| The increase of temperature and CO2 concentration are two major trends in climate change.Temperature and CO2 concentration are important environmental factors affecting crop growth.The radiation use efficiency of crop directly determines crop dry matter production and yield.With the increase of temperature and CO2 concentration at the same time,the change of crop radiation use efficiency is one of the key scientific issues that need to be addressed to accurately assess the impact of climate change on crop yield.Rice is one of the main staple food crops in China.To investigate the change of radiation use efficiency of rice canopy under the increase of temperature and CO2 concentration conditions,four years field experiments were conducted using the large-scale Temperature Increase by Free Air CO2 Enrichment(T-FACE)platform located at Kangbo Village,Baimao Town,Changshu City,Jiangsu Province(31°30’N,120°33’E).The rice varieties tested were Oryza sativa L.cv Changyou 5(in 2013,2014,2017,2020)and Oryza sativa L.cv Yangdao 6(in 2020).In these experiments,four treatments were designed:control(CT),atmospheric CO2 concentration increased to 500 μmol mol-1 or 590 μmol mol-1(C+T);canopy temperature increased by 2.0℃(CT+);atmospheric CO2 concentration increased to 500 μmol mol-1 or 590 μmol mol-1 combined with canopy temperature increased by 2.0℃(C+T+).There were 3 replicates per treatment and 12 plots in total.Based on the observed data of photosynthesis rate of different leaf rank,leaf area index,specific leaf area,dry matter partitioning index of shoot to leaf,canopy light interception and light extinction coefficient,the effects of temperature and CO2 concentration on rice canopy radiation use efficiency were analyzed and their causes were identified.The main results obtained in this study are as follows.1.Elevated CO2 concentration increased the photosynthesis rate of different leaf rank,the fraction of light intercepted and cumulative intercepted light by rice canopy.Thus,the canopy radiation use efficiency and dry matter accumulation in aboveground parts were increased.Under the increase of CO2 concentration,the initial light-use efficiency(ε)and light-saturated gross photosynthetic rate(Ag,max)of first and third(only in 2017)leaves increased at heading and grain-filling.For Changyou 5,under the increased CO2 concentration,the ε of first and third leaves increased by,respectively,8.2%-28.6%and 20.8%at heading,and 6.2%～23.3%and 4.3%at grain-filling;the Ag,max of first and third leaves increased by,respectively,17.9%～30.5%and 12.3%at heading,and 7.9%～27.1%and 11.0%at grain-filling.For Yangdao 6,the increased CO2 concentration enhanced the ε of first leaf by 30.0%and 35.2%,respectively,and enhanced the Ag,max of first leaf by 7.8%and 17.4%,respectively,at heading and grain-filling.Under the increase of CO2 concentration,the leaf area index increased at heading and grain-filling,Changyou 5 increased by 3.8%～16.7%and 3.4%～12.8%,respectively;Yangdao 6 increased by 4.7%and 10.6%,respectively.Under the increase of CO2 concentration,the canopy light extinction coefficient decreased at heading and grain-filling,Changyou 5 decreased by 0.6%-9.1%and 3.7%-6.7%respectively;Yangdao 6 decreased by 8.5%and 11.0%,respectively.Under the increase of CO2 concentration,the leaf area index increased and the canopy light extinction coefficient decreased which increased the fraction of light intercepted(Changyou 5 increased by 0.5%～1.1%,Yangdao 6 increased by 0.8%)and the cumulative intercepted light by rice canopy(Changyou 5 increased by 0.1%～0.5%,Yangdao increased by 2.1%).Thereby,the canopy radiation use efficiency(Changyou 5 increased 1.8%～20.4%,Yangdao 6 increased 15.1%)and dry matter accumulation in aboveground parts(Changyou 5 increased 2.3%～20.9%,Yangdao 6 increased 12.8%)increased from jointing to grain-filling.2.Elevated temperature decreased the photosynthesis rate of different leaf rank,the fraction of light intercepted and cumulative intercepted light by rice canopy.Thus,the canopy radiation use efficiency and dry matter accumulation in aboveground parts were decreased.Under the increase of temperature,the ε and Ag,max of first and third leaves decreased at heading and grain-filling.For Changyou 5,under the increase of temperature,the ε of first and third leaves decreased by,respectively,1.8%～16.0%and 5.3%at heading,and 5.3%～10.0%and 40.1%at grain-filling;the Ag,max of first and third leaves decreased by,respectively,3.9%～15.1%and 5.6%at heading,and 6.2%～14.8%and 32.8%at grain-filling.For Yangdao 6,the increased temperature reduced the ε of first leaf by 13.6%and 13.7%,respectively,and reduced the Ag,max of first leaf by 28.6%and 9.1%,respectively,at heading and grain-filling.Under the increase of temperature,the leaf area index decreased at heading and grain-filling,Changyou 5 decreased by 2.0%～10.3%and 6.8%～8.8%(except