecular variations at many steps; ultimately, plant growth and production get hampered by these stresses.11 To cope with temperature stress, plants modify their morphology, physiology, biochemistry, and gene expression, which alters their growing activities to grow and tolerate HS and CS conditions.15 Temperature extremes (HS and CS) can cause overgeneration of reactive oxygen species (ROS) viz singlet oxygen (1O2), hydroxyl (OH) radicals, superoxide (O2−), and hydrogen peroxide (H2O2).17 Especially, ROS are exceptionally sensitive to cellular objects and can cause critical oxidative impairment to metabolic events and thus restricting plant growth and development under temperature stress.17–19Plants are required to breed and flourish to prolong their survival under temperature stress. Thus, there are numerous advances for retaining a balance between plant growth, development, and stress tolerance.20 To improve plant growth and productivity under stress conditions, chemical approaches, i.e., exogenous treatment with several substances of both natural and synthetic origin, like phytohormones, osmolytes, gaseous molecules, etc., are getting much attention.21–23 These substances help in mitigating the adverse effect of stress conditions.Due to the sessile nature of plants, they are unable to escape temperature stress merely by moving to an appropriate stress-free environment. Therefore, plants have evolved a series of mechanisms to cope with undesirable stress conditions by modifying their developmental, physiological, biochemical, and molecular processes to changing environments.24 MET mediates diverse functions, including organogenesis, flowering, photosynthesis, reproduction, circadian rhythm, fruit ripening and plant acclimatization to changing environments.25 Recent evidence on MET studies highlights the beneficial role of MET in plant stress tolerance and is considered as a regulatory hub of phytohormones.26–28 So, MET appears to be multi-regulatory molecules, as small molecular weight indoleamine  that takes part in various abiotic stresses, including HS and CS.29–31 Plants’ introduction to HS/CS triggers a rise in endogenous MET levels, resulting in up-regulation of MET biosynthesis genes and increased MET content. Exogenous MET improves plant tolerance against HS and CS in two aspects, i.e., either by directly scavenging ROS molecules or indirectly by improving antioxidant enzyme activities, photosynthetic efficacy, metabolite contents, and up-regulating the expression levels of MET-related and stress-inducible genes in plants.31–34 Thus, firstly we discussed MET biosynthesis and metabolism in response to stress conditions. The beneficial role of exogenous MET in understanding how plants respond, adapt and tolerate temperature stress has been widely explained. MET crosstalk with other molecules has also been documented under HS and CS conditions. Finally, the genetic engineering of MET-related genes has also been highlighted, indicating that the modifications in MET biosynthesis and endogenous levels can significantly improve the temperature stress tolerance by rising transgenic plants.Among various substances, melatonin (MET) is considered as a master regulator. MET is an indole compound derived from serotonin (5-hydroxytryptamine); it was first discovered in animal tissues playing an important role in multiple physiological responses.2.35 MET is an indolic compound derived from serotonin whose biosynthetic pathway has been widely investigated in animals and plants.33 MET has a potent natural antioxidant capacity which justifies its exogenous supply in plants subjected to stressed conditions biostimulating the plant growth under adverse conditions.33MET (N-acetyl-5-methoxytryptamine) was firstly isolated in 1958 in the bovine pineal gland.36 In plants, mitochondria and chloroplasts are the main organelles in which MET is synthesized , tryptophan hidroxylase (TPH), tryptamine 5-hydroxylase (T5H), serotonin N-acetyltransferase (SNAT), N-acetylserotonin methyltransferase (ASMT), and caffeic acid O-methyltransferase (COMT) .29,34 Th40 The first-step reaction is the decarboxylation of tryptophan into tryptamine through TDC. One rate-limiting step in MET biosynthesis is the participation of TDC.41 The second-step reaction is the hydroxylation of tryptamine via the cytochrome P450 enzyme T5H to generate serotonin.39 It is necessary to point out that besides the conversion of tryptophan to serotonin via TDC and T5H, there is another pathway reported in St. John’s wort (Hypericum perforatum) in which tryptophan is converted to 5-hydroxytryptophan through TPH and then by TDC to generate serotonin.42 Once