Study of PAR interception, energy balance studies and microclimatic profiles in potato (Solanum tuberosum) cultivars under varying planting dates

The crop experiments were carried out during Rabi season 2016-17 & 2017-18. The experiment conducted at Research farm adjacent to agromet observatory (Latitude: 29° 10’ N; Longitude: 75° 46’ E & Altitude: 215 m), Department of Agricultural Meteorology, CCS HAU, Hisar, Haryana. The main aim and objectives of this work were to be accesses the microclimatic condition of potato under varying planting dates and varieties. Results revealed that micrometeorological parameters observed more favourable in 3rd week of October (D2) planted crop as compared to other planting dates. The absorbed PAR was recorded higher in 3rd week of October (D2) sown crop and Kufri Pushkar then other treatments at phenophases, respectively higher in during both seasons. PAR transmitted (watt/m2) at bottom, Soil heat flux pattern was observed in decreasing trend from advanced vegetative stage to tuber bulking stage may be due to more cover of ground and absorption percentage of PAR. The consumption of PAR, bright sun shine and assimilation rate were more in second planting dates. Among planting dates, the net radiation (Rn) was higher at early vegetative stage in 3rd week of October (D2) and 1st week of November (D3) during both crop seasons. The major portion of Rn (Net radiation) was utilized by latent heat of vaporization (LE). The 2nd week of October (D1) was received higher values of soil heat flux (G) during both the crop seasons. The minimum value of G was observed at tuber bulking stage in 1st week of November (D3) during both crop seasons. LE was higher at all phenophases in 3rd week of October (D2) at early vegetative stage during both the crop seasons. G was highest at initiation of tuber and early vegetative stage in 3rd week of November (D4) respectively, during both season. Temperature profiles were inverse throughout the day within the canopy at all phenophases. Over the top of the crop canopy the temperature profile was lapse. The relative humidity profiles were lapse inside the crop canopy throughout the day at 9:00 hours where it was iso-humic. Further concluded that the microclimate profile condition pattern was optimum in second sown crops and produced higher tuber yield.


Introduction
The potato (Solanum tuberosum L.) is the third most important food crop in the world after rice and wheat and consumed by more than a billion people worldwide. Globally potato has an annual production of around 388 million tons (Statista, 2019) [19] . The potato is a crop which has always been the 'poor man's food'. It belongs to the Solanaceae family of flowering plants. In India it is cultivated over an area of approximately 21.42 lakh ha with a production of 513.10 lakh metric ton and productivity 23.3 metric ton per ha (Anonymous, 2018) [3,4] . In Haryana, potato is cultivated over an area of approximately 0.363 lakh ha with a production of 8.97 lakh metric ton and productivity 26.58 metric ton per ha (Anonymous, 2017) [3,4] . Potato is regarded as a high potential food security crop because of its ability to provide a high yield of high-quality product per unit input with a shorter crop cycle (mostly<120 days) than major cereal crops (Adane et al., 2010) [1] . One hectare of potato can yield two to four times the food quantity of grain crops. It is a fundamental element in the food security for millions of people across South America, Africa, and Central Asia. Potato tubers give an exceptionally high yield per acre and are consumed as different recipies (Feustel, 1987;Talburt, 1987) [7,20] . A different meteorological element governs growth, development, production and quality of potato crop. Potato is grown in many different environments, but it is best adapted to temperate climates (Haverkort, 1990) [12] . Tuber growth is sharply inhibited in temperatures below 10 °C and above 30 °C, while optimum yields are obtained where mean daily temperatures ranges between 18 to 20 °C range (Haris et al., 2015) [9] . Potato is also frost sensitive and severe damage may occur when temperature drops below 0 °C   [10,11] . The first step to optimise tuber yield and quality is to understand crop responses to environmental and management factors. For a crop free of pests and diseases, weather is the primary determinant of crop yield. When crops are grown in non-limiting moisture, temperature and nutrient conditions, yield responds linearly to the amount of solar radiation intercepted (Monteith, 1977;Allen and Scott, 1980) [2,13] . The effects of climate change on crop production can be complex. Depending on the temperature regime and the crop, high temperatures can lead to low yields due to increased development rates and higher respiration. Although in some regions changes in yield are strongly influenced by changes in temperature, radiation, sometimes induced by changes in cloudiness which influence potential yield of potato (Hijmans, 2003) [10,11] . The microclimate of plant communities varied with energy balance, turbulent exchange and thermal status of the soil surface. Available net radiation is the balance between incoming and outgoing radiation. The balance of energy at the earth's surface is closely associated with the overlying atmospheric boundary layer (Roxy et al., 2014) [16] . The partitioning of net radiation into latent heat flux of a vegetated system, as well as into other components of energy balance is closely linked to changes in land use and system water availability. Apart from its intrinsic importance in energy balance studies, net radiation is a key input variable in climatologically models to assess evapotranspiration, and its precise estimate is essential for water resource management on regional scale (Villa Nova et al., 2006;Ryu et al., 2008;Pereira et al., 2011) [15,17,21] , for frost prediction studies, and air pollution monitoring (Fritschen and Fritschen, 2007) [8] . Canopy leaf area is used to estimate the interception of PAR. Increases in plant mass (g/plant) are computed by multiplying light interception by a constant value for radiation use efficiency (grams of biomass per mega joule of intercepted PAR). (Kooman and Haverkort, 1995;Shaykewich et al., 1998) [12,18] . The radiation transfer within the crop canopy play's crucial role in energy balance and turbulent transfer processes. Vertical profile of PAR, air temperature and humidity in case of horizontally uniform crop canopy the radiation transfer within the crop canopies plays a crucial role in many aspects of crop growth and development. Firstly the shortwave radiation is the governing component of canopy energy balance influencing leaf, soil, and within canopy air temperature. Second, together with friction, it is a driving force of turbulent transfer within the canopy. Finally, intensity of PAR, as a part of shortwave radiation spectrum, rates intensity of photosynthesis which directly influences the exchange of CO2 between the crop canopy and the atmosphere (Wolfe and Thornton, 2011) [22] . Measurement of canopy temperature with infrared thermometers has been an efficient tool for irrigation scheduling in semi-arid and arid conditions (Evett et al., 2000) [6] and hence moisture status of soil as well as plant can also be indicated by canopy temperature of the crop. Keeping in view the above facts, detailed micrometeorological studies are necessary to understand the crop-weather interaction under various planting dates. While scanning the literature one fails to find much relevant information on micrometeorological studies on potato particularly in Haryana conditions and therefore the study has been planned to generate relevant information on those aspects which play crucial role in development of both source and sink and thereby decides the quantum of final production. The study and use of micrometeorological processes to problems of agrometeorological importance has greatly increased the application of the physical processes regulating the natural environment of crop plants.

