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

With the continuous increase of the greenhouse effect, high temperature is becoming a major harm to crop production. To understand the mechanism involved in the effect of high temperature on rice (Oryza sativa L.) growth and 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, three heat-tolerant rice cultivars/lines of Huanghuazhan, T226 and 082 and three heat-sensitive ones of Shuanggui 1, T219 and Xieqingzao B were used as materials, the effects of high temperature on rice yield and quality and their physiological mechanisms were investigated. The ways to reduce the injury of high temperature to rice growth and development were studied including chemical application and irrigation and nitrogen (N) management. The main results are as follows:1.Effect of high temperature on grain yield of rice and its physiological mechanismHigh temperature during meiosis significantly reduced the number of spikelets per panicle, seed-setting rate, and 1000-grain weight. High temperature during heading and early grain filling significantly reduced seed-setting rate and 1000-grain weight, leading to a significant reduction in grain yield, with a more reduction in Shuanggui 1 than in Huanghuazhan. The effect of high temperature on grain yield was much less during mid grain filling than during heading and early grain filling. High temperature significantly reduced spikelet-fertilized rate, seed-setting rate, grain-filling percentage and 1000-grain weight of inferior spikelets in all cultivars and of superior spikelets in heat-sensitive cultivars, with less effect on superior spikelets in heat-tolerant ones. The effect was also less during heading and early grain filling than during mid grain filling. Under high temperature, lower leaf-temperature, higher activities of antioxidant enzymes, ascorbic acid (ASA) and reduced glutathione (GSH)) contents, lower malonyldialdehyde (MDA) and hydrogen peroxide (H2O2) contents and lower Superoxide anion (O2.-) production rate in leaves, and higher ribonucleic acid (RNA) and adenosine triphosphate (ATP) contents and lower ethylene production rate in young panicles, and higher root activity would be important physiological traits leading to a higher grain yield for a heat-tolerant rice cultivar. 2.Effect of high temperature on rice quality and endosperm structureThe high temperature treatment during heading and early grain filling significantly reduced milling quality, shortened the gel consistency, and increased amylose content, chalkiness degree and protein content. The effect was much less during mid grain filling than during heading and early grain filling. The high temperature treatment increased prolamine protein content and rice nutrition elements contents in grains. High temperature during heading and grain filling decreased the size of amyloplasts, either complex or single, increased the size difference between complex and single amyloplasts, and loosened their arrangements, which resulted in fewer complex amyloplasts and more single amyloplasts. The earlier the high temperature treatment during the grain filling period, the more serious was the effect on the endosperm structure. The effect was more on inferior spikelets than on superior spikelets, and was more on Shuanggui 1 than on Huanghuazhan.3.Responses of hormones in rice to high temperatureHigh temperature during seedlings and meiosis significantly reduced zeatin (Z), zeatin riboside (ZR) and indole-3-acetic acid (IAA) contents and increased abscisic acid (ABA) content in leaves, young panicles and grains. The decrease or increase in hormonal levels was more in inferior spikelets than in superior spikelets. The effect of high temperature was more during heading and early grain filling than during mid grain filling. The contents of Z+ZR, IAA, ABA were higher in heat-tolerant cultivars than in heat-sensitive cultivars. The contents of Z+ZR, IAA, ABA and ABA/(Z+ZR+IAA+ABA) in grains were significantly correlated with grain-filling rate, indicating that higher Z+ZR,IAA and ABA contents would also be important physiological traits for a heat-tolerant rice cultivar.4.Polyamines in rice and their relationships with grain filling of inferior and superior spikelets and in responses to high temperature(1) Superior spikelets exhibited higher contents of free sperdimine (Spd) and free spermine (Spm) than inferior spikelets at the early grain filling stage. The maximum contents of free Spd and free Spm were significantly correlated with the maximum grain filling rate and grain weight. Application of Spd and Spm to panicles significantly increased grain filling rate and grain weight of inferior spikelets, implying that the poor development of inferior spikelets is attributed, at least partly, to the low polyamine contents. (2) High temperature significantly reduced free Spd, whereas it significantly increased free Spm contents in leaves for both heat-tolerant and heat-sensitive cultivars, and increased free Spd content in young panicles for a heat-tolerant cultivar. The decrease or increase was more in heat-sensitive cultivars