Response Of Indica Grain Yield And Grain Quality To Experimental Warming In A Double Rice Cropping System And Its Mechanism

According to IPCC,compared with the 1850-1900 period,global surface mean temperature is predicted to increase 1.5? between 2030 and 2052.A large number of studies indicate that climate warming will have a serious impact on rice production and food security.Indica rice is one of the most important stable food crops in China.In 2018,the planting area and grain yield of indica rice in China accounted for 66%and 63%,respectively.As one of the most important rice cropping systems in China,the planting area and yield of double cropping rice accounted for 34.9%and 29.7%,respectively.Therefore,in 2018 and 2019,we performed a field warming experiment with free air temperature increase(FATI)facility in the double rice cropping system of China.Four indica rice cultivars(Early rice:Xiangzaoxian45 and Qiliangyou2012;Late rice:Jiuxiangnian and Taiyou398)were treated with FATI facility during the whole growth period to study the effects of warming on grain yield and quality and its mechanism,systematically.The main conclusions were as follows:In double rice cropping system,a single infrared heater could form an effective warming area about 2.7 m2.Under warming condition,the diurnal,daytime and nighttime canopy temperature of early rice from transplanting to maturity were increased 1.4-1.9?,0.9-1.2? and 1.9-2.6?,respectively;and 1.8-2.1?,1.1-1.6? and 2.4-2.9? for late rice.Warming caused the canopy temperature increase amplitude at nighttime was greater than that at daytime,so it reduced the diurnal temperature range.Under warming condition,rice canopy daily average temperature dynamic trends during the whole growth period and diurnal temperature trends of the critical growth period(15 days after transplanting,heading stage and 15 days after heading)were basically consistent with the ambient temperature treatment.Therefore,the FATI system in this study could meet the requirements of experimental research on the response of double rice cropping system to climate warming.Experimental warming had no effects on yield,yield components and harvest index for both early and late rice.The effects of experimental warming on rice growth period were asymmetrical.Under warming condition,the time from transplanting to heading and from heading to maturity were significantly shortened of early rice,and the shorter time before heading was longer than that after heading.For late rice,experimental warming prolonged the time from transplanting to heading,but shortened the time from heading to ripening,significantly.The responses of dry matter accumulation and translocation of early and late rice to experimental warming were inconsistent.Under warming condition,dry matter accumulation of early rice at tillering stage was increased significantly,but decreased at maturity stage.For late rice,experimental decreased dry matter accumulation significantly at tillering stage.but had no effects on dry matter accumulation at maturity stage.The effects of experimental warming on nitrogen accumulation and nitrogen use efficiency also varied among seasons and cultivars.Experimental warming increased the nitrogen accumulation at tillering stage of early rice,but had no significant effect on the nitrogen accumulation and nitrogen use efficiency at heading stage and maturity stage.For late rice,the nitrogen accumulation of two cultivars at tillering stage was significantly reduced under warming condition,but the nitrogen accumulation at heading stage and maturity stage,and nitrogen use efficiency were inconsistent.Experimental warming changed the grain filling process and reduced the final weight of early rice top grains,increased it of bottom grains;but for late rice,experimental warming had no significant effects on the final weight of top and bottom grains.Under warming condition,the chalky grain rate,chalkiness,protein content,amino acid content,rice flour RVA peak viscosity,breakdown and gelatinization temperature all showed an increase trends,amylose content and setback showed downtrends,but the milled rice rate and head rice rate varied among the seasons.In addition,the decrease of milled rice rate of early rice was mainly due to the decrease of top grain milled rice rate.The response of the appearance quality of early rice top grains to experimental warming was less than that of top grains,but the response of late rice top grains to experimental warming was greater than that of bottom grains.The response of protein content in the top grains for both early and late rice to experimental warming was less than that in the bottom grains,while amylose content,peak viscosity,breakdown,setback,and gelatinization temperature in the top grains were greater than that in the bottom grains.The contents of total starch and amylopectin in milled rice were not significantly changed under warming condition,but the amylose content was decreased and the protein content was increased.Compared with the ambient temperature treatment,experimental warming increased total starch and amylose accumulation in grains for both early and late rice during early grain filling phase,which was related to the increase of ADP-glucose pyrophosphorylase(AGPase)and soluble starch synthase(SSS)activity,but had little relationship with granules bound starch synthase(GBSS)and starch branching enzyme(SBE)activity.Experimental warming increased the protein accumulation in grains for both early and late rice,especially in the early grain filling stage,which was mainly related to the higher glutamate synthase(GOGAT)activity in grains,but less related to glutamine synthetase(GS)activity.Under warming condition,the structural characteristics of rice starch changed significantly,with a larger granule average diameter,higher starch relative crystallinity,and longer amylopectin average chain length,these resulted in starch with lower swelling power,water solubility,and pasting viscosity,but higher gelatinization temperatures and gelatinization enthalpy.Therefore,in the double rice cropping system,experimental warming had no significant effects on indica grain yield and yield components.The appearance quality and cooking quality were deteriorated for both early and late rice under experimental warming condition,which was beneficial to improve the eating quality and nutritional quality,but the milling quality was varied among seasons.The results of this study could provide scientific basis for the cultivation of double cropped-rice with high quality and high yield under climate warming condition in the future.