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High temperatures (HT) before heading strongly inhibit the development of spikelets in rice. Spermidine (Spd) can improve rice’s resistance to HT stress; however, the mechanism underlying this effect has not been elucidated. This study investigated several parameters, including yield, superoxide anion (O 2  .- ), protective enzyme activities, and polyamine content, in a heat-sensitive genotype, Shuanggui 1. The yield and yield components decreased dramatically when subjected to HT stress, while this reduction could be partially recovered by exogenous Spd. Spd also slowed the generation rate of O 2  .-  and increased protective enzyme, superoxide dismutase (SOD) and catalase (CAT) activities both under normal and high temperatures, which suggested that Spd may participate in the antioxidant system. Furthermore, genes involved in polyamine synthesis were analyzed. The results show that HT before heading significantly increased the expression of arginine decarboxylase  OsADC1 , Spd synthase  OsSPDS1  and  OsSPDS3  and had little effect on the expression of the S-adenosylmethionine decarboxylase  OsSAMDC2  and ornithine decarboxylase  OsODC1 . In addition, exogenous Spd considerably reduced the expression of  OsSAMDC2 ,  OsSPDS1  and  OsSPDS3  under HT but not the expression of  OsADC1 . The above mentioned results indicate that the exogenous Spd could help young rice spikelets to resist HT stress by reducing the expression of  OsSAMDC2 ,  OsSPDS1  and  OsSPDS3 , resulting in higher levels of endogenous Spd and Spm, which were also positively correlated with yield. In conclusion, the adverse effect of HT stress on young spikelets seems to be alleviated by increasing the amounts of Spd and Spm, which provides guidance for adaptation to heat stress during rice production.

Spermidine Enhanced Free Polyamine Levels and Expression of Polyamine Biosynthesis Enzyme Gene in Rice Spikelets under Heat Tolerance before Heading