Abstract: In order to investigate the effects of high temperature at early grain filling stages (5–9 d after anthesis) on grain starch formation of wheat (Triticum aestivum L.), cultivars Jimai 20 (weak heat tolerance) and Lumai 21 (strong heat tolerance), were used with a heat stress treatment in the field by plastic sheds. Starch accumulation, starch granule distribution, and changes of enzymes activities involved in starch synthesis were examined. The results showed that heat stress after anthesis remarkably reduced starch accumulation at maturity. The total starch and amylopectin contents decreased significantly, but amylose content increased in heat treatments compared to control. Accordingly the ratio of amylose to amylopectin of heat treantment was significantly higher than that of control. The effect of high temperature on starch accumulation in Jimai 20 was greater than that in Lumai 21. The volume, number, and surface area percentage of A type starch granule were enhanced significantly, but those of B type starch granule reduced greatly under high temperature. After 5 d heating stress, the sucrose content, the activities of sucrose synthase (SS), adnosine diphosphate glucose pyrophosphorylase (AGPP), soluble starch synthase (SSS), and granule-bounded starch synthase (GBSS) increased in heat-treated grains of Lumai 21, but changed slightly in treated grains of Jimai 20 with no significant difference to control. These indexes of Jimai 20 and Lumai 21 became lower than those of control at 15 and 20 d post anthesis (6 and 11 d after heat stress removal), respectively. The GBSS activity was affected slightly by heat treatment. The results indicate that the changes in grain sucrose content and the activities of SS, AGPP, SSS, and GBSS are companied with the changes in grain starch accumulation, and high temperature restrains starch synthesis at middle and late grain filling stages, which is caused by inferior sucrose providing capacity and low activities of enzymes (SS, AGPP, SSS, and GBSS) involved in starch synthesis.

Key words: Winter wheat (Triticum aestivum L.), High temperature, Starch, Starch granule, Enzyme activity