近年来极端高温天气频发，对我国水稻生产造成极大损失，花期高温不仅显著降低水稻结实率，千粒重也显著下降，严重影响到稻米品质的形成。粒重降低，以往研究多认为主要在于高温对水稻“源”和“库”的影响。然而，申请者的试验表明开花期高温对水稻剑叶光合作用及籽粒灌浆关键酶活性的影响均不大。在“源”和“库”没有明显变化的情况下，粒重显著下降，表明高温影响籽粒充实的主要原因不在于“源”和“库”的限制，而在于“流”的不畅，尤其是光合同化物在茎鞘及颖果韧皮部中的运输及卸载。鉴此，本项研究以日本晴（热钝感）及其突变体（热敏感）为材料研究高温影响水稻同化物在韧皮部运输的作用机理，即高温胁迫对水稻同化物在水稻茎鞘韧皮部及颖果背部维管束韧皮部中的运输及卸载的影响。从超微结构及分子表达水平揭示高温下影响水稻同化物运输的主要限制因素，研究结果不仅为水稻耐热性研究提供新的研究思路，还可为培育大穗耐热型水稻品种提供生理学基础。

Heat stress frequently occurs at flowering stage of rice these years that seriously damages the grain yield. This stress not only signifcanly decrease the seed-setting rate but also the kernel weight and grain quality. Previous results suggested that heat stress during flowering stage impaired the photosynthesis of flag leaves and grain filling key enzyme activities were the main reasons for reducing the 1000-grain weight of rice. However, the results in our experiment indicated that heat stress occurred at flowering stage caused little adverse effect on the photosynthesis of flag leaves and Q enzyme activity of grains at the mid and late grain filling stage. There was little changes showed in the “source” and “sink” whereas the kernel weight was significantly decreased when encountered heat stress suggested that“flow”rather than “resource” and “sink” was the main cause in decreasing 1000-grain weight. Thereby, two rice genotypes with different heat resistance(Nipponbare and its mutant, HTS) were used as materials in this experiment to study the effect of heat stress on the translocation of photoassimilates in the phloem of stem, sheath and caryopsis and apoplasmic unloading in the grains. The main factor in limiting the assimilate translocation in the phloem will be clarified in ultrastructure and molecule level, and the results provide not only the new ideas to the researches of heat stress but also the physiological basis for rice breeding with large panicle and heat resistance.