The Mechanism Of Salicylic Acid In Alleviating Damage On The Rice Induced By Heat Stress

Extremely high temperatures that have occurred frequently in recent years have caused a lot of damage on the grain yield of rice, which could directly influence our food security. Therefore, more research should be focused on improving cultivation techniques for alleviating heat stress. Results from previous studies indicated that salicylic acid(SA) could enhance heat tolerance in plants, but little evidence of this could be found on rice. Thus, in this study, rice plants were subjected to heat stress through temperatures of 39-43℃ during the reproductive stage of rice for 10-15 d to study(1) the difference responses of spikelets and flag leaves to heat stress at the anthesis of rice;(2) The effect of SA on the floret differentiation, degradation, and fertility of rice under heat stress. The rice plants were sprayed with SA diluted to 0, 10, 100, 1000, 10000 and 50000μmol/L before being subjecting to heat stress. The results indicated that:1. The heat stress significantly decreased the seed-setting rate in both rice genotypes N22 and GT937,but more damage was found in spikelets than in flag leaves. In flag leaves, the difference in photosynthetic rate of the heat-stressed and control was insignificant. Moreover, the difference of temperatures in spikelets and flag leaves was attributed to differences in heat dissipation. Under heat stress, the transpiration rate was significantly higher in flag leaves than in spikelets, and the temperature in flag leaves were 38?C but only 35℃ in spikelets. These results suggest that organ temperature is controlled mainly through transpirational cooling, and also that heat stress is an indirect result of oxidative stress rather than a direct result of heat damage.2. Although the spikelet temperatures did not differ significantly between the two genotypes under heat stress, spikelets of GT937 were more severely damaged than those of N22, which might result from the differences in the antioxidant capacities between genotypes. Results showed that under heat stress, a slight reduction of superoxide dismutase(SOD), peroxidase(POD), and catalase activities(CAT) of spikelets was found in N22, while significant reduction was found in GT937 compared to the control.3. SA could alleviate the damage on the spikelet differentiation induced by heat stress during the floret differentiation stage of rice. When super rice Yongyou12 sprayed with 100μmol/L SA underwent heat stress, it had significantly increased grains per panicle by 80 compared to rice without SA. Higher activities of SOD, POD, CAT, and APX as well as lower MDA concentration in spikelets were also seen in plants sprayed with 100μmol/L SA. In addition, this treatment also increased the concentration of IAA and especially BR in spikelets when subjected to heat stress, Accordingly, the soluble sugar concentration in plants treated with 100μmol/L SA was higher than those without SA.4. The rice plants under heat stress at the pollen mother cell meiosis stage sprayed with 100μmol/L SA attained more grains per panicle and higher seed-setting rate of Changyou1 compared to rice plants without SA. This may ascribe to the role of SA in increasing the pollen viability and the drastically increasing secondary branch numbers. Moreover, increased activity of SOD, POD, CAT and APX as well as lower concentrations of ABA and MDA in spikelets of rice was shown in the SA 100μmol/L treatment than those without SA under heat stress.5. At the pollen mother cell meiosis stage of rice, SA significantly reduced the pollen sterility of heat susceptible rice Changyou1 induced by heat stress. Interestingly, there was little difference in pollen viability among the treatments of 100, 1000 and 10000 μmol/L SA. However, higher activities of SOD,POD, CAT and APX as well as lower MDA concentrations in anthers of rice were most prominent in the treatment of 10000μmol/L. This could greatly reduce programmed cell death(PCD) in the tapetum of rice.