Studies On The Photosynthetic Physiological Mechanisms Of Elevated Atmospheric CO₂ Concentration And Temperature Affecting Yield And Quality In Rice

Atmospheric elevated CO₂ concentration and temperature are two principal features of global climate change that impact agro-ecosystems.Rice is one of the important staple crops.However,few studies have investigated the effect of elevated CO₂ concentration and temperature on photosynthesis,growth,yield,and grain nutritional quality,especially in cold-temperate regions.This study provides a data base and mechanistic evidence for the prediction of the impacts of global climate change on rice yield and quality,which would be important for the future management and maintenance of rice productivity.Thus,in this study,pot experiments were conducted in open-top chambers（OTCs）from 2021 to 2022.Rice cultivars included Shishoubaimao（SH）,Hejiang 19（HJ）,and Longjing 31（LJ）in this study.The treatments comprised ambient CO₂ concentration and temperature（CK）,elevated CO₂ concentration（700 ppm,e C）,warming（ambient+2°C,e T）,and elevated CO₂ concentration plus warming（e Ce T）.The objective of this study is to examine the effect of elevated CO₂ and warming on rice morphological traits,photosynthetic characteristics,yield,and nutritional quality in grain.In addition,Illumina sequencing technology was applied to analyze the underlying mechanisms.The main results and conclusions are as follows:1.Compared with the control,elevated CO₂,warming and elevated CO₂ plus warming decreased rice plant height by 4.5%,6.7%,and 5.5%,but increased the number of effective tillers by 18.9%,13.6%,and22.6%,respectively.Elevated temperature significantly increased the area of the flag leaf by 14.7%,while elevated CO₂ alone and elevated CO₂ concentration plus warming did not significantly affect the area of the flag leaf.2.Photosynthetic characteristics data showed that elevated CO₂ concentration significantly increased the net photosynthetic rate,intercellular CO₂ concentration,and water use efficiency of rice leaves,while elevated temperature significantly decreased the net photosynthetic rate of HJ and LJ at the anthesis stage,but no significant effect at the tillering and filling stages.The CO₂ and temperature co-elevation increased net photosynthetic rate and water use efficiency by 11%-67%and 6.2%-144.4%,respectively.Elevated CO₂ and elevated CO₂ plus warming altered chlorophyll fluorescence parameters and increased J_max:V_c,maxand A_n,max,which respond to CO₂ concentration and light intensity.3.Transcriptome sequencing analysis showed that,compared with the control,elevated atmospheric CO₂ concentration resulted in up-regulated expression of 1079 genes and down-regulated expression of 943genes,elevated temperature led to the up-regulated expression of 1623 genes and down-regulated expression of 610 genes,and elevated CO₂ concentration plus warming caused up-regulated expression of4715 genes and down-regulated expression of 1892 genes.Elevated atmospheric CO₂ concentration significantly stimulated the expression of genes involved in the metabolic pathways of photosynthesis and carbon fixation in photosynthetic organisms.Although the effect of warming on these pathways was not significant,elevated CO₂ concentration plus warming significantly promoted the up-regulation of genes in both pathways.These results indicated that elevated atmospheric CO₂ concentration and temperature have a beneficial effect on photosynthesis in rice.4.Elevated atmospheric CO₂ concentration,warming,and elevated CO₂ concentration plus warming improved aboveground dry matter accumulation in rice,resulting in significant increases of 27.5%,11.5%,and 25.1%in aboveground biomass and 32.8%,17.0%and 19.5%in grain yield,respectively（P<0.05）.Climate change had no significant effect on 1000-grain weight,while it significantly increased the number of grains per rice plant,which contributed more to the grain yield increase.However,elevated CO₂ plus warming significantly reduced filled grain percentage,the reasons for which still need to be further investigated.5.The mineral elements（K,Ca,Mg,Fe,Mn,Cu and Zn）were analyzed in the grains of different rice cultivars.Elevated CO₂,warming and elevated CO₂ plus warming significantly increased Ca concentration in the grain by 29.6%,44.4%,and 65.1%,and significantly decreased Cu concentration in the grain by29.6%,26.1%and 38.0%,while the effects on the concentrations of other mineral elements were not consistent,with three patterns of increase,decrease and no change.However,the concentration of the total mineral element in the grains decreased by 0.7%-4.8%.Similarly,grain protein concentration decreased by7.0%-7.2%under elevated CO₂,warming,and elevated CO₂ plus warming.6.The analysis of non-structural carbohydrates in rice grains under climate change showed that elevated CO₂,warming,and elevated CO₂ plus warming increased the concentration of soluble sugar in the grains by 17.7%,10.6%,and 25.4%,respectively,compared to the control.The concentration of sucrose,which accounts for a greater proportion of soluble sugar in rice grain,increased by 18.2%,11.0%,and26.1%,respectively.Elevated CO₂,warming and elevated CO₂ plus warming have not an effect on the concentrations of starch and non-structural carbohydrate in HJ but increased the concentration of starch by5.4%,7.0%,and 5.0%,and the concentration of non-structural carbohydrates by 5.5%,7.1%and 5.4%in SH and LJ,respectively.Climate change altered the stoichiometric component of rice.Elevated CO₂ and temperature increased the ratio of non-structural carbohydrates to mineral elements,and CO₂ and temperature co-elevation increased total non-structural carbohydrates（TNC）:Cu by 36%-103%.However,TNC:Ca decreased by 31%-47%,and TNC:protein increased by 11%-12%.In summary,elevated CO₂ and warming slightly decreased the height of rice plants,but increased the number of effective tillers and the area of flag leaves,improved photosynthetic efficiency,promoted the expression of photosynthesis-related genes,increased net photosynthetic rate and water use efficiency,improved photosynthesis and stimulated dry matter accumulation,consequently increased yield.However,the climate change increased the accumulation of non-structural carbohydrates,but decreased the concentration of mineral elements and protein,which in turn changed the chemical composition of rice grains and posed a threat of deterioration in the nutritional quality of rice.