HIS STUDY aimed to study the performance of durum and bread .... wheat genotypes in relation to grain filling process, under water and heat stresses. The experiment was laid out in a split plot design at Fuka Research Station, Faculty of Desert and Environmental Agriculture, Matrouh Governorate in 2011/2012 and 2012/2013 seasons. Environmental conditions were considered as the main plots and included four environments (normal, drought, heat, heat and drought conditions). Sub plots were assigned

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... to 14 durum and bread wheat genotypes. The results revealed significant variations between studied environments for number of grains/spike, 100-grain weight and grain yield. However, number of fertile tillers/m 2 was insignificantly influenced by environments. Wheat genotypes significantly differed for the studied traits, whereas, the environment x genotypes interaction significantly affected number of fertile tillers and grain yield only in both seasons. Grain yield, as an average of the two seasons, was reduced by 26.03, 43.07 and 58.28 % at drought, heat and combined drought and heat, respectively, compared to normal conditions. All wheat genotypes suffered with application of combined drought and heat stresses in both seasons. Wheat genotypes varied in their response to heat stress conditions where the Beni suef 3 and Sohag 3 (durum wheat) and Sids 1 and Giza 168 (bread wheat) showed comparatively higher tolerance to heat stress. Stability analysis revealed a differential response of wheat genotypes (b value) to individual or combined drought and heat stress.