Effect Of High Temperature Stress On Rice Grain Yield And Quality, And Mitigation Through Application Of Chemical Regulators, And Its Mechanism

The mean temperature might rise up to range of 2.0-4.5 ℃ worldwide by the end of this century. Beside from this, a prediction has been made that rise in minimum night temperature will be at a quicker rate as compare to the maximum day temperature. Rising temperatures caused by high temperature stress not only affect the crop growth process, but also lead to direct changes in other environmental factors and pose indirect effect on yield and quality of rice has been observed, so at the present stage, it aroused public attention. Breeds, including through breeding and biotechnology to improve high temperature tolerance of rice help to mitigate the negative effects of high temperature, however, progress in this area have been slow. By adopting different methods like sowing, water and nutrient management can also to some extent mitigate the effects of high temperature on rice yield and quality, but in most cases, these techniques are influenced by many factors, such as crop rotation, irrigation and other constraints like their applications are hard to applied to large area. When high temperature occurs, timely application of chemical control substances relieve high temperature damage to rice, but currently exogenous application of chemical and carbonaceous substances (including biochar, growth regulators) effect improving the high temperature resistance of rice research is very limited. So this experiment study was designed to find out suitable combination of growth regulators along with biochar fertilization to enhance the performance of rice plant under heat stress.For this reason, this study using high-temperature tolerance of different types of rice, artificial green house temperature control by setting different combinations of temperature, a comparative study of different plant growth regulators, antioxidant and bio carbon under high temperature stress influence and mechanism of the formation for growth development, yield and quality of rice.Two indica rice (Oryza sativa L.) cultivars viz., IR-64 and Huanghuazhan (HHZ) with almost similar plant architecture but different responses to temperature, set of temperature processing including ambient temperature (AT), high night temperature (HNT) and high day temperature (HDT) and four different combinations of ascorbic acid (Vc), alpha-tocopherol (Ve), brassinosteroids (Br), methyl jasmonates (MeJA), and triazoles (Tr) along with a nothing applied control were used. Rice plants were cultivated under natural conditions at Huazhong institute technology of artificial climate chambers during rice growing season (15th May to 25th September) of 2013 and 2014. In addition, a separate experiment of different biochar and phosphorus (P) fertilization treatments during 2014 was conducted. Application of exogenous growth regulators and biochar fertilization on rice growth, photosynthetic rate, dry matter accumulation, pollen characteristics (like pollen germination, anther dehiscence etc), metabolites synthesis, hormonal contents and antioxidant activities were investigated under different temperature treatments. Most of the data were recorded at flowering and maturity stage apart from one or two parameters at panicle imitation stage. The main results are as follows.(1) The research results showed that high temperature significantly reduced pollen fertility, rate of anther dehiscence, pollen accepting on stigmas, pollen germination rate and metabolites synthesis. High night temperature effects were more pronounced. Exogenous application of various plant growth regulators assuaged the adverse effects of high temperature and Vc+Ve+MeJA+Br was found the best combination than the other treatments for every studied characteristic. Variations were also apparent between cultivars and Huanghuazhan performed better than IR-64 under high-temperature stress, with higher pollen fertility, better anther dehiscence, and greater pollen retention and germination rates. The greater tolerance of Huanghuazhan to high temperature was related with higher synthesis of metabolites in this cultivar.(2) High temperature stresses have significant effect on growth and lower production of rice. Research showed that high temperature stress severely affected rice morphology, and also reduced leaf area, above and below ground biomass, photosynthesis, and water use efficiency, while increased leaf water potential of both rice cultivars. Grain yield and its related attributes like 1000 grain weight except number of panicles, were reduced under high temperature thus lead to reduced production. The Huanghuazhan performed better than IR-64 under high-temperature stress with better growth and higher grain yield. Exogenous application of PGRs was helpful in alleviating the adverse effects of high temperature. Among PGR combinations, the Vc+Ve+MejA+Br was the most effective treatment for both cultivars under high temperature stress. The highest grain production by Vc+Ve+MejA+Br treated plants was due to enhanced photosynthesis, spikelet fertility and grain filling, which compensated the adversities of high temperature stress.(3) High temperature stress degrades the grain quality of rice during both year; nevertheless, exogenous application of plant growth regulators (PGRs) may alleviate such negative effects of high temperture. Results showed that high-temperature stress was detrimental for grain appearance and milling qualities, and both HDT and HNT reduced grain length, grain width, grain area, head rice percentage and milled rice percentage, while increased chalkiness percentage and percent area of endosperm chalkiness in both cultivars compared with ambient temperature (AT). Significantly higher grain breakdown, set back, consistence viscosity and gelatinization temperature, while significantly lower peak viscosity, trough viscosity and final viscosity was recorded under high temperature stress compared with AT. The HNT was more devastating for grain quality than HDT. Exogenous application of PGRs assuaged the adverse effects of high temperature in both rice cultivars and the Vc+Ve+MejA+Br was the best combination for both cultivars under high temperature stress.(4) A high-temperature stress severely reduced pollen fertility, anther dehiscence, pollen retention and germination rate, which is the main cause of decrease seed setting rate. Heat resistant variet Huanghuazhan perform better as compared to heat sensitive variety IR-64. Fertilizer can effectively alleviate the high temperature effect on pollen fertility and viability. The biochar+P application outperformed the other treatments in protecting the studied pollen characteristics from heat stress in two rice cultivars. Mean while biological activated carbon mixed with phosphate fertilizer promotes plant growth and enhance yield. Highest grain production and better grain quality in biochar+P treatments might be due to enhanced photosynthesis, water use efficiency, and grain size, which compensated the adversities of high temperature stress. The result of this study show that plant growth and development of rice plant under high temperature stress were seriously affected but with exogenous application of chemical (Vc+Ve+MeJA+Br) along with Biochar+ Phosphate fertilizers and other measures help to reduce the adverse effects of high temperature on rice. In addition, investigation on metabolites synthesis, endogenous hormone balance, genotypic differences would help to contribute to the richness of rice plants, mechanism of heat stress and ease of exploration and mechanism of knowledge, heat resistance and disaster reduction to provide technical guidance for rice under high temperature stress.