generated serotonin, there are two pathways to catalyze the conversion to MET. One of these pathways is the conversion of serotonin to N-acetylserotonin via serotonin-N-acetyltransferase, which is then converted to MEL through the catalytic process via N-acetylserotonin methyltransferase (ASMT) or caffeic acid O-methyltransferase (COMT) (pathway known as NM). The second pathway is based on the conversion of serotonin to 5-methoxytryptamine catalyzed by ASMT/COMT, and finally, the synthesis of MET via SNAT (pathway known as MN).44There are four-step reactions involved in MET biosynthesis.45 The mode of action of MET in plants is dependent on the subcellular localization of the enzymes involved. For instance, SNAT is present in mitochondria and chloroplasts. The cytoplasm possesses TPH, ASMT/COMT, and TDC, while T5H is present only in the endoplasmic reticulum.47 Considering this fact, some authors have reported that N-acetylserotonin is generated in chloroplast and then moved to the cytoplasm for O-methylation to synthesize MET. Conversely, other authors have noted that the methoxylation of serotonin to produce methoxytriptamine occurs in the cytoplasm. Then, this molecule is transported to the chloroplast for acetylation by SNAT to generate MET.45To describe biosynthesis and mode of action of MET, in rice, it was reported that the presence of a reverse MET pathway where N-acetylserotonin is converted to serotonin through N-acetyl- serotonin deacetylase (ASDAC).3.The plant needs an ideal temperature for normal growth and development. Changing temperature can inhibit plant growth and severely affect its developmental mechanisms. Especially, CS and HS devastatingly conquer plant growth and development by unsettling several morphological, biochemical, physiological, molecular and cellular processes . Therefo3.1.48 Cold stress also affects the photosynthetic machinery inducing photo-inhibition at both PSI and PSII.49 Melatonin (N-acetyl-5-methoxytryptamine), first reported in plants in 1995, protects plants against various environmental stresses, including CS  exerts multiple effects and leads to changes in biochemistry, physiology, and molecular biology in plants .4,5,48 Cuding CS .57 In maD. odorifera seedlings with exogenous MET and Ca2+ (0.6 mM + 5 mM CaCl2) improved the plant growth and relieved injuries.79 Exogenous MET and Ca2+ enhanced photosynthetic and antioxidant activities and decreased lipid peroxidation, electrolyte leakage, and ROS generation under CS (3°C). In addition, exogenous MET and Ca2+ intensely raised the phytohormones levels like gibberellin (GA3) and auxin (IAA); however, abscisic acid (ABA) was decreased. The GA3 and IAA contents were improved respectively by 22% and 17% in seedlings when treated with combined MET-Ca2+ than MET alone.79 In a study, hulless barley seeds were grown under different temperatures  and treated with MET (1 µM) solution for 12 hours before the seeds germinated.80 Exogenous MET (1 µM) treatment alleviated the growth inhibition caused by CS and also restored the circadian rhythmic oscillation of circadian clock genes HvCCA1 and HvTOC1, whose circadian rhythmic phenotypes were lost by CS. The findings confirmed that MET treatment also lessened the soluble sugars and malondialdehyde (MDA) contents.80 Pepper seedlings and flowering were pre-treated with 5 µM MET and then were exposed to CS at 5°C/10°C (night/day) for 3 days.61 The results revealed that MET treatment improved CS tolerance by improving photosynthetic parameters, water relations, and antioxidant enzyme activities while lowering H2O2 and MDA contents and membrane permeability.61 In another study, the exogenous role of MET was investigated on soybean seedlings when exposed to CS.81 The results revealed that CS enhanced oxidative damage by ROS accumulation, affecting the growth and development of soybean. However, 5 µmol L−1 MET treatment alleviated the oxidative damage by increasing the transcript abundance of antioxidant-related genes.81Under CS, treatment of 82 The findings revealed that MET treatment lessened the cell structure damage and lightened the increase in chilling injury incidence, MDA content, and membrane permeability under CS conditions. Moreover, MET activated the antioxidant defense system to fight oxidative damage by up-regulating the expression levels of CaSOD, CaPOD, CaCAT, and CaAPX genes.82 During cold storage of pomegranate fruit at 4°C for 120 days, different doses of MET  confer CS tolerance.83 The results confirmed that treatment with MET attenuates the H2O2 accumulation by improving GR, APX, CAT, and SOD activity, maintaining membrane integrity by hindering LOX and PLD