Effects Of Free Air CO₂ Enrichment And Temperature Increase On C/N Ratio And Source-sink Relationships Of Rice

Climate change is mainly characterized by atmosheric CO2 concentration elevation and temperature increase,which have a direct impact on crop yields.The C/N ratio and sourcesink relationship of plants affect crop yield by affecting leaf photosynthesis,senescence rate,and dry matter distribution.How does the C/N ratio and source-sink relationship change under the conditions of elevated CO2 concentration and temperature?How will it affect yield?This is one of the scientific questions that need to be answered accurately to assess the impact of climate change on crop growth and development and to identify how to deal with climate change.Rice is one of the most important food crops in China,quantifying the effects of CO2 enrichment and temperature increase on C/N ratio and source-sink relationships of rice may provide a reference for rice to deal with climate change and cultivation management.In this study,the rice cultivar Changyou 5 was used as the experimental material.We designed four experimental treatments,that was,an untreated plot with ambient condition as control(CT),one with target atmospheric CO2 up to 500 ppm(C+T).one with warming of canopy temperature by 2℃ above ambient(CT+),and one with combined CO2 enrichment and warming(C+T+)to analyse the responses of C/N ratio and source-sink relationships of rice to elevated CO2 and increased temperature in T-FACE system during 2014 and 2017.The main research results were as follows:(1)Elevated CO2 and increased temperature treatment increased C/N ratio of shoots,stems,and leaves at all growth stages,but decreased the C/N ratio of panicles after heading stage during two growing seasons.Elevated CO2 treatment significantly increased dry weight of rice plants at all growth stages in 2014 and 2017,and the dry matter accumulation rate was greater than the nitrogen accumulation,resulting in an increase in the C/N ratio of vegetative organs.Under the increased temperature treatment,the nitrogen accumulation decreased more than the biomass accumulation,resulting in an increase in the C/N ratio of vegetative organs,but the C/N ratio of panicles was reduced after heading of rice.The C/N ratio of panicles of rice plants may be more susceptible to temperature,and elevated CO2 may exacerbate the negative effect of temperature on the C/N ratio of panicles.(2)Elevated CO2 and increased temperature treatment decreased the carbon source and sink,nitrogen source and sink,and increased carbon and nitrogen sourcesink difference of rice plants during two seasons.Elevated CO2 treatment increased the carbon sink,nitrogen source,nitrogen sink,and carbon source-sink difference of rice plants during two seasons,this was related to the increased CO2 concentration promoting the dry matter accumulation of rice plants.But increased temperature treatment significantly decreased the carbon source and sink,nitrogen source and sink,increased carbon and nitrogen source-sink difference of rice plants during two seasons,resulting reduced accumulation of carbon and nitrogen assimilation of rice.Since temperature had a greater impact on source and sink of rice carbon and nitrogen than CO2,especially the source-sink relationships of nitrogen were more sensitive to the response of temperature,so the simultaneous increase of CO2 concentration and temperature had an similar effect on the source-sink relationships of carbon and nitrogen of rice with warming,but CO2 showed a certain compensation effect.The nitrogen source-sink difference of rice was higher than the control under the increased temperature and the elevated CO2 and increased temperature treatment in the two seasons,indicating that the increased temperature may increase the demand for nitrogen accumulation before heading,which reflected that nitrogen uptake prior to heading was crucial for reducing rice yield losses due to climate change.(3)Elevated CO2 and increased temperature treatment increased C/N ratio of vegetative organs and decreased C/N ratio of panicles,which accelerated the senescence of the source and sink,thus reduced yield of rice.The temperature was the main factor for yield reduction of rice.Elevated CO2 concentration promoted the increase in yield by increasing the C/N ratio of vegetative organs and the source and sink of carbon and nitrogen.Increased temperature treatment significantly reduced the yield of rice in two seasons by shortening the growth period,reducing the source(increasing C/N ratio of stems and leaves)and lowering the sink(decreasing C/N ratio of panicles).Under low-temperature and lowlight weather conditions,elevated CO2 and increased temperature sharply increased the C/N ratio of leaves and reduced the C/N ratio of panicles,which exacerbated leaves senescence,blocked dry matter production and reduced storage capacity,resulting the degree of rice reduction in 2014 relative to the control was higher than in 2017.This study clarified the effect of elevated CO2 concentration and increase temperature on the C/N ratio、carbon and nitrogen sources、sinks and sorce-sink difference of rice.This study clarified the reason why elevated CO2 concentration and increased temperature decereased rice yield from the perspective of carbon-nitrogen metabolism balance and source-sink relationships.The reason was related to the increase on the C/N ratio of vegetative organs and the decrease on the CON ratio of panicles,which accelerated the senescence of the source and sink..The research results can provide a theoretical basis for rice production to deal with climate change.