Effects Of Warming And Nitrogen Application At Grain Filling Stage On Rice Quality And Physicochemical Properties Of Starch

Rice is one of the main food crops in China,and starch is the main carbohydrate and storage material in rice,which determines the quality of rice.At present,climate warming has brought many serious impacts on rice.The Intergovernmental Panel on Climate Change（IPCC）predicts that climate warming will continue to rise in the future,and its frequency and intensity will continue to increase.Elevated temperature during grain filling stage is one of the important environmental factors limiting the formation of rice quality,and nitrogen application is an important means to maintain the normal growth of rice and resist environmental stress.In addition,the morphology,crystal and molecular structure of rice starch affect the physicochemical properties of starch,which in turn determine the quality of rice.Therefore,it is of great significance to study the effect of nitrogen application on rice starch under climate warming.In this study,Wuyunjing 24 was used as material to simulate the climate warming scenario by the free-air temperature enhancement（FATE）facility at Danyang experimental station of Nanjing Agricultural University in 2019 and 2020.Four treatments were set up under the actual field conditions:ambient temperature as the control,（CK）;elevated temperature,（ET）;ambient temperature with nitrogen application,（CKN）;and elevated temperature with nitrogen application,（ETN）.The effects of nitrogen fertilizer application on grain filling,quality,physicochemical properties and starch chain length distribution of rice under open field warming were analyzed.The main results are as follows:1.Effects of warming and nitrogen application on photosynthesis of rice leaves and material accumulation.Under warming condition,net photosynthetic rate decreased,stomatal conductance and transpiration rate increased at 20 days after flowering,indicating that the increase of temperature led to the accumulation of reactive oxygen species,which accelerated the degradation of chlorophy II and leaf senescence.Compared with ambient temperature,the SPAD value of leaves reduced under increased temperature conditions,but nitrogen fertilizer application under warming significantly increased the SPAD value.With the progress of grain filling,the dry matter weight ratio of stem and leaf significantly decreased,and the dry matter weight ratio of ear significantly increased.2.Temperature and nitrogen fertilizer significantly affected rice grain filling characteristics and yield.The grain weight of rice showed a’S’curve,and the rapid growth time of superior spikelet weight was about 5-10 days earlier than that of inferior spikelet.Warming could accelerate grain pre-filling,increase the initial growth potential of superior and inferior spikelet,promote grain development,and greatly shorten the time for grain to reach the maximum filling rate.At the same temperature level,the application of nitrogen fertilizer delayed the time of superior and inferior spikelet reaching the maximum filling rate,indicating that increasing nitrogen fertilizer can delay grain development and prolong filling time.Warming significantly reduced rice yield,but nitrogen application could increase rice yield under increased temperature conditions,which could effectively make up for the loss of rice yield under warming.3.Application of nitrogen fertilizer under elevated temperature could improve rice quality.Compared with ambient temperature,the brown rice rate,milled rice rate,head rice rate,length and width of rice decreased;grain thickness and chalkiness increased,appearance quality deteriorated under elevated temperature.However,increasing nitrogen application under warming could effectively improve the milling quality and chalkiness traits of rice.The content of albumin,globulin,prolamin,total starch and amylose in rice decreased and the content of glutelin and total protein increased under elevated temperature.Warming and nitrogen fertilizer significantly increased the content of total amino acids,indicating that temperature increased the eating quality and nutritional quality of rice.4.Elevated temperature and application of nitrogen fertilizer had a significant effect on the physical and chemical properties of rice starch.Ten days after anthesis,the structure of starch grains arranged closely under ET treatment than CK,indicating that early warming accelerated grain filling.With the progress of grain filling,the arrangement of starch grains changed from tight to loose and large voids under ET and ETN treatments,resulting in increased chalky grains and poor appearance quality.Under the same nitrogen level,with the increase of temperature,the peak viscosity,hot paste viscosity,breakdown and gelatinization temperature showed an upward trend,while the setback and consistence decreased.Heating and nitrogen application did not change the crystalline type of rice starch,but ET increased the relative crystallinity,reduced the swelling potential and solubility of starch and affected the physicochemical properties of starch.5.Effect of warming and nitrogen fertilizer on starch properties and quality by changing starch chain length distribution.The polymerization degree(X_AM1,X_AM2)and the ratio of two peak heights(H_AM2/AM1)of amylose peaks 1 and 2 significantly reduced with the increase of temperature.Nitrogen application at elevated temperature could effectively improve the peak height of H_AM2.The difference of amylopectin chain length distribution（CLDs）was mainly located in the middle-long chain region.The AP5 peak of ET was the highest,followed by ETN,indicating that the application of nitrogen fertilizer under warming could reduce the peak height of middle-long chain.Both warming and nitrogen application increased the proportion of amylopectin long chain and decreased the proportion of short chain.Starch CLDs and structural parameters had no significant correlation with RVA parameters.Solubility was significantly positively correlated with X_AM1,H_AM2,H_AM2/AM1,amylose content（AC）and A chain.There was no significant correlation between starch CLDs and rice appearance quality,but it was significantly correlated with milling quality.The more the percentage of medium and long branched chains of amylopectin was,the worse the milling quality of rice was.The chain length distribution was significantly correlated with AC,indicating that the CLDs affected the eating quality of rice.Combined with the above results,the free-air temperature enhancement（FATE）system was used to simulate climate warming.It was found that a slight increase in temperature during grain filling stage affected the development of starch granules,resulting in poor rice quality and reduced yield.Increasing nitrogen fertilizer could improve rice quality.The increase of temperature changed the crystal structure and chain length distribution of starch,and positively correlated with AC,indicating that the physicochemical properties and chain length distribution of starch mainly contributed to the eating quality of rice.This provides a new understanding of rice quality formation mechanism under climate warming and how to take breeding and agronomic measures to maintain starch quality.