activity. Therefore, exogenous MET is a safe strategy for providing CS tolerance in pomegranate fruit.83A recent study investigated the effect of 100 μM MET on bell pepper while storing at 4°C for 20 days and afterward shelf at 20°C for 3 days.84 MET treatment  ameliorated chilling injury in cut anthurium flowers by 11, 29, 51, and 31%, respectively, than untreated flowers. Furthermore, MET triggers the proline synthesis and enhances ROS avoiding and scavenging activity, increasing GR, CAT, APX, and SOD activities.84 In another study, the authors studied the impact of exogenous MET treatment on chilling injury in tomatoes during cold storage.85 They found that 100 μM of MET application alleviated chilling injury and provided enough intracellular ATP by higher H+-ATPase, Ca-ATPase, cytochrome C oxidase (CCO), and succinate dehydrogenase (SDH) enzyme activity. It also protected membrane integrity due to a higher unsaturated/saturated fatty acids ratio as a consequence of higher FAD3 and FAD7 genes expression, which coincided with lower PLD and LOX genes expression and enzyme activity.85 Cold stress decreased the biomass, photosynthetic pigments, and mineral nutrients of Medicago truncatula plants. The authors studied that exogenous MET and Rhizobium inoculation (RI) attenuated CS-induced injuries and reduced oxidative damage.48 MET pretreatment also enhanced the antioxidative ability by improving the activities of APX (42%), CAT (140%), SOD (50%), and POD (8%), also increased osmolyte accumulation, nutrient uptake, and nitrate reductase activity.48Cut anthurium flowers could develop chilling injury during low-temperature storage, manifested as spathe browning.86 The results showed that 1000 µM of MET treatment exhibited the lowest flesh browning and decayed incidence after 45 days of storage at 0°C. Moreover, exogenous MET enhanced the activities of APX, SOD, GR, and CAT while lowered the activities of lipoxygenase and phospholipase D.86 Another study demonstrated that the plum fruit treatment with 1.0 mmol L−1 MET mitigated the fruit chilling injury by reducing flesh-reddening and ‘ethylene burst.’ The MET application also repressed fruit softening and maintained energy status. In contrast, MET reduced the accumulation of cold-induced secondary metabolites.87 Likewise, MET application in kiwifruit alleviated chilling injury during cold storage.88 MET treatment strongly repressed the activity of lignin metabolism enzymes  and the expression of structural genes, whereas improved the activity of antioxidant enzymes  and the accumulation of antioxidant substances (AsA and GSH). The outcomes propose that MET actively participates in cold tolerance and lignin accumulation through enzyme activity regulation in postharvest kiwifruit.88Recent studies also showed that MET plays a vital role in maintaining the fruit’s health during cold storage. For instance, during cold storage at 0°C for 45 days, sweet cherry fruits were treated with different MET levels . MET treatment retarded the fruit senescence and improved the antioxidant potential.89 MET treatment showed lower weight loss, cell membrane permeability, and chilling injury index also delayed the decreases of fruit firmness, sucrose, total soluble sugar, vitamin C, titratable acidity, and total soluble solids. These findings suggest that MET treatment could improve chilling tolerance and retain the quality of cold-stored guavas.89 Likewise, MET treatment confers chilling tolerance in mango fruit.90 The MET application increased polyamine accumulation and GABA shunt pathway activity.90 MET application also increased the accumulation of endogenous polyamines such as ADC and ODC and lowered DAO and PAO activities in the peel and pulp.90 In a recent study, the authors investigated the effects of exogenous MET on the ripening and decay incidence of plum fruit.91 The results presented that MET could slow the ripening process as directed by the firmness, respiration rate, and ethylene production and reduced the weight loss and decay incidence of plum fruit in room storage.91 In addition, MET application triggered the phenylpropanoid pathway by enhancing the activities of phenylalanine ammonia-lyase (PAL), 4-coumarate-coenzyme A ligase (4CL), cinnamate-4-hydroxylase (4CH) and peroxidase (POD), and accompanied by higher contents of total phenols and lignin, which might be contributed to improving the temperature tolerance in plum fruit during storage.91 In a nutshell, all these examples suggest that exogenous MET significantly improves CS tolerance by regulating several physiological, biochemical, and molecular mechanisms in different fruits and crop plant species.Another