Effect Of High Temperature And Soil Drying On The Yield Quality And Quantity Of Rice And Its Physiological Mechanism

Temperature and soil drying are the most important ecological factors to the rice growth. With the continuous increase of the greenhouse effect, high temperature and soil drying are becoming major harm to crop production. To understand the mechanism involved in the effect of high temperature and soil drying on rice(Oryza sativa L.) growth and to take counter measures to reduce damage of heat stress to rice would have great significance in promoting rice production and ensuring food security. In this study, the effects of high temperature, soil drying and high temperature plus soil drying on rice yield and quality and their physiological mechanisms were investigated. Cultivation techniques to reduce the injury of high temperature to rice growth and development were studied. The main results are as follows:1. Effect of high temperature during heading and grain filling on the yield quality and quantity of rice and its physiology reasonTwo mid-season indica cultivars, Huanghuazhan (a heat-tolerant rice cultivar) and Shuanggui1(a heat-sensitive rice cultivar), were pot-grown and subjected to high temperature treatments (mean temperature during the day/night at35.1℃/20.8℃) during early grain filling (0-10d after heading) and mid grain filling (11-20d after heading), and natural temperature (the mean temperature during the day/night at29.0℃/20.8℃) was taken as the control. The grain yield and quality of rice were investigated. The results showed that the high temperature treatment significantly reduced seed-setting rate, grain yield, milled rice, head rice, gel consistency and the break down viscosity of both cultivars. The high temperature also increased chalky kernel, chalkiness and the setback viscosity. The reduction rate was greater for the heat-sensitive cultivar than for the heat-tolerant cultivar, with a more reduction under the treatment during early grain filling than during mid grain filling. The high temperature obviously increased ethylene evolution rate, abscisic acid (ABA) contents and starch synthase (StS) activity, decreased contents of zeatin+zeatin riboside (Z+ZR), indole-3-acetic acid (IAA) and starch branching enzyme (SBE) activities in spikelets of Shuanggui1and Huanghuazhan during early grain-filling stage, but the increase or reduction was less for heat-tolerant cultivars than for heat-sensitive cultivars. These results suggested that higher concentrations of Z+ZR, IAA, ABA and activities of StS and SBE, lower ethylene evolution rate in spikelets were contributed to higher grain yield and better quality of rice for a heat-tolerant rice cultivar. 2. Effect of soil moisture on the yield quality and quantity of rice and its physiological mechanism(1) Hormones in rice spikelets in responses to water stress during meiosisTwo rice cultivars, Han A-3(HA-3, drought-resistant) and Wuyujing7(WY-7, drought-susceptible), were used and subjected to well-watered (WW) and water-stressed (WS) treatments during meiosis (15-2d before heading). Leaf water potentials of both cultivars markedly decreased during the period as a result of WS treatment, but panicle water potentials remained constant. The percentage of sterile spikelets in WS plants was increased by48.5to50.9%for WY-7, whereas only by12.6to12.8%for HA-3. Concentrations of Z+ZR, IAA and gibberellins (GAi+GA4) in spikelets showed no significant differences either between the WW and WS treatments or between the two cultivars. ABA, ethylene, and1-aminocylopropane-1-carboxylic acid (ACC) were all enhanced in spikelets by the water stress, with more enhanced for ethylene than for ABA in WY-7, whereas elevated ABA balanced ethylene production in HA-3. The spikelet sterility was significantly reduced when amino-ethoxyvinylglycine (an inhibitor of ethylene synthesis) or ABA was applied to the panicles of WS plants at the early meiosis stage. Application of ethephon (an ethylene-releasing agent) or fluridone (an inhibitor of ABA synthesis) had the opposite effect. The results suggest that antagonistic interactions between ABA and ethylene may be involved in mediating the effect of water stress that affects spikelet fertility. A higher ratio of ABA to ethylene would be a physiological trait of rice adaptation to water stress.