Physiological Response Mechanism To Soil Water Deficit And Re-watering Of Plants Under Elevated CO₂ Concentration

Global atmospheric carbon dioxide concentration（[CO₂]）have dramatically increased due to fossil fuel combustion and quickening of the industrial processes in recent decades.This increase in[CO₂]might be accompanied by shifting precipitation patterns and increasing extreme precipitation events.This projected global[CO₂]elevating is expected to have effects on terrestrial ecosystem structure and function.Understanding how elevated[CO₂]and soil water deficit-rewatering affects plant leaf structure and function is of great significance to responding the climate change and ensuring the stability of terrestrial ecosystem.However,there is a lack of comprehensive study on underlying mechanisms of elevated[CO₂]and soil water deficit on the plant physiological processes are far from understood.One of this study objectives was clarified the controversial conclusion that whether elevated[CO₂]can alleviate the adverse effects of water deficit on plants by Meta-analysis.Then,the pot-experiments of progressive soil water deficit and re-watering were conducted in environmental growth chambers on oilseed rape,white clover and alfalfa plants with elevated[CO₂].The objective of this study was to explore the underlying mechanisms of the elevated[CO₂]regulate the stomatal traits of plants to alleviate the impact of water deficit on physiological properties.Meanwhile,this study also explored the impact of the interaction between elevated[CO₂]and water deficit on leaf structure,physiology and biochemistry of oilseed rape plants.Finally,this study clarified the contribution of stomatal limitation（l_s）,mesophyll limitation（l_m）and biochemical limitation（l_b）to photosynthesis.The main conclusions were as follows:（1）The effects of elevated[CO₂]on water deficit on plant gas exchange were quantified.Elevated[CO₂]were demonstrated to stimulate the growth of plants,counteracting the negative impacts of water deficit on plant leaf net photosynthesis（A_n）.Elevated[CO₂]significantly increased A_n by 11.9%and 16.4%under well-watered and water deficit conditions.The interaction effect between water deficit and elevated[CO₂]resulted in an 8.3%decrease in A_n.Meanwhile,water deficit and elevated[CO₂]increased leaf water uses efficiency(WUE_leaf)by24.5%and 29.3%,respectively.Increases in WUE_leaf under the water deficit condition were contributed to the transpiration rate（T_r）positive effect,whereas increases in WUE_leaf under the elevated[CO₂]condition may be due to the common effect of A_n and T_r.Very strong CO₂fertilization effects on A_n were observed when the[CO₂]to be>800μmol mol^-1 under water deficit condition.（2）The responses of gas exchange and stomatal characters of three green fertilizer plants to elevated[CO₂]and progressive soil water deficit and re-watering were explored.Water deficit decreased the A_n,stomatal conductance（g_s）and T_r of oilseed rape and white clover plants,but did not significantly affect the gas exchange of alfalfa.Under elevated[CO₂],A_nof the three plants did not change with soil moisture decreasing and re-watering.There was a highly asymmetrical response of stomatal aperture on the adaxial and abaxial surfaces to mild water deficit.Elevated[CO₂]made the spatial distribution pattern of stomata on the adaxial leaf surface more regular in oilseed rape subjected to mild water deficit,suggesting that e[CO₂]improves the regularity of stomatal distribution to balance[CO₂]and water availability.（3）The underlying mechanism of the effect of mesophyll limitation on gas exchange and water use efficiency in oilseed rape plants was revealed under elevated[CO₂]and water deficit.Mesophyll conductance（g_m）decreased 68%when soil moisture decreases to<35%field capacity compare the well-watered.In addition,elevated[CO₂]significantly decreased 61%in g_m.The decrease in leaf water potential and leaf water content may be leads to decrease in g_munder water deficit condition.Elevated[CO₂]decreased the g_m mainly due to increase the area of palisade cells and spongy cells,because larger cells increase the resistance of CO₂ diffusion between cells.In addition,a quadratic function was established to describe the relationship of the intrinsic water use efficiency with g_m/g_s.The optimal value of g_m/g_s for intrinsic water use efficiency was 3.1 under ambient[CO₂],and 2.1 under elevated[CO₂]condition.（4）The response of biochemical factors to elevated[CO₂]and water deficit was explored,and the contribution of stomatal limitation（l_s）,mesophyll limitation（l_m）and biochemical limitation（l_b）to photosynthesis was quantified.Short-term soil water deficit did not affect the maximum carboxylation rate(V_cmax)or maximum electron transport rate(J_max)of oilseed rape,suggesting that water deficit did not affect the biochemical mechanisms of oilseed rape,and the decrease in A_n resulted from stomatal rather than non-stomatal limitations.Elevated[CO₂]alleviates the inhibition of water deficit on plant photosynthesis mainly by promoting the regeneration of Ru BP,but not related to changes in chlorophyll content.Under ambient[CO₂],the l_b（63.4%）have the greatest contribution to photosynthesis,while l_s（11.7%）have the smallest impact on photosynthesis under full irrigation treatment.The l_swas gradually increase with soil water decreasing.When soil water decreased to<35%field capacity,the contribution of l_s to photosynthesis increased to 30.5%.Elevated[CO₂]increased the contribution of l_s to photosynthesis under full irrigation treatment.The contribution of l_b to photosynthesis limit gradually decreased,while l_m gradually increased with oil water decreasing under elevated[CO₂].The results of this study indicated that elevated[CO₂]alleviates the inhibition of water deficit on plant gas exchange by changing the spatial distribution of stomata on the adaxial surface of leaves.In addition,this study also found that the impact of water deficit on plant photosynthesis is greater than the stimulating effect of elevated[CO₂]on plants,and thus many current climates change models based on earlier results of“doubling–CO₂”experiments may overestimate the CO₂ fertilization effect on terrestrial ecosystem.These results may also be helpful for improving current process-based ecological models to more accurately predict the structure and function of terrestrial ecosystems under future rising atmospheric[CO₂]and climate change scenarios.