study evaluated the influence of MET application on the chilling injury and quality of guavas during storage at 4 ± 1°C.3.2.93 HS also disturbs the respiration and photosynthesis-related metabolic processes leading to disruption of the carbon economy of the plants under HS condition  is one of the major limiting factors for plant growth and causes a severe decline in global crop production. Heat stress accelerates crop senescence in extreme environments, decreasing crop productivity and even may cause death.ondition .93–95 MEnditions .69 After heat shock, the molecular analysis showed that MET treatment effectively lessened the oxidative stress by regulating the over-accumulation of superoxide (O2•−) and hydrogen peroxide (H2O2), reducing the lipid peroxidation content and less membrane injury index. So, MET alleviated HS injuries by improving antioxidant defense mechanisms, such as the ascorbate–glutathione cycle, and reprogramming the NO biosynthesis and PAs metabolic pathways.69 Cherry radish was cultured at 35°C/30°C day/night high temperature and applied different MET concentrations, i.e., 0, 11.6, 17.4, 29.0, 34.8, and 67.0 mg L−1.71 The results showed that MET treatment significantly increased cherry radish biomass by 13%, and the soluble solid and soluble protein were improved by 9% and 18%, respectively. MET treatment also enhanced the activities of antioxidant enzymes, ascorbate, Chl a, carotenoid contents, ABA, and auxin contents. Thus, MET application positively affected cherry radish growth under HS.71Pretreatment of tomato seedlings with 100 μM MET and then were exposed to HS (42°C) for 24 h.96 MET treatment reduced the polyphenol to free amino acid ratio by finely refining the concentrations of polyphenols and amino acids. The qRT-PCR analysis discovered that MET amplified the transcript levels of catechins biosynthesis genes, including CsCHS, CsCH1, CsF3H, CsDFR, CsANS, CsLAR, and CsANR, under HS.96 Pretreatment of wheat seedlings with 100 µM MET followed by exposure to HS resulted in efficiently reduced oxidative stress by avoiding the over-accumulation of H2O2, dropping the MDA content, and growing proline biosynthesis.72 Moreover, the activities of antioxidant enzymes, like SOD, CAT, and POD, were increased substantially. Furthermore, the expression of ROS-related genes TaSOD, TaPOD, and TaCAT, and anti-stress responsive genes, such as TaMYB80, TaWRKY26, and TaWRKY39, were also induced in MET-treated seedlings.72In a recent study, it was found that MET promotes photosynthesis and biomass accumulation in tea plants and improves tea quality under HS.Mentha × piperita L. and Mentha arvensis L. plants, a group of researchers assessed the chemical profile changes of essential oil and antioxidant enzymes activity in response to HS.97 The findings revealed that MET treatment plays an essential role in regulating physiological processes and alleviating HS. External MET applications also triggered a significant rise in enzymatic antioxidants, including SOD, POX, CAT, and APX peroxidase, and non-enzymatic antioxidants like ascorbic acid and vitamin E, causing reduced ROS levels and lipid peroxidation under HS.97 In another study, it was reported that irrigation treatment with 20 µM of MET significantly alleviated HS-induced pollen inactivation in tomatoes.98 MET treatment alleviated the ROS synthesis by up-regulating the transcription and activities of antioxidant enzymes under HS conditions. The results suggest that MET protects pollen activity and reduces ROS accumulation by inducing antioxidant enzyme activities under HS.98 In a recent study, regulatory networks of MET in chrysanthemum seedlings were explored in response to HS.99 The findings revealed that MET treatment decreased the H2O2 content and MDA accumulation, promoting the proline and soluble protein, GSH, and AsA accumulation, and antioxidant enzymes activities, including SOD and POD CAT, and APX under HS. Moreover, MET improved the dry weight, fresh weight, photosynthesis rate, Chl content, and gas exchange parameters.99In 2 assimilation and photosynthetic pigment content under HS. MET protects the PSI and PSII reaction center and lessens photo-inhibition.100 Likewise, HS (42°C) enhanced the ROS and lowered the photosynthesis efficiency in soybean.101 While pretreatment with 100 µmol of MET improved the photosynthetic pigment (Chl a and Chl b), plant growth and reduced the oxidative stress via scavenging superoxide and H2O2 and reducing the electrolyte leakage and MDA contents.101 A recent study with meta-analysis revealed that MET treatment improves the maize root length, plant height, leaf area, shoot weight, fresh and dry