Effect Of High-temperature During Heading And Grain Filling On Grain Quality And Endogenous Homornes Of Rice

Heading/flowering and early grain filling stages of rice (Oryza sativa L.) are the most sensitive periods to high temperature. Increase in the population, rapid development of industry, and deforestation have resulted in continuous increases in the carbon dioxide concentration, greenhouse effect, and global temperature. Extreme climate, such as high temperature in summer, occurs more frequently and keeps longer in many regions of all over the world, leading to adverse effects on crop growth and development and quality deterioration of rice. However, little is known how heat stress during heading and grain filling stages affects grain quality of rice. This study investigated the effects of high temperature on grain quality and responses of endogenous hormones to the high temperature during heading and grain-filling stages. Two rice cultivars with different heat tolerance, Huanghuazhan (heat- tolerant cultivar) and Shuanggui 1 (heat-sensitive cultivar) were pot-grown and subjected to high temperature (mean temperature during the day>35℃) and natural temperature (the mean temperature during the day<33℃). The main results are as follows:1. Effect of high temperature during heading and grain filling on grain yield The high temperature treatment obviously decreased grain filling percentage, grain weight, and grain yield of both cultivars. The reduction rate was greater for the heat-sensitive cultivar than for the heat-tolerant cultivar, and was greater for the treatment of high temperature imposed at heading and the early grain filling stage(0~10 d after heading) than for the treatment imposed at the mid grain filling stage(11~20 d after heading).2. Effect of high temperature during heading and grain filling on grain quality The high temperature treatment decreased milling quality, cooking quality, appearance quality, and nutrient quality. The effect was greater for the high temperature treatment imposed at the heading and the early grain filling stage than the treatment imposed at the mid grain filling stage, and was the greater for the heat-sensitive cultivar than the heat-tolerant cultivar.3. Effect of high temperature during heading and grain filling on ethylene evolution rate of grainsEthylene in grain was released very rapidly during the early grain-filling period, but it dropped rapidly with the filling process. The high temperature treatment increased the release rate of ethylene from grains. The increase was more for the high temperature treatment imposed at the heading and the early grain filling stage than the treatment imposed at the mid grain filling stage, and was the more for the heat-sensitive cultivar (Shuanggui 1) than the heat-tolerant cultivar (Huanghuazhan).4. Effect of high temperature during heading and grain filling on cytokinins, indole-3-acetic acid, and abscisic acidHigh temperature stress decreased the content of zeatin + zeatin riboside (Z+ZR) and indole-3-acetic acid (IAA), but increased the content of abscisic acid (ABA) in the grains during the grain-filling period. The increase or decrease was more for the high temperature treatment imposed at the heading and the early grain filling stage than the treatment imposed at the mid grain filling stage, and was the more for the heat-sensitive cultivar than the heat-tolerant cultivar.5. Effect of high temperature during heading and grain filling on polyaminesHigh temperature at heading and the early grain filling stage increased free spermidine (Spd), spermine (Spm) and putrescine (Put) contents in grains during the early grain-filling stage and increased the maximum free polyamine contents. The increase was more in inferior spikelets than in superior spikelets and more in heat-sensitive cultivars than in heat-tolerant ones. High temperature reduced free Spd and Spm contents in inferior spikelets at the mid grain-filling stage, and put content was varied with cultivars. The effect of high temperature was more on inferior spikelets than on superior spikelets.6. Role of irrigation patterns in reducing harms of high temperature to riceUnder either normal or high temperature, compared with the control (conventional irrigation), the alternate wetting and mild soil drying irrigation (MD) significantly increased grain filing percentage, 1000-grain weight, grain yield, brown rice, milled rice and head rice, and reduced chalky grains and chalkiness degree. The MD also increased the break down viscosity and decreased the setback viscosity. The the alternate wetting and severe soil drying irrigation (SD) showed the opposite effects. The two cultivars behaved the same. These results indicate that the MD could maintain a higher grain yield and better quality of rice when subjected to high temperature. The reduction of relative humidity in the panicle canopy, decreased reactive oxygen (O2.-) production rate, elevated contents of ascorbic acid and reduced glutathione, and increased concentrations of cytokinins in leaves and spermidine and spermine in grains account for the reducing harms of high temperature to rice under the MD regime.