The Impacts Of Drought Stress On Grain Quality And Wx Gene Expression In Rice

Rice is one of the most important food crops. It is the staple food for more than half of the global population. Rice is a crop with the largest water consumption and drought stress can cause severe lose in yield and adversely affect grain quality. Global climate change has increased frequency of flood and drought disasters, and drought stress has become a serious threat to rice production and human food security. Therefore, rice drought resistance study has become one of hot topics in rice science. Many researches have been done on drought stress and rice yield in China and abroad. However, there are few researches on the effects of drought stress on rice grain quality, and no systematic research has been reported up to present. Using grain quality data of a total of6sets of experiments in different years (2006-2008), different locations (Wuhan, Hubei province and Lingshui, Hainan province), different drought stress intensity (moderate drought: field trials,37varieties tested; severe drought:field tests with movable rain-out shelter,18varieties tested), systematic studies on the relationships between drought stress and rice quality traits have been made in this thesis, In addition, using PVC pipes with adjustable soil moisture content, a pot experiment with drought stress during grain filling stage was done and quantitative analysis was made on the dynamic changes of Wx gene expression and amylose content in rice endosperm. The main results were as follows:1. Variance analysis on6sets of different field tests on rice processing quality traits (brown rice rate, white rice rate, and whole polished rice rate), exterior appearance quality traits (grain shape, chalky rice rate and chalkiness degree), cooking and nutrition qualities (gel consistency, gelatinization temperature, amylose content and protein content) showed that there are extremely significant interaction effects between different years, different locations, different drought stress intensity and different varieties. This indicates the effect of water stress on rice quality traits has genotypic differences, and it is also related to other environmental factors.2. Drought stress has negative effects on rice processing quality traits. The brown rice percentage and white rice percentage are relatively stable under drought stress, but the whole polished rice rate dropped significantly. Under drought stressed and control (normal water management) treatments, the average brown rice percentages ranged from77.31%to77.31%and77.30%to80.65%respectively, and polished rice rate ranged from65.17%to70.70%and65.71%to70.50%respectively in6sets of drought stress tests. No significant difference has been found in5sets of tests. However, both brown rice rate (79.31%) and white rice rate (67.71%) under moderate drought stress were about1%lower than the brown rice rate (80.30%) and white rice rate (68.73%) of control in Hainan trial in2007, which reached significant and extremely significant levels respectively, the average whole polished rice rate ranged from55.97%to63.54%and61.42%64.14%respectively under drought stressed and control treatments, the whole polished rice rates under drought stress were lower than that of control. The differences between the treatments in5out of the6sets of experiments reached significant or extremely significant levels.3. Drought increased the variability of all exterior appearance traits. The variability of grain shape caused by drought is relatively small, while that of chalkiness is larger. Average grain width under the drought stressed and control treatments in6sets of tests were2.0-2.4mm and2.2-2.4mm respectively, no significant difference was detected between the treatments. Average grain length under drought stressed and control treatments were6.0-6.5mm and6.5-6.6mm. The grain length under drought stress were lower than that of control, and4out of the6trials reached extremely significant level. Average length/width ratio under drought stressed and control treatments were2.8-3.0and2.8-2.9respectively, no significant difference was detected between the treatments. Average chalky rice rate under drought stressed and control treatments were21.4%-47.3%and28.0%-50.8%respectively. And average chalkiness degree under the drought stress and control treatments were3.2%-15.9%and9.9%22.9%respectively. The differences in both chalkiness indexes reached significant or extremely significant levels.4. Drought stress also has an impact on rice cooking and nutrition traits. Amylose content decreased while protein content increased under drought stress. Alkali value decreased (gelatinization temperature increased) along with decreased gel consistency under severe drought stress. Average amylose content under drought stressed and control treatments were16.6%-17.3%and17.2%17.7%respectively. The average amylose contents under drought stress were lower than that of control, and4out of the6trials reached extremely significant level. Average protein content under drought stressed and control treatments were8.9%-9.9%and9.4%-10.8%. The average protein contents under drought stress were higher than that of control, and5out of the6trials reached extremely significant level. The average alkali value were6.5-6.8and6.5-6.5under drought stressed and control treatments, no significant difference was detected between moderate drought stressed and control treatments, while extremely significantly reduction in alkali value were found in2trials with severe drought stress. Average gel consistency under severe drought stressed and control treatments were41.8-44.0mm and50.0-53.0mm respectively, the reduction of gel consistency has reached extremely significant level.5. Genotyic difference was found in the impacts of drought stress on rice grain quality. There are significant differences among the tested varieties in their range of variation in grain quality traits. For example, relatively small changes were observed in whole polished rice rate of C1027, chalkiness degree of Aisanlu, and amylose content of PRIMAVERA under different drought stress conditions.6. The Wx gene expression patterns in endosperm are generally consistent to the amylose accumulation patterns. Wx gene expression in rice endosperm significantly increased in the5th day after flowering, and reached to its maximum in the10th day, and then gradually declined, which was consistent to the amylose accumulation during grain filling stage. Drought stress treatments decreased the relative expression quantity of Wx gene in endosperm and reduced the synthesis and accumulation rate of amylose, and*eventually resulted in a reduction of amylose content in rice. This indicates that drought stress affects grain quality traits via regulating the expression of related genes.