Heat stress-reduced kernel weight in rice at anthesis is associated with impaired source-sink relationship and sugars allocation

Heat stress is one of the most limiting factors in rice plant growth and development. However, the impairment in the whole-plant carbohydrate partitioning at anthesis caused by heat stress is seldom documented. Two rice genotypes with different heat tolerance, namely Nipponbare (NIPP) and its high-temperature susceptible (HTS) mutant were subjected to heat stress of 40 °C for 12 d at anthesis. Heat stress significantly decreased the spikelet fertility and kernel weight, and a larger decline was found in HTS than in NIPP plants compared with their respective controls. However, there was no significant difference in photosynthesis in flag leaf and starch branching enzyme activity in grain between the control and heat-stress groups. Thus, heat stress-induced decline in kernel weight was most likely due to the inhibition of the assimilate distribution rather than the limitation of the source and sink. This assumption was confirmed by the distribution of dry mater weight, nonstructural carbohydrates (NSC) and starch in grain, leaf and sheath-stem observed under heat stress. Indeed, the sucrose transporter (SUT) genes, sucrose synthase (SUS) genes and phytohormones in grain and the callose deposition in plasmodesmata of leaf and sheath were involved in the process. However, the patterns of change in phytohormones in leaf and sheath-stem were different from those of carbohydrates, but exogenous sucrose alleviated the heat-stress effect on the spikelet fertility, kernel weight, dry matter weight accumulation and allocation. Thus, sugars rather than phytohormones might act as a signal molecule to mediate the source-sink relationship in rice under heat stress.