Mechanism Of Heat Stress Decreasing The Spikelet Fertility And Assimilate Translocation In Rice At Anthesis

Ascribed to the increase in the greenhouse gases, extreme high temperature stress occurred frequently these years that significantly decreased the grain yield of rice at anthesis. A great number of researches has been conducted on this filed, while little were showed on the genetic in heat resistance of rice, plant organ temperatures, IAA metabolism of pollinated pistil and assimilate translocation in the phloem of rice. Thus, in this experiment, the rice plants with different heat resistance were subjected in heat stress at 39-43℃ for 15 d at anthesis to 1) analyze the heat resistance in the rice maintainer lines and restorer lines widely used in the three-line hybrid breeding in China as well as the relationship in heat resistance between the maintainer lines and restorer lines and their F1 hybrid rice; 2) study the effect of heat stress on the seed-setting rate and kernel weight of superior and inferior spikelet in rice; 3) reveal the relationship between the IAA metabolism of pollinated spistil of rice and its heat resistance; 4) discuss the inhibition of heat stress on the assimilate translocation in the phloem of rice. The results indicated that: 1. According to the heat stress indexes, these rice plants were divided into thermal resistant, semi-thermal resistant, semi-thermal sensitive and thermal sensitive lines. In general, higher heat resistance was observed in rice maintainer lines than restorer lines in this experiment. The correlation analysis indicated that the heat stress index was significantly correlated with seed-setting rate and abortive grain rate under heat stress, but not under natural conditions. Heat resistance in three-line hybrid rice was controlled by the recessive genes in this experiment. Thus, both parents should possess high temperature tolerance to develop heat-tolerant F1 hybrid rice. Furthermore, the heat stress index of F1 hybrid rice combinations significantly correlated with the heat stress index of rice restorer lines but not with the heat stress index of rice maintainer lines. This result suggested that the male parent played a more important role in heat-tolerant hybrid rice than the female parent. 2. Heat stress causes more damage to the superior spikelet than inferior ones, because of higher decrease in seed-setting rate and kernel weight were showed in the former than the latter under heat stress, which ascribed to the temperature difference between the superior and inferior spikelets. The temperatures of superior spikelet was higher by 3-5℃ than those of inferior ones under heat stress. 3. IAA could significantly increase the heat resistance in the heat sensitive rice cultivars, while little effect was showed to the heat resistant rice. Compared with control, higher seed-setting rate and pollen tube length were observed in the IAA-treatment when subjected to heat stress during 0-2h pose anthesis of rice. Meanwhile, higher decrease in IAA concentration of pollinated pistil was showed in heat sensitive rice than those of heat resistant ones, which may ascribe to the significant increase in IAO activities of pollinated pistil when subjected to heat stress. Furthermore, changes in the concentration of flavonol were not involved in the IAA metabolism of pollinated pistil under heat stress. 4. 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. However, the kernel weight was significantly decreased. This suggested that “flow” rather than “resource” and “sink” was the main cause in decreasing 1000-grain weight. Further, the block of assimilate unload in grains may be maindly responsible for the decrease in kernel weight under heat stress. Because the relative expression of the sucrose transporters OsSUT1 and OsSUT2 in the grains were decreased under heat stress, especially the latter, while those in the flag leaves, sheath and stem were higher than the control. Additionally, heat stress seems to cause little effect on the leaf plasmodesmata and phloem sieve tube elements in sheath, stem and neck of panicle of rice base on their ultrastructures.