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Heat stress has become an increasingly important factor in limiting wheat yields. In northern China, high temperature (>30°C) during the grain filling is one of the major constraints in increasing wheat productivity. We used two winter wheat (Triticum aestivum L.) cultivars with different sensitivities to heat stress (Jimai 22 'JM22', low sensitivity and Xinmai 26 'XM26', high sensitivity) to study the various aspects of photosynthetic characteristics during the grain filling stage under heat stress. The results showed that photosynthesis rates (P n ) in flag leaves of XM26 decreased faster than in JM22 under heat stress during the grain-filling stage. P n decreased more rapidly under heat stress than without stress, by up to 69.9% and 59.3%, respectively, at 10 days following heat stress (10 DAS). This decline of P n was not caused by heat-induced stomatal limitation, but rather by a decline in Rubisco activity and a functional drop in photosystem II (PSII). After heat stress, the grain yield of JM22 decreased by 6.41%, but XM26 decreased by 11.43%, when compared with their respective controls. Heat stress also caused an alteration of mesophyll cell ultrastructure. Injury caused by heat stress to organelles in XM26 was more severe than JM22. Moreover, the JM22 cultivar showed some self-repair capacity following heat stress injury. These results indicate that declines in photosynthetic performance caused by heat stress were cultivardependent. Compared with XM26, the JM22 cultivar had superior heat stability in terms of PSII function and carboxylation activity, both of which are susceptible to heat stress.

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