BREEDING FOR YIELD STABILITY IN UNPREDICTABLE ENVIRONMENTS - SINGLE TRAITS, INTERACTION BETWEEN TRAITS, AND ARCHITECTURE OF GENOTYPES

Attempts to identify individual traits to use as an indirect measure for grain yield have shown some degree of success in environments where crop yields are affected by no stress or only by predictable stresses. However, analytical breeding has been largely unsuccessful in the case of a) stressful environments characterized by low yields due to high variability in the frequency, timing, duration and severity of a number of climatic stresses, and b) breeding programs where the major objective is greater yield stability defined as a reduction in the frequency of crop failures. Experimental evidence suggests that, when environmental variability is high due to unpredictable differences in frequency, timing and severity of various climatic stresses, each time different combinations of several traits are likely to confer 'drought resistance'. Interaction among traits in determining overall crop response to variable stresses is expected to occasionally enhance the importance of a specific trait in a specific stress situation. In this context it becomes difficult to consider 'drought resistance' as a character with its own identity in terms of inheritance. Assessment and verification of traits based on the use of isogenic lines tends to oversimplify the interactions between traits, as the approach provides information on the effect of a specific trait only in a specific genetic background. At the population level of organisation, the dilemma between selection for individual traits and specific combinations of traits can be translated into the dilemma between selection for individual genotypes and specific combinations of genotypes. The evidence that natural selection under stress conditions has not been able to identify either a single trait or a single genotype with a given 'adapted' architecture of traits, is discussed in relation to germplasm development philosphies to stabilize yield in unpredictably stressed environments.