Two successive cycles of divergent selection for cell membrane thermostability (CMS) were carried out from within F₃ families of five populations of wheat under heat stress as well as under drought + heat stresses. Another cycle of divergent selection for stomatal frequency was also applied under drought + heat stress to the F₄ families selected for CMS in order to generate F₅ families with different combinations of high CMS-high stomata, high CMS –low stomata, low CMS-high stomata and low CMS-low stomata. The descended F₆ families with such combinations were field evaluated for grain yield per plant and 1000 grain weight under favorable, heat stress and drought + heat stress conditions. Significant positive responses to selection for CMS were obtained in the five populations which averaged 26.29% and 26.53% in the F₄ and F₅ high CMS selections, respectively and 25.21% and 24.30% in the F₄ and F₅ selections for low CMS. Positive responses of 19.57% and 8.71% were obtained in the high and low stomatal frequency F₅ selections, respectively, under heat stress. Selection for higher CMS did not produce any significant correlated responses in grain yield per plant under favorable or drought stress conditions although CMS was strongly correlated with grain yield per plant under drought +heat stresses (r = 0.80, p <0.01) as well as under drought stress (r = 0.64, p <0.01). However, positive correlated responses were obtained in 1000 grain weight under drought + heat stresses which averaged 6.45% but not under drought stress alone. Meanwhile, positive correlated responses to selection for high stomatal frequency were displayed in grain yield per plant as well as in 1000 grain weight under both favorable and heat stress conditions.
Among the four combinations of CMS and stomatal frequency tested, the F₆ families with the high CMS-high stomata were the top in grain yield per