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The aim of this study was to investigate the hormonal alterations triggered in different staygreen wheat cultivars by postanthesis heat stress resulting in yield losses at harvest.In the field experiments, plants of Wennong 6 (a staygreen cultivar) and Jimai 20 (control cultivar) were used and subjected to heat stress treatment from 1 to 5 days after anthesis (DAA).Meanwhile, the two cultivars were sprayed daily with 10 mg L-1 of 6-benzylarninopurine (6-BA) between 1 and 3 DAA under heated and non-heated conditions.The grain yield of Wennong 6 was higher than that in Jimai 20 under both heated and non-heated conditions.The grain yield of Wennong 6 was reduced by 19.65 %.In contrast, the values in Jimai 20 was reduced by 26.22%.Heat stress significantly (P < 0.05) decreased the number of kernels per spike and grain yield.Although zeatin riboside (ZR) content was decreased by heat stress, gibberellin (GA3), indole-3-acetic acid (IAA) and abscisic acid (ABA) contents were increased at 3 to 15 DAA.Application of 6-BA significantly (P < 0.05) increased grain-filling rate, endosperm cell division rate,endosperm cell number, and 1,000-grain weight under heated condition.6-BA application increased ZR and IAA contents at 3 to 28 DAA, but decreased GA3 and ABA contents.In the indoor experiments,kernels weight and inferior kernels number per spike of two cultivars were significantly (P < 0.05)decreased when the spikes were cultured under 35/20℃ conditions.Kernels weight were significantly increased with adding ZR in culture medium under both 25/20℃ and 35/20℃ conditions.Exogenous ZR increased inferior kernels number per spike of two cultivars under 35/20℃ condition.These results show that manipulating ZR supply can affect kernels number and weight under heat stress.Correlation analysis showed that endogenous contents of ZR, ABA and IAA in kernels were positively and significantly (P <0.05) correlated with grain-filling rate, whereas GA3 content was negatively correlated with grain-filling rate and endosperm cell division rate at 3 to 15 DAA.We may summarize the mechanism of grain yield reduction under heat stress as follows: heat stress (i) decreased grain sink strength by reducing the rate of endosperm cell division and number of endosperm cell owing to the decreases in ZR content and the increases in GA3 content at the early endosperm cell division stages and (ii) affected the grain carbohydrate accumulation by decreasing the grain-filling rate.In our study, the declines in endosperm cell division rate and grain-filling rate were mitigated by exogenous cytokinins, especially under heat stress treatment.Therefore, alleviation of heat stress in wheat grain yield by exogenous cytokinins could be related to expanding grain sink strength by increasing endosperm cell number and accelerating grain carbohydrate accumulation by accelerating grain filling.