Theoretical Aspects of Selection for Yield in Stress and Non‐Stress Environment 1

The question of choice of selection criterion when lines are grown in stress and non‐stress environments is examined from a theoretical standpoint in this paper. Tolerance to stress is defined as the difference in yield between stress and non‐stress environments, while mean productivity is the average yield in stress and non‐stress environments. Equations are developed for the genetic correlations of tolerance and mean productivity with one another and with yields in stress and non‐stress environments in terms of the ratio of genetic variances and the genetic correlations between yields in stress and non‐stress environments. These equations show that selection for tolerance to stress will generally result in a reduced mean yield in non‐stress environments and a decrease in mean productivity. Selection for mean productivity will generally increase mean yields in both stress and non‐stress environments. Tolerance and mean productivity show negative genetic correlations whent he genetic variance in stress environmentsi s less than the genetic variance in non‐stress environments. This result provides an explanation for the positive correlations often reported between regression coefficient stability and mean productivity; a line with high tolerance to stress normally would have a low regression coefficient stability and genetic variances in stress environments are generally lower than in non‐stress enviornments.