Material and Methods
An experiment was conducted in Rabi season, 2016-17 and 2017-18 at research farm, Department of Agricultural Meteorology, CCSHAU Hisar, Haryana. The field area was adjacent to Agrometeorological observatory at 29° 10' N latitude, 75° 46' E longitude and altitude of 215.2 m. The main plots treatments consisted of four planting dates viz. 2 nd week of October (D1), 3 rd week of October (D2), 1 st week of November (D3) and 3 rd week of November (D4) and the subplots consisted of three varieties: V1 -Kufri Bahar, V2 -Kufri Pushkar and V3 -Kufri Surya. The forty eight treatment combinations were tested in split plot design with four replications. Each plot was 5.0 meters wide and 3.6 m long. There were 6 rows in all potato plots. After field preparation and pre sowing irrigation, potato crop was sown from seed potatoes; -small tubers. All seed tubers were presprouted. Tubers were kept for 25-30 days before planting in sufficiently humid and lighted room in a wooden box. The distance between tubers was 20 cm, and the distance between rows 60 cm apart at a depth of 5 to 10 cm manually. All intercultivation practices were kept uniform in the entire plots. The recommended dose of nitrogen (150kg N ha -1 ), phosphorus (50 kg P2 O5 ha -1 ) and potassium (100 kg ha -1 ) were applied. Full dose of DAP (245 g/plot), MOP (375 g/plot) and half dose of nitrogen were applied before sowing and remaining ½ N was top dressed after 25-30 days after sowing at earthing up. All the necessary cultural practices and plant protection measures were followed uniformly for all the treatments during the entire period of experimentation.

Micrometeorological observations
The following micro-meteorological observations were recorded in the experimental field during early vegetative, initiation of tuber and tuber bulking phases with clear sky at hourly interval from 0800 to 1700 hours. a. PAR observations b. Diurnal energy balance components c. Temperature and humidity profile studies

A. PAR observations
Photosynthetically Active Radiation (PAR) was taken after 30 days of planting at 30 days interval. PAR was measured during noon hours at top, middle and bottom of canopy with the help of Line Quantum sensor. The reflected radiation was obtained by keeping the sensor inverted above the crop canopy and the transmitted radiation at the ground was obtained by keeping the sensor on ground across the rows diagonally at random sites. The vertical PAR observations were taken hourly basis interval on clear weather condition or in clear sky days from 0900 to 1700 hour. The day time vertical canopy PAR pattern was observed on occurrences of different phenophases.  Transmitted radiation (%): It is the ratio of transmitted PAR to the total incidence on the crop surface and multiplied by 100.
 Reflected radiation (%): It is the ratio of reflected radiation by crop with the total incidence PAR over crop surface and multiplied by 100.  Absorbed PAR: It is calculated by the formula as below: APAR = 100transmitted -reflected The IPAR (expressed as the percentage of the incidence PAR) was obtained by keeping the sensor above the canopy and absorbed radiation (A) was determined using the following established relationship.
Absorbed radiation (A) = Incidence radiation on the canopy (IPAR) -Reflected radiation by the canopy -Transmitted radiation