than in heat-tolerant ones. The changes in free putrescine (Put) content varied with plant organs and cultivars. High temperature during heading and early grain filling increased free Spd, Spm and Put contents in grains at the early grain filling stage and enhanced the maximum free Spd and Spm content. The enhancement 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 later grain filling stage. Free polyamine contents in grains were reduced at the early treatment and increased at the late treatment when subjected to high temperature during the mid grain filling. The effect of high temperature was more in inferior spikelets than in superior spikelets. These results suggest that little change in free Spd and Spm content in the leaves during seedlings and in young panicles during meiosis would benefit rice to resist against high temperature.5. Effect of high temperature on enzymatic activities in grainsHigh temperature during heading and early grain filling significantly reduced ATPase activity in grains for a heat-sensitive cultivar. High temperature during mid grain filling significantly increased ATPase activity in grains for a heat-tolerant cultivar. High temperature during heading and early grain filling significantly enhanced activities of adenine diphosphoglucose pyrophosphorylase (AGPase), soluble starch synthase (SSS) and isoamylase (ISA) and starch content of Shuanggui 1, T219 and T226, and the result was reversed when subjected to high temperature during mid grain filling. High temperature during heading and grain filling significantly reduced granule-bound starch synthase (GBSS) activity and the ratio of amylose to total starch content, and significantly increased starch branching enzyme (Q) activity in grains of the three cultivars. High temperature during mid grain filling markedly increased GBSS and SSS activity and ratio of amylose to total starch content in Huanghuazhan, and showed no significant effects on activities of AGPase, ISA, Q enzyme and starch content. These results indicated that the response of enzymatic activities to high temperature varied with the treatment period, tolerance of cultivars, grain positions, and kinds of enzymes. A heat-tolerant cultivar would have higher activities of ATPase, GBSS and Q enzyme in grains under high temperature.6. Effect of high temperature on protein expression in leaves of riceHigh temperature made the diversity of protein expression in leaves: new proteins were produced, or some proteins were not expressed or up-regulated or down-regulated. The protein expressions in heat-tolerant or heat-sensitive cultivars were not consistent during the treatment period. The change of protein expression was less in a heat-tolerant cultivar than in a heat-sensitive cultivar. Mass spectrometry analysis showed that the differences in proteins were mainly related with photosynthesis, signal transduction and putative uncharacterized protein. These proteins were not expressed or down-regulated in a heat-sensitive cultivar, while new proteins were produced or proteins were up-regulated in a heat-tolerant cultivar, indicating that expressions of proteins are closely associated with the heat-tolerance of a cultivar.7.Regulation techniques to reduce the harms of high temperature(1) When subjected to high temperature during the seedling period, application of brassinolide (BR) significantly enhanced the activities or expression levels of protective enzymes in the leaves, and therefore effectively protected seedlings from damage from high temperature. The cultivars differing in heat-tolerance exhibited deferent responses to high temperature when BR was applied. Under high temperature during meiosis, application of Spd, ABA and BR could reduced O2 .- production rate, increased Z+ZR, IAA and ABA contents in leaves, enhanced Spd and Spm contents and reduced Put content in young panicles, resulting in increases in the number of spikelets per panicle, seed-setting rate, grain weight and grain yield. (2) Compared with the control (panicle fertilizer 0.46 g N pot-1), the panicle fertilizer treatment (0.92 g N pot-1) and the panicle fertilizer plus grain fertilizer treatment (0.46g N pot-1 was applied the panicle initiation and 0.46g N pot-1 at the initial of heading) increased grain yield of Huanghuazhan and Shuanggui 1. The panicle fertilizer treatment increased brown rice, milled rice and head rice of Huanghuazhan. The panicle fertilizer plus grain fertilizer treatment increased head rice of Huanghuazhan and reduced gel consistency of the two cultivars. The panicle and grain fertilizer treatment increased protein content and decreased chalky grains and chalkiness degree. The panicle fertilizer treatment improved eating quality of Huanghuazhan. (3)Under either normal or high temperature, compared with the control (well watered), alternate wetting and moderate soil drying (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 alternate wetting and severe soil drying (SD) showed the opposite effects. These results indicate that MD could increase grain yield and improve quality of rice when subjected to high temperature.