root weight, CAT, POD, SOD and APX activities, protein, and soluble sugar under stress conditions.102 In contrast, MET treatment lowered the levels of H2O2, O2−, MDA, and electrolyte leakage. In conclusion, MET relieves oxidative damage by refining stress tolerance, regulating the antioxidant defense system, and improving leaf Chl content compared to control.102 The effect of MET priming on the photosynthetic electron transport of PSII against heat stress was evaluated in tall fescue. The results revealed that MET weakened the electron transfer efficiency of PSII per light reaction center at the receptor and donor sides. In contrast, it improved the number of reaction centers per unit cross-sectional area. In brief, MET regulates the photoelectric conversion of PSII of tall fescue under heat stress and improves its survival rate after heat shock.103 In conclusion, exogenous MET treatment improves plant growth and development by inducing the antioxidant defense mechanisms, interacting with other chemicals, and reprogramming the biochemical metabolism.In a recent study, the authors found that 100 μM of MET application enhanced tomatoes’ CO4.4.1.104 In certain plants, genetic engineering has allowed for the integration of specifically desired features. Isolating a gene of interest, ligating that gene with a suitable vector to generate a recombinant-DNA molecule, and then transferring that gene into the plant genome to develop a novel function are all examples of genetic engineering.105 Transgenic technology has been dubbed “agriculture’s fastest expanding technology.” It refers to a collection of approaches for transferring desirable gene(s) across taxonomic boundaries into a specific plant via non-conventional means from any source .107 The cornerstone and essence of sustainable agriculture are combating numerous sorts of abiotic stressors like temperature. The primary benefits of transgenic technology are that the genes regulating many agronomically significant features may be derived from any organism – plants, bacteria, etc., and used to alter plants. Novel features from any background can thus be easily integrated into the target plant.107Biotechnology has given marvelous and fantastic scope for crop development, crop protection, crop quality management, and enhancing other agricultural attributes.108 CRISPR/Cas9 is more rapid, less expensive, precise, and highly effective in editing genomes even at the multiplex level than other genome editing technologies like transcriptional activator-like effector nucleases (TALENs) and zinc finger nucleases (ZFNs). CRISPR/Cas9 has shown the most significant promise for addressing emerging challenges in agriculture.109 Targeted knock-out/in, substitution, insertion, and deletion mutations created by CRISPR/Cas9 system have explored the regulatory functions of genes and their impact on other biochemical processes and helped to improve crops under abiotic stresses by their scavenging capacity.108 CRISPR/Cas9 can be easily programmed to include double-strand breaks at any desired target site. CRISPR/Cas9-based genome editing helps to create novel cultivars with desirable traits such as yield improvement, enhancement in yield quality and higher resistance to biotic and abiotic stresses.110 Moreover, through this genetic engineering methodology, genetic breeders can eliminate unfavorable traits or add favorable traits in a straightforward process needing only one single generation.108 Recently, CRISPR/Cas9-based genome editing have been widely utilized to improve temperature tolerance in different plant species.111–115The clustered regularly interspaced short palindromic repeats (CRISPR)/associated protein 9 (Cas9) system is a valuable genome editing tool having specificity, high efficiency, and processing a wide range of applications.4.2.116 Tryptophan decarboxylase (TDC), the first gene involved in MET production, has been overexpressed in rice. MET buildup in TDC3 transgenic lines is seed-specific; transgenic seeds had 31-fold greater MET concentrations than wild-type seeds.117Genetic engineering of MET-encoding genes is a fascinating area that can also help improve the temperature stress tolerance in plants. However, only a few studies have looked at the effect of MET in heat-stressed crops at the genetic/molecular level. MET increased the production of heat shock proteins in tomato cells in response to HS, which controlled cell structure and protects cellular protein structure and stability under HS.118 This suggests that MET has a function in temperature regulation. Microarray examination of global gene expression patterns in transgenic rice seedlings overexpressing