B. Diurnal energy balance components
Diurnal net radiation was measured at top of the canopy at early vegetative phase, initiation of tuber and tuber bulking stages.  Solar radiation: The amount of solar radiation received by crop was measured with the help of pyranometer (Medoes and Co., Australia) connected to a digital multivoltmeter. The measurements were made at one meter height above crop. While making measurements the pyranometer was kept horizontally so as to follows the cosine law.  Net radiation: Net radiation was measured at one meter height above crop canopy with net radiometer (Medoes and Co., Australia) connected to a digital multivoltmeter.  Soil heat flux: Soil heat flux was measured with the help of three soil heat flux plate (Medoes and Co., Australia) which were kept at 5 cm soil depth in cropped field and connected to a digital multivoltmeter.

Computation of energy balance
The energy balance of a crop was computed by the following equation:

Rn = G + A + LE + Mi
Where, Rn = Net radiation, mW cm -2 G = Soil heat flux, mW cm -2 A = Sensible heat, mW cm -2 LE = Latent heat of vapour flux, mW cm -2 Mi = Miscellaneous energy used in physiological processes of plant, mW cm -2 (This parameter is generally neglected because of its low value of less than 2%) The latent heat flux was calculated using the following formula: Where, β is Bowen ratio and is inferred from the measurements of dry and wet bulb temperature at two heights and is represented as below: β = 0.66 x dt/de (Denmead and Mclory, 1970) [5] Where, dt = Temperature gradient between two heights de = Vapour pressure gradient between two heights The sensible heat flux (A) was calculated from the energy balance equation using measured Rn and G and calculated LE values and is given as: The net radiation was also quantified daily and weekly basis, and pattern was evaluated.

C. Temperature and humidity profile studies
For recording various micrometeorological observations, following methods were adopted:  Dry and wet bulb temperatures were measured at 8:00 AM, 2:00 PM and 6:00 PM at three levels of crop canopy: top, middle and bottom with the help of Assmann Psychrometer at different phenophases of pigeonpea. Psychrometric tables are used to find out relative humidity in the crop by using these values.  Air and canopy temperature (°C) were measured through Infrared (IR) thermometer and Relative humidity (percentage) measured by digital psychrometer. The profile of air temperature, wind speed, humidity and canopy temperature pattern were measured at different phenophases and further quantified the pooled bases vertical pattern of these parameters.

Results and Discussion
Photosynthetically active radiation (PAR) of solar radiation viz., reflected (R), absorbed (A) and transmitted (T) of potato recorded at different phenophases are presented in

Energy balance components (Wm -2 )
The energy balance components namely, net radiation (Rn), soil heat flux (G), latent heat of vaporization (LE) and sensible heat flux (A) were studied at different phenophases for two crop seasons (Figure 1 and 2). The values of energy fluxes at different phenophases of potato were calculated from the integration of diurnal hourly observations. All the energy fluxes vary considerably during crop growth stages and within the seasons.

Net radiation (Rn)
The

Soil heat flux (G)
The

Temperature profile
The temperature profiles shown in Fig 5 to 8

Relative humidity profile
The humidity profiles shown in Fig 9 to 13 indicated that the relative humidity was higher inside the crop canopy than above the canopy in all the treatments i.e. the relative humidity profiles were lapse inside the crop canopy throughout the day but profiles were near iso-humic at 9:00 hours at different phenophases during both crop seasons. The maximum relative humidity of the day was observed at 9:00 hours. The relative humidity was lowest at noon time.

Conclusion
It may be concluded that the micrometeorological parameters observed more favourable in 3 rd week of October (D2) planted crop. The absorbed PAR was recorded higher in 3 rd week of October (D2) sown crop and Kufri Pushkar then other treatments at all phenophases higher in during both crop seasons. The reflected and transmitted radiation was highest in 3 rd week of November (D4) at all phenophases during both the years. Among different planting dates, the net radiation was higher at early vegetative stage in 3 rd week of October (D2) and 1 st week of November (D3) during both crop seasons. Among all treatments, the major portion of net radiation was utilized by latent heat of vaporization. The 2 nd week of October (D1) was received higher values of soil heat flux during both the crop seasons. Temperature profiles were inverse throughout the day within the canopy at all phenophases. Over the top of the crop canopy the temperature profile was lapse. The relative humidity profiles were lapse inside the crop canopy throughout the day at 9:00 AM hours where it was iso-humic.