for the increased by 4.3%in 2014),respectively;Yangdao 6 decreased by 9.5%and 12.0%,respectively.under the increase of temperature,the canopy light extinction coefficient increased at heading and grain-filling,Changyou 5 increased by 1.2%～11.9%and 2.8%～7.2%;Yangdao 6 increased by 8.8%and 10.3%,respectively.Under the increase of temperature,the leaf area index decreased and the canopy light extinction coefficient increased which significantly decreased the fraction of light intercepted(Changyou 5 decreased by 1.5%～4.5%,Yangdao 6 decreased by 1.1%)and the cumulative intercepted light by rice canopy(Changyou 5 decreased by 4.3%～7.5%,and Yangdao 6 decreased by 3.6%).Thereby,the canopy radiation use efficiency(Changyou 5 decreased by 11.1%～25.7%,Yangdao 6 decreased by 19.1%)and the dry matter accumulation in aboveground parts(Changyou 5 decreased by 17.1%～29.1%,Yangdao 6 decreased by 25.4%)decreased from jointing to grain-filling.3.The combination of increased CO2 concentration and temperature increased the photosynthesis rate of different leaf rank,and decreased the fraction of light intercepted and cumulative intercepted light by rice canopy.The increased leaf photosynthetic rate could not cancel out the decreased the fraction of light intercepted and the cumulative intercepted light by rice canopy which eventually led to the decrease of canopy radiation use efficiency and dry matter accumulation in aboveground parts of rice.Under the increase of CO2 concentration and temperature conditions,the ε and Ag,max of first and third leaves increased at heading and grain-filling.For Changyou 5,under the increased CO2 concentration and temperature conditions,the ε of first and third leaves increased by,respectively,3.2%～21.6%and 18.0%at heading,and 4.4%～22.0%and 13.5%at grain-filling;the Ag,max of first and third leaves increased by,respectively,8.4%～26.3%and 9.9%at heading,and 5.3%～22.4%(except for the decrease by 13.2%of Ag,max in 2017)and 8.2%at grain-filling.For Yangdao 6,the increased CO2 concentration and temperature conditions enhanced the ε of first leaf by 29.3%and 10.5%,respectively,and enhanced the Ag,max of first leaf by 39.2%and 15.2%,respectively,at heading and grain-filling.Under the increase of CO2 concentration and temperature conditions,the leaf area index of Changyou 5 increased by 0.5%～10.4%(except for the decrease by 2.8%in 2013)at heading,but decreased by 0.8%～4.1%(except for the increase by 9.2%in 2014)at grain-filling;The leaf area index of Yangdao 6 decreased by 3.4%and 5.3%at heading and grain-filling,respectively.Under the increase of CO2 concentration and temperature conditions,the canopy light extinction coefficient increased at heading and grain-filling,Changyou 5 increased by 0.3%～2.1%(except for the decrease by 1.3%in 2020)and 1.9%～5.9%at grain-filling;Yangdao 6 increased by 5.0%and 4.7%,respectively.Under the increase of CO2 concentration and temperature conditions,the leaf area index decreased at grain-filling and the canopy light extinction coefficient increased which decreased the fraction of light intercepted(Changyou 5 decreased by 0.1%～3.4%,Yangdao 6 decreased by 0.6%)and the cumulative intercepted light by rice canopy(Changyou 5 decreased by 1.6%～6.3%,Yangdao 6 decreased by 3.1%).Thereby,the canopy radiation use efficiency of rice(Changyou 5 decreased by 0.3%～20.2%,Yangdao 6 increased by 2.1%)and the dry matter accumulation in aboveground parts(Changyou 5 decreased by 6.3%～23.0%,Yangdao 6 decreased by 5.1%)decreased from jointing to grain-filling.The results of this study show that the promotion effect of elevated CO2 concentration on the canopy radiation use efficiency could not offset the negative impact of elevated temperature on the canopy radiation use efficiency.The increase of CO2 concentration and temperature conditions tended increasing rice leaf photosynthetic rate,but decreased canopy light interception through reducing leaf area index and increasing canopy light extinction coefficient.The increase of CO2 concentration and temperature conditions decreased leaf nitrogen accumulation after heading which accelerated the senescence of rice leaves and shortened the growth period of rice,resulted in leaf area index reduction and canopy light extinction coefficient increase.Under increase of CO2 concentration and temperature conditions,the increased leaf photosynthetic rate could not cancel out the decreased the fraction of light intercepted and the cumulative intercepted light by rice canopy which eventually led to the decrease of canopy radiation use efficiency of rice.The results of this study indicate that applying nitrogen fertilizer at heading stage so as to increase leaf nitrogen content and avoiding leaf senescence can be an effective measure to mitigate climate change on rice photosynthetic productivity. |
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