the sheep SNAT gene resulted in an endogenous rise in MET levels relative to the wild type.119 Because of the increased levels of endogenous MET, the MET-rich transgenic rice plants showed higher oxidative stress resistance against cold. According to the microarray analysis, the endogenous MET increase in rice resulted in a moderate level of gene expression changes, with 464 genes differentially expressed.120 According to a previous report, genome-wide transcriptome analysis showed that nearly 4000 transcripts were differentially expressed as a result of MET exposure in transgenic bermudagrass (Cynodon dactylon). MET had many favorable impacts on transgenic bermudagrass under CS (4°C).121 Moreover, it was reported sheep (Ovis aries) SNAT (OaSNAT) gene is expressed in tobacco leaves. OaSNAT was found to be transiently expressed and exhibited a bright red fluorescence, indicating that the animal’s OaSNAT protein was expressed in the plant under HS (30°C).119High temperatures and dark conditions increased MET levels of the final two enzymes in the MET production process.ASMT overexpressed tomato plants was greatly reduced compared to WT plants after 9 hours of HS (40°C) treatment.116 These results imply that either exogenous MET or endogenous MET manipulation via ASMT overexpression decreases HS-induced protein aggregation and ubiquitination. Heat stress significantly increased transcript levels of HSP17.4, HSP20, HSP20-1, HSP21, HSP70, and HSP90 in WT plants within 3 hours. More importantly, the expression of these HSPs was increased in MET-treated and ASMT plants following HS, implying that MET activated HSPs to aid in the refolding of denatured proteins.116 Under HS (28°C), MET concentrations in transgenic rice seeds were 31 times greater than those in WT seeds. Both in homologous and ectopic TDC overexpression lines, the level of MET intermediates was likewise elevated and improved the HS tolerance in transgenic rice lines.122 In plants treated with MET, the relative normalized expression of COR15a was seven times greater, and a four-fold induction was seen in MET-treated plants after 0.5 hours of CS treatment.123 These results suggest that the up-regulation of CS-related genes in transgenic plants by MET may stimulate the biosynthesis of cold-shielding compounds and improve plants growth treated with exogenous MET under CS.A previous study discovered that the development of ubiquitinated protein in the insoluble protein aggregates of MET treated and TDC3 transgenic rice line, which was able to synthesize MET when compared to three separate transgenic rice plants that expressed each TDC isogene separately for MET production.117 While the WT and other TDC genotypes showed considerable expression of TDC under HS (28°C), there was specific TDC overexpression for each TDC isogene.117 In another study, preincubation with MET prior to CS treatment (2–3°C) protected carrot suspension cells from dying, as detected by trypan blue staining.124 When cells were preincubated with 86 nm MET before being exposed to cold, there was only a small drop in cell viability, after a 7-day CS, the proportion of TUNEL-positive cells reduced from 83.6 to 12.8% as MET concentrations increased.124Interestingly, MET treatment also plays key role in gene expression and cell suspension under temperature stress. For instance, it was reported that pIPKb002-TDC3 binary vector was used to create the AANAT and ovine HIOMT genes.125 The average mass ratio of transgenic lines declined from 100% to around 65% after 4 hours of HS treatment. This is substantially higher than the WT, which experienced a reduction from 100% to 50%. In fact, the three transgenic lines had a higher mass ratio than the WT at all times during HS (20°C) treatment, and transgenic plants overexpressing ovine HIOMT were more drought tolerant.125 According to scientists, at – 20°C, MET triggered the synthesis and breakdown of cellulose, pectin, and xylan, suggesting that MET may alleviate the effects of temperature stress based on the trials above.30 Previously, it was opined that the leaves of WT and snat mutant lines were inoculated with Pst-avrRpt2 to see how decreased MET production caused by the knockout of SNAT gene function affected pathogen resistance in Arabidopsis thaliana under HS (28°C).126 The knockout mutant lines were more susceptible to pathogen assault than WT plants, suggesting the direct involvement of MET as a signal molecule in pathogen resistance, mainly under HS conditions.126Furthermore, a previous study found that when tomatoes were subjected to a drought treatment at 20°C, WT plants lost water faster than transgenic ‘Micro‐Tom’ tomato