In rice, high-temperature stress (HT) during flowering results in decreased grain yield via a reduction in spikelet fertility; however, the effect of plant water status on spikelet fertility under HT remains unknown. To investigate the relationship between spikelet water status and spikelet fertility under HT, two experiments were performed under temperature-controlled conditions using four genotypes with varying tolerance to HT. Rice plants were exposed to HT for seven consecutive days during the flowering stage under three soil water treatments (soil water potential 0, −20, and −40 kPa), as well as under hydroponic conditions in a separate experiment. HT significantly decreased spikelet fertility, pollen fertility, and anther dehiscence under each of the three water treatments. HT significantly increased the spikelet transpiration rate, and this change was accompanied by a significant decrease in the internal temperature of the spikelets. HT decreased pollen grain diameter in heat-sensitive genotypes. HT had varying effects on the water potential of panicles and anthers but increased anther soluble-sugar concentration. Different aquaporin genes showed different expression profiles under HT, and the expression levels of PIPs for plasma membrane intrinsic proteins and TIPs for tonoplast intrinsic proteins increased in anthers but decreased in glumes. Correlation analyses showed that anther dehiscence and pollen (spikelet) fertility were tightly associated with anther water status, and the expression levels of almost all anther aquaporin genes were significantly correlated with anther dehiscence under HT. In summary, an increased spikelet transpiration rate and decreased internal spikelet temperature were associated with alleviation of the effects of HT in rice genotypes with varying degrees of heat tolerance, and the response of spikelet water status to HT, involving increased total expression of aquaporins and soluble sugar content, thereby improved pollen fertility, anther dehiscence, and spikelet fertility, especially in heat-resistant genotypes. The heat-resistant genotypes N22 and SY63 may adopt different approaches to reduce heat damage.

With global warming, rice plants may be subjected to heat stress more regularly during the heat-sensitive flowering stage, causing spikelet sterility and grain yield loss. Stigma exsertion is considered to increase pollen reception and promote female reproductive success. The aim of this study was to investigate the role of stigma exsertion on spikelet fertility at high temperatures. Five rice cultivars (Liangyoupeijiu, Shanyou 63, Huanghuazhan, Nagina 22, and IR64) with differing degrees of stigma exsertion were cultivated and exposed to high temperature at anthesis. Heat-tolerant cultivars did not always show a high percentage of spikelets with exserted stigmas, and vice versa. Irrespective of the presence of more pollen grains on exserted stigmas, spikelets with exserted stigmas did not show greater spikelet fertility than spikelets with fewer exserted stigmas or hidden stigmas under heat stress. GA₃ application augmented the percentage of spikelets with exserted stigmas; however, it did not increase spikelet fertility under heat stress. Spikelet fertility of whole panicles was negatively correlated with the percentage of spikelets with exserted stigmas, but positively with that with hidden stigmas. Viability of the hidden stigmas was less reduced than that of exserted stigmas under heat stress, suggesting that hidden stigmas have an advantage in maintaining viability. Heat stress delayed anther dehiscence and reduced the viabilities of both exserted stigmas and pollens, thereby causing low spikelet fertility. Together, these results suggest that high spikelet fertility does not depend on stigma exsertion and that enclosed stigma generally contributes to higher spikelet fertility and heat tolerance under high-temperature conditions during flowering in rice.