The Mechanism Underlying Nitrogen Affecting Heat Tolerance In Rice Plants Associated With The Energy Homeostasis

Rice is China's largest grain crop,and its output accounts for about 40%of the country's total grain output.For a long time,China's rice production has faced huge problems:with the change of global climate,high temperature heat damage has become an important factor limiting the potential of rice production.It is of great significance to study the physiological mechanism of rice growth in response to high temperature stress and put forward cultural regulation measures to cope with high temperature.This study analyzed the effects of high temperature on the growth of rice at different growth stages,the effect of nitrogen application on the heat tolerance of rice under high temperature conditions,and explored its mechanism from the view of carbohydrate metabolism and energy balance.The main results are as follows:1.The rice seedlings with 45 days old were subjected to 35?,40? and 45? constant temperatures for 48 hours while constant temperature of 30? was set as the control.The results showed that constant temperature of 30 ? had little effect on rice growth,while constant temperatures of 40 ? and 45 ? can be regarded as high temperature stress treatment.Under high temperature,the leaf growth of rice plants was significantly affected,and the accumulation of carbohydrates including sucrose,fructose,glucose,and starch in the leaves was significantly reduced.Activities of nitrogen metabolic enzymes including nitrate reductase,glutamic acid synthase,and glutamine synthetase,antioxidant system enzymes including catalase,peroxidase,and superoxide dismutase significantly decreased.The content of hydrogen peroxide and malondialdehyde significantly increased,indicating that the leaves were significantly damaged by peroxidation under high temperature.The abscisic acid content in the leaves was increased,while the contents of auxin,zeatin and gibberellin were significantly decreased.Changes of the above-mentioned indicators caused by high temperature compared with the control under medium nitrogen(MN)and high nitrogen(HN)turned to be smaller than zero nitrogen(0N)and low nitrogen(LN).It indicates that rice plant growth(carbohydrates accumulation,N metabolism related enzyme activities and hormones that stimulates growth)and high temperature damage to leaves(antioxidant enzyme activities,hydrogen peroxide and malonaldehyde contents)were alleviated with the increase of nitrogen levels.The results show that adding a certain amount of nitrogen at the seedling stage can alleviate the oxidative stress injury caused by high temperature.The physiological metabolisms and growth status of rice plants were inclined to maintain balanced while N was additionally supplied.2.Rice plants were subjected to high temperature treatments during the tillering growth period(45 days after transplanting)for 10 days.The results showed that the high temperature of 40/40?(day/night)significantly reduced the tiller numbers,plant height,leaf area,dry weight of stems and leaves,accumulation of carbohydrates,and phosphorus content In addition,leaf thickness,leaf angle,and nitrogen content increased significantly at high temperatures.With the decrease of the night temperature(40/35? and 40/30?)or the increase of nitrogen levels(MN and HN)under high temperature treatment,the reduction of the above-mentioned rice growth-related parameters and physiological indicators caused by high temperature during the daytime get smaller.It shows that the decrease of night temperature(increased temperature difference between day and night)and the increase of nitrogen application can alleviate the damage to rice plant under high temperature during tillering.3.The results of different high temperature during the tillering growth period showed that,the reduction of night temperature(40/35? and 40/30?)under high temperature treatment significantly reduced the temperature of plant organs and increased leaf chlorophyll content while compared with the high temperature of 40/40?(day/night).Activities of key enzyme involved in nitrogen metabolism and antioxidant system,leaf photosynthetic electron transfer,NAD+,NADH and Adenosine tri-phosphate(ATP)contents were significant increased.And the changes in the above-mentioned physiological parameter values caused by high temperature were significantly reduced while the nitrogen levels(MN and HN)increased compared with lower nitrogen levels(ON and LN).It shows that the decrease of night temperature(increased temperature difference between day and night)and the increase of nitrogen application under high temperature stress can increase energy and carbohydrates synthesis to maintain growth during the tillering stage under high temperature stress by regulating energy.4.High temperature treatment(38/30?)was performed during the meiotic phase of rice reproductive growth for 7 days.The results showed that:Compared with the control(30/25?),the high temperature treatment during meiosis significantly reduced the total number of spikelets per panicle,seed setting rate and grain weight,and thus significantly reduced the final yield.The fertility of pollen grains was significantly reduced under high temperature treatment.Carbohydrate accumulation,antioxidant system enzyme activities,zeatin,auxin,and gibberellin contents in spikelets were significantly reduced,while the contents of malondialdehyde,hydrogen peroxide,and ABA were significantly increased.Besides,NAD+,NADH and ATP levels were significantly reduced under high temperature treatment.And the change of the above indexes caused by high temperature was significantly smaller at the MN level than HN,LN and 0N.It shows that too high or too low nitrogen levels during meiosis are not conducive to fertilization and yield formation.Maintaining a certain level of nitrogen can effectively alleviate energy imbalance and carbohydrate metabolism obstruction caused by high temperature.