overexpressing ovine OsCOMT) gene in modulating rice grain yield.127 The results revealed that OsCOMT is involved in the production of MET.127 According to transgenic assays, OsCOMT dramatically slowed down the senescence of leaves at the grain-filling stage by preventing the breakdown of Chl and chloroplast, which boosted the effectiveness of photosynthesis. Briefly, MET-mediated leaf senescence and vascular development offer a potential method for genetically enhancing rice grain output.127 In watermelon, 16 putative O-methyltransferase (ClOMT) genes were identified. Among them, ClOMT03 (Cla97C07G144540) was considered a potential COMT gene (renamed ClCOMT1) based on its high identity (60.00–74.93%) to known COMT genes involved in MET production, expressed in nearly all tissues, and upregulation during abiotic stressors.128 Watermelon calli with ClCOMT1 overexpression had considerably higher MET concentrations than those with ClCOMT1 deletion using the CRISPR/Cas-9 technology. These results suggest that ClCOMT1 plays an essential role in MET biosynthesis in watermelon. Cold, drought and salt stress also increased MET concentrations while up-regulated ClCOMT1 expression in watermelon. ClCOMT1 is a positive regulator of plant tolerance to abiotic stimuli, as shown by the improved tolerance of transgenic Arabidopsis to such challenges when ClCOMT1 was overexpressed.128 So far, MET-encoding genes have not been widely edited using CRISPR/Cas system; therefore, more efforts are required to apply the genome editing application to fully unravel the MET role against temperature stress. The use of CRISPR/Cas9 for modifying plant genomes is fast growing. To offset the harmful effects of climate change and secure the future food security of growing populations in tropical nations, the CRISPR/Cas9 system is evolving into a user-friendly tool for the generation of non-transgenic genome-edited crop plants.Through the dual regulation of leaf senescence and vascular development, a recent study discovered the involvement of a caffeic acid O-methyltransferase (5.2S).129–131 The interactions among MET, ROS, NO and H2S demonstrate the responses to abiotic stress through a series of signaling networks, which confer the plant adaption and tolerance. Moreover, MET is able to interact jointly with H2O2 and NO, resulting in post-translational changes and regulation of hormonal activities.129–132 There is also established crosstalk between MET and phytohormones such as auxin, cytokinin, gibberellins, ethylene and ABA.133 In this section, we demonstrated the significance of MET interaction, regulation of redox homeostasis via antioxidant system, and protection of plants from oxidative stress-induced cellular injuries in response to temperature stress.Melatonin (MET) acts as a multifunctional molecule that is widely involved in plant growth, development and stress responses. Numerous studies reveal that MET interacts with other signaling molecules, including ROS, nitric oxide (NO) and hydrogen sulfide  for 24 h.69 After HS, MET-treated plants showed an increase in the endogenous levels of several polyamines such as putrescine, spermidine, and spermine. Moreover, the findings revealed an increase in NO content.69 Working on the same crop, the foliar sprays with 100 μM MET in tomato plants subjected to HS (38/28°C and photoperiod of 16/8 h) for 5 days resulted in a decline in ABA concentration in tomato leaves.135 The mRNA levels of several genes such as NCED1, NCED2, and AAO3 involved in ABA biosynthesis were assessed, showing a significant increase under HS and MET application.135 On the contrary, GA content in leaves was elevated under HS with MET application. The determination of relative expression of GA biosynthesis genes (GA20ox1 and GA20ox2) were downregulated under HS, while the responses were reversed after the addition of MET supplementation with HS conditions in tomato plants.135Tomato is one of the most important horticultural crops worldwide, and both HS and CS reduce the optimal physiological performance of tomatoes.101 The irrigation with a volume of 30 mL of 100 µM MET twice daily for 6 days in soybean plants subjected to HS (42°C) for 3 and 7 days resulted in an increase in proline, phenolics compounds, and flavonoids concentration as well as in polyamines such as spermine, spermidine, and putrescine.101 Moreover, the application of MET reduced ABA content whereas increased the level of salicylic acid.101 On the other hand, a study reported that the supply of 100 µM MET under HS in wheat seedlings resulted in an overaccumulation of proline concentration.72According to recent reports, there are other studies focused