Study On The Response And Adaptability Evaluation Of Three Green Manure Plants To CO₂ Concentration And Water

As typical green manure plants,oilseed rape（Brassica napus）,white clover（Trifolium repens）and alfalfa（Medicago sativa）are widely cultivated in orchard grass production and returning farmland to forest（grass）projects in arid and semi-arid regions.The dominant factor in the growth and development of green manure plants in arid and semi-arid regions is water,and soil water often presents a variable and low-water state.According to the prediction of relevant models,the global CO₂ concentration has shown a rising trend in recent years,which is bound to affect the growth of plants in arid and semi-arid regions.Therefore,the study was conducted to explore the effects of elevated CO₂ concentration and water change on the physiological characteristics of the three green manure plants,to provide theoretical support for more accurate simulation of the changes in grassland ecological processes under climate change.An experiment was conducted in an artificial climate chamber that can precisely control the CO₂ concentration with two CO₂ treatments,400（natural atmospheric）and 800μmol·mol^-1（doubled）,and 4 groups of soil moisture gradient,80%field water holding capacity（FC）（full irrigation control group）,55%～60%FC（mild water deficit）,35%～40%FC（moderate water deficit）,<35%FC（severe water deficit）.In addition,different degrees of rewatering treatment after drought were set,that is,when the soil moisture is lower than 35%FC,replenishing water to plants:rewatered to 80%FC（DW-1）,rewatered to 60%FC（DW-2）,rewatered to 40%FC（DW-3）.We studied the effects of water deficit on photosynthetic performance and water use efficiency,and the effects of rewatering after drought on physiological characteristics of three green manure plants under different CO₂ concentrations.In addition,we comprehensively evaluated the adaptability of green manure plants to the variable and low water environment by combining principal component analysis and AMMI model.The main conclusions are as follows:（1）The effects of water stress on photosynthetic performance and water use efficiency（WUE）of three plants under doubled CO₂ concentration were analyzed.Under the same CO₂concentration,compared with the full irrigation control group,when soil moisture was less than 40%FC,the chlorophyll content and gas exchange parameters of three plants were decreased significantly（P<0.05）.The doubled CO₂ concentration increased the net photosynthetic rate（P_n）of white clover and alfalfa under different water deficit conditions.However,it significantly reduced the P_n of oilseed rape under full irrigation conditions by21.5%（P<0.05）,and there was no significant effect on the P_n of oilseed rape under<35%FC treatment.In addition,the doubled CO₂ concentration also significantly increased leaf WUE of white clover and alfalfa under water deficit conditions（P<0.01）,and significantly increased the individual WUE of the three plants under moderate water deficit by 25%～33%（P<0.05）,while increasing the stable carbon isotope resolution(?¹³C)of the three plants under each water treatment by 6.7%-14.2%.Further analysis showed that leaf WUE of the three plants had weak positive correlation with?¹³C,while individual WUE had strong correlation with?¹³C.（2）The compensation effect of the physiological parameters of three plants at rewatering after drought under different CO₂ concentrations was discussed.There were great differences in the recovery ability of the three plants in different degrees of rewatering after drought.Under the atmospheric CO₂ concentration,rewatering to DW-1 and DW-2,the gas exchange parameters,chlorophyll fluorescence parameters,leaf water relationship and growth parameters of the three plants all showed a certain compensation effect.In addition,the actual photo chemical efficiency（Y（II））of white clover under DW-1 treatment,and the stomatal conductance（gs）of oilseed rape under DW-2 treatment all showed supercompensation effect.But rewatering to DW-3 treatment,only the initial fluorescence（F_o）of white clover,the transpiration rate（T_r）and photochemical quenching coefficient（q P）of alfalfa returned to control levels.Through the analysis of the effect of doubled CO₂concentration on the compensation effect of rewatering,it was found that the doubled CO₂concentration increased the gas exchange parameters such as T_r of oilseed rape and white clover in DW-1 and DW-2 treatments,and increased the P_n of alfalfa in DW-3 treatment.Moreover,it was conducive to the recovery of some chlorophyll fluorescence parameters of the three plants under DW-2 and DW-3 treatments,but only improved the biomass recovery ability of white clover under DW-1 treatment.（3）The stability and growth adaptability of the three plants in the variable and low water environment under different CO₂ concentrations were clarified.The biplot of the AMMI model indicated that the growth of oilseed rape and alfalfa had better adaptability to the drought environment compared with the white clover,and the doubled CO₂ concentration enhanced the adaptability of the white clover under the DW-2 treatment.Through quantitative calculation,under the same CO₂ concentration,alfalfa had the best stability and the strongest growth adaptability in the variable and low water environment.In addition,compared with atmospheric CO₂ concentration,the doubled CO₂ concentration increased the average growth value of the three plants by 10%～20%,but decreased the stability of white clover and alfalfa in the variable and low water environment.The grey correlation analysis showed that the biomass,leaf relative water content and leaf water content of the three plants can better reflect the growth status of plants under variable and low water environment.In summary,under the same CO₂ concentration,when the soil moisture content was less than 40%FC,the photosynthetic performance of plants is inhibited,and when rewatered to60%FC and above,the growth of the three plants could be largely recovered.Elevated CO₂concentration can improve the adverse effects of mild water deficit on the photosynthetic performance and WUE of the three plants,and improve the recovery ability of some gas exchange parameters and chlorophyll fluorescence parameters of the three plants at rewatering after drought,but it is necessary to pay attention to the unstable factors caused by elevated CO₂ concentration on plant productivity.The alfalfa has better adaptability under variable and low water environment,and it can be given priority in the selection of green manure plants in arid and semi-arid regions.