wn in two environments, a heavily infected and a non-infected field.
This concept paper focuses on a new approach that can be used effectively during the early or later generations of a breeding program and, notably, in a nonstop manner to secure the continuous adaptation of cultivars to their target environments. The homeostasis or yield stability of a cultivar is a genetically controlled trait that can be selected for without sacrificing higher yields, and it is essential to bridge the gap between actual and potential yields, particularly in resource-poor farming areas. The concept of Coefficient of Homeostasis incorporates tolerance to a multitude of biotic and abiotic stresses, including the stress of higher planting densities. Genotypes that possess improved homeostasis are better able to exploit randomly distributed environmental resources (e.g., in rainfed agriculture where erratic rainfalls can nevertheless sustain production), and respond better to even limited amounts of fertilizers or chemical interventions for plant protection.
Importantly for the resource-poor farming areas, the new Equation A permits unbiased selection for high and stable yield on a single plant basis, a concept that leads to the creation of density-neutral cultivars. The denser the seeding rates in low-input agriculture, the higher the possibility that many plants in the crop stand will not make it to the end of their cycle to produce seeds because of the intense competition for resources within the stand. However, a genotype with improved homeostasis is capable of satisfactory production even under low seeding rates, thus, exploiting limited resources in a most efficient manner. Cultivars that are density-neutral have the capacity to fully exploit lower planting densities, maintaining high crop yields at lower seeding rates. This implies that farmers in resource-limiting environments can achieve high crop yields using a wider density range. An advantage that enables to avoid reseeding in the case of not-satisfactory stands and most importantly, to effectively exploit drought-prone fields using lower seeding rates. It is envisioned that the crop performance of the varieties of the future will be density-independent.
Of particular interest also is that the superiority of genotypes selected in the absence of interplant competition is linked to their ability to develop deep and extensive root systems and exhibit fast early germination and growth . These properties enable plants to better withstand drought conditions and suffer less from weeds by covering the field before emergence and establishment of the weed majority.
In light of the growing evidence about the frequency, importance, and heritable nature of adaptive variation as a result of the genotype by environment (GE) interaction, the process of nonstop selection emerges as a key step to capture the plasticity of the genome and exploit this untapped, yet big reservoir of yield increasing potential. Even without considering the harsh reality of climate change, target environments are in a constant, often unpredictable, flux, and cultivated genotypes should be given the opportunity to keep up with adapting to their ever-changing microenvironments. The evidence is strong that the annual genetic gain from implementation of nonstop intracultivar selection can be larger compared to the average 1% annual genetic gain reported by most conventional programs [4,6,45,46,47,48,49,50,51,52,53,54]. As selection on a single plant basis is rather inefficient with conventional methods, the unique properties of the new equations can increase the efficiency of selection facing the confounding effects of competition, soil heterogeneity, heterozygosity, and GE interaction on single plant yields, while exploiting the constantly released adaptive variation. The outlined concepts demonstrate also a new dimension in breeding methodology, where the critical role of the environment is fully recognized and exploited during the selection process, since field selections based on the values of the two equations, bypa