on the amelioration of exogenous feeding with MET in temperature-stressed plants.Lolium perenne L.).66 This positive effect can be related to the decrease in ABA synthesis, mainly ascribed to the down-regulation of two ABA biosynthesis genes (LpZEP and LpNCED1). Though, a study found an increased level of auxin and ABA in cherry radish subjected to HS and an exogenous supply of MET.71 This study is a bit exceptional in terms of ABA induction in response to MET-mediated HS. Moreover, they reported a positive effect of MET in temperature-stressed plants.71 Regarding other biomolecules, the exposure of wheat plants to HS (40°C) for six hours for 15 days was ameliorated by the exogenous application of MET, which led to an increase in hydrogen sulfide contents.136 Regarding the interaction with ABA, it was also reported that the exogenous application of MET under CS in Elymus nutans resulted in an increase in ABA synthesis.53The ameliorative effect of exogenous application of MET under HS has also been reported in perennial ryegrass (CsNCED1 and CsNCED2) as well as genes involved in ABA catabolism (CsCYP707A1 and CsCYP707A2).56 A recent study reported that CS significantly reduced root length, plant height, leaf surface area, and Chl contents, and increased the ROS levels that led to induced oxidative damage, lipid peroxidation and electrolyte leakage. However, these parameters were significantly reverted after supplementation of MET enhanced stress tolerance.137 Recently, it has been reported that there is a correlation between MET with MeJA, which modulate H2O2 levels and increase CS tolerance in watermelon.138 In short, all these examples suggest that MET interacts and crosstalk with hormones, polyamines, and other gaseous molecules to improve temperature stress tolerance by regulating several physio-biochemical and molecular mechanisms in plants. In the near future, more in-depth studies should be carried out to fully reveal the mode of action that can help plants to cope with temperature stress conditions.Under CS, the exogenous MET (200 µM) in cucumber seedlings led to an increase in the polyamines contents  and also modulated the expression of the key ABA biosynthesis genes  causes multiple problems at the molecular and physiological levels in plants, resulting in massive productivity losses worldwide. MET is a nontoxic indolic molecule that plays important roles in animals  and plants . The current review has focused on the metabolism and biosynthesis of MET in plants, the beneficial effects of the exogenous supply of MET in crops exposed to low and high temperatures, the recent engineering techniques in melatonin biosynthesis in crops as well as the interaction of this molecule with other hormones under changing temperature conditions. Exogenous MET application improves plant growth under temperature stress by maintaining membrane integrity, improving photosynthetic pigment synthesis, and increasing water and nutrient uptake. MET treatment alleviates the cold-induced osmotic imbalance by enhancing the accumulation of hormones, osmolytes, and secondary metabolites. MET mitigates the detrimental effects of heat stress on the photosynthetic machinery of plants. The production of MDA and H2S, Ca2+, H2O2, NO, and other signaling molecules and the MAP kinase cascade pathway are essential in MET-induced plant tolerance to stress. In addition, MET-mediated genetic engineering could provide a promising approach to unraveling the molecular basis of temperature stress tolerance. Genetic modifications can also improve the MET synthesis in transgenic plants and improve tolerance to temperature stress. Future research is required to know the genetic mechanisms and metabolic pathways involved during recovery under stress conditions on exposure to MET. The information about this topic is vast; nevertheless, there is a complexity of the metabolic pathways and their synthesis in two organelles (chloroplasts and mitochondria) that should be considered. A strong effort is required to elucidate these bottlenecks and provide clarification on several fundamental points. Future research should focus on a deeper understanding of the core MET metabolism by generating overexpressing and knockout mutants via CRISPR/Cas system in different crops of the genes involved in the metabolic pathways.MET works synergistically and/or antagonistically to phytohormones as well as other molecules for regulating plant growth, development, and defense response. MET crosstalk and interacts with hormones, polyamines, and gaseous molecules to mitigate the damages caused by temperature stress. The current research outcomes specify that MET, H