(2) Effect of soil moisture during heading and grain filling on the yield and quality of riceTwo rice cultivars, Wuyujing3(japonica) and Yangdao6(indica), were grown in pots. Three soil moisture treatments, well-watered (WW, well-watered), moderate soil-drying (MD, soil water potential at-10～-30kPa), and severe soil-drying (SD, soil water potential at-40～-60kPa), were imposed during grain-filing. Compared with WW, grain weight, peak viscosity and breakdown values were significantly increased, as well as chalkiness and setback values were reduced under MD. The results were reversed for SD treatments. Activities of sucrose synthase (SuS), adenine diphosphoglucose pyrophosphorylase (AGP) and starch synthase (StS) in grains in the middle and late grain-filling stages were significantly enhanced, whereas ethylene revolution rate of grains were decreased, under MD. The SD had the opposite effects. The results indicate that MD can improve rice quality, resulting from the physiological mechanism of enhancing activities of SuS, AGP and StS in grains.3. Effect of high temperature and soil drying on the yield and quality of rice and its physiological basesSeven rice cultivars, Huanghuazhan (an indica heat-tolerant cultivar), Shuanggui1(an indica heat-sensitive cultivar), Huhan15(an indica drought-resistant cultivar), Yangdao6(an indica inbred), Liangyoupeijiu (a two-line indica hybrid cultivar), Yangfujing8and Yangjing4038(japonica inbreds), were grown in pots and subjected to high temperature treatments (HT, maximum temperature during the day at37.5℃), soil drying treatments (SD, soil water potential at-30±10kPa) and high temperature plus soil drying treatments (HT+SD) during meiosis (15-2d before heading), heading, and early grain filling (0-10d after heading), and the natural temperature plus well-watered treatment was taken as the control (CK). The results showed that high temperature, soil drying or high temperature plus soil drying treatments during meiosis and during heading significantly reduced pollen fertility rate, fertilization rate, anther dehiscence rate, spikelets per panicle, seed-setting rate and grain yield of all the cultivars. High temperature or soil drying treatment during meiosis or grain filling stage significantly reduced milled rice rate, head rice rate, gel consistency, break down value and short chain in debranched amylopectin, whereas increased chalky grains, chalkiness and setback values. The increase or reduction was less for heat-tolerant cultivars and drought-resistant cultivars than for other kinds of cultivars. Both heat-tolerant cultivars and drought-resistant cultivars showed stronger root activity and antioxidative defense system, greater net photosynthetic rate of the flag leaf, higher activities of the key enzymes involved in sucrose-to-starch metabolic pathway, and more gene expressions in grains, which contributed to maintaining a higher grain yield under the high temperature or drought stress.4. Role of irrigation patterns in reducing harms of high temperature to riceTwo mid-season indica cultivars, Huanghuazhan (a heat-tolerant rice cultivar) and Shuanggui1(a heat-sensitive rice cultivar), were grown in pots. Both high temperature (mean temperature during the day/night at35.1℃/20.8℃) and normal temperature (mean temperature during the day/night at29.0℃/20.8℃) treatments and three irrigation patterns, alternate wetting and moderate soil drying (MD, plants were re-watered when soil water potential reached15kPa), alternate wetting and severe soil drying (SD, plants were re-watered when soil water potential reached30kPa), and well-watered (control, continuously flooded), were conducted during heading and grain filling. The grain yield and quality of rice were investigated under these treatments. The results showed that under either normal or high temperature, compared with the control, the MD significantly increased seed-setting rate,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 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 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.5. Role of nitrogen fertilizer in reducing harms of high temperature to riceTwo rice cultivars, Huhan15(indica) and Yangjing4038(japonica), were grown in pots. Three nitrogen fertilizer rate, low nitrogen (LN,0.5g N per pot), middle nitrogen (MN,1.0gN per pot), and high nitrogen (HN,2.0g N per pot), were conducted during panicle initiation stage. Both high temperature (mean temperature during the day/night at35.1℃/20.8℃) and normal temperature (mean temperature during the day/night at29.0℃/20.8℃) treatments were conducted during heading and grain filling. The grain yield and quality of rice were investigated under these treatments. The results showed that under the same temperature, especially under high temperature, compared with the LN, the MN and HN significantly increased spikelets per panicle, seed-setting rate,1000-grain weight, grain yield, head rice, break down viscosity and short chain in debranched amylopectin, and reduced chalky grains and setback viscosity, and the effect was the most significant at MN. The two cultivars behaved the same. These results indicate that a proper nitrogen fertilizer rate could maintain a higher grain yield and better quality of rice when subjected to high temperature. Increases in photosynthetic rate of the flag leaf, root activity, and activities of the key enzymes involved in sucrose-to-starch metabolic pathway in grains were closely associated with the reducing harms of high temperature to rice under higher nitrogen rate.