HEAT TOLERANCE IN SPRING WHEAT - I - ESTIMATING CELLULAR THERMOTOLERANCE AND ITS HERITABILITY

High temperature is a major environmental stress factor limiting wheat (Triticum aestivum L.) productivity. Improvement of heat tolerance in wheat is an important breeding objective. Genetic variation in cellul ar thermotolerance among 56 spring wheat cultivars was evaluated at th e seedling stage of growth by cell membrane thermostability (CMS) and triphenyl tetrazolium chloride (TTC) assays. A subset of eight lines w as also evaluated at the flowering stage using the same assays. With b oth assays Average thermotolerance tended to decrease from the seedlin g to the flowering stages. However, thermotolerance was well correlate d between growth stages among the eight cultivars for both CMS (r = 0. 92; p = 0.004) and TTC (r = 0.84; p = 0.050). The correlation between TTC and CMS among the eight cultivars tested at the seedling and the f lowering growth stages was significant (r = 0.74; p = 0.031 and r = 0. 75; p = 0.029, respectively). The same correlation was less strong, th ough still significant (r = 0.32; p = 0.014) across 56 cultivars at th e seedling stage. In a study of the cross V747 (tolerant)/Barkaee (sus ceptible), broad sense heritability was estimated at 89% for TTC. Most of the genetic variance was additive. CMS in seedlings of 16 cultivar s was positively and significantly (p less than or equal to 0.05) corr elated with yields of these cultivars in each of four hot environments in Mexico, Sudan, India, and Brazil. The same correlations for TTC we re positive but nonsignificant.