Research On The Whole-plant Water And Carbon Process And Their Coupling Mechanism Of Beijing Platycladus Orientalis

Whole-plant instantaneous and short-term water and carbon processes and their coupling relationships can reveal the mechanism of whole-plant long-term pattern,and bridge the gap of comparison across different spatial scales.In this study,Platycladus orientalis saplings-typical and widely distributed tree species in Beijing mountainous areas,were selected and cultivated in growth chambers for a month,with orthogonal treatments?three CO₂ concentrations?C_a?×five soil volumetric water contents?SWC??.The transpiration,carbon sequestration and respiration rates in response to different treatments were obtained,in order to reveal the whole-plant instantaneous and short-term?time integral?water and carbon processes;the response patterns of water-carbon coupling,represented by water use efficiency?WUE?,was observed and its threshold was determined.In addition,leaf and whole-plant physiological and morphology properties were measured,and the relationship between leaf and whole-plant water-carbon coupling was investigated,allowing the WUE model scaled up from leaf to whole-plant for estimating WUE_i-P and WUE_s-P.The main results and conclusions are as follows:?1?The whole-plant daytime transpiration rate?E_P?was significantly influenced by SWC,but not by C_a.E_P increased with the increase of SWC,reaches maximum values at 70-80%field capacity?FC?,and then slightly decreased.Under different soil water conditions,there were no clear variation trend of E_P in response to elevated C_a.The whole-plant nighttime transpiration rate?E_d?was also significantly influenced by SWC,reaching maximum values at well-watered conditions?70%-80%of field capacity,FC?;while the E_d was not significantly influenced by changes in C_a.The whole-plant cumulative transpiration over a day-night cycle??E_P?was significantly influenced by SWC,and its variation trend with SWC was similar to that of E_P.However,the influence of elevated Ca to?E_P was not significant.?2?The whole-plant daytime net photosynthetic rate(P_n,P)was significantly influenced by Ca and SWC.At high C_a(600 and 800?mol·mol^-1,C₆₀₀and C₈₀₀),P_n,P increased with the increase of SWC until SWC exceeded 70%-80%FC.While at ambient C_a(400?mol·mol^-1,C₄₀₀),P_n,P was improved with the increase of SWC until it exceeded 60%-70%FC.Under different SWC,the response trend of P_n to C_awas different.Variations in Ca and SWC significantly influenced whole-plant nighttime respiration rate?R_P?.The increase of SWC improved R_P.Except at severe drought?35%-45%FC?,R_P decreased with the elevation of C_a.The cumulative carbon sequestration over a day-night cycle(?P_n,P)was also significantly influenced by changes in C_a and SWC.The response trend of?P_n,P to SWC was similar to that of P_n,P.The?P_n,P was generally improved as C_a rose.?3?The whole-plant instantaneous(WUE_i-P)and short-term water-use efficiency(WUE_s-P)were significantly influenced by C_a and SWC,both of which increased with the increase of C_aexcept at severe drought.At C₆₀₀ and C₈₀₀,WUE_i-P and WUE_s-P reached maximum at 50%-60%FC,but further increase of SWC decreased them.In contrast,at C₄₀₀,WUE_s-P reached maximum values at 60%-70%FC,while WUE_i-P reached maximum at 50%-60%FC.The leaf instantaneous WUE(WUE_i-L)was significantly linearly correlated with WUE_i-P and WUE_s-P?P<0.05?,and the correlation coefficients R² were 0.93 and0.73,respectively.Without the attendance of saplings having very small total leaf area(C₈₀₀×35%-45%of FC),the R² between WUE_i-L and WUE_i-P as well as between WUE_s-P increased to 0.99 and 0.96respectively,indicating that WUE_i-L was better predictor of WUE_i-P and WUE_s-P when the canopy structure were similar.Compared with WUE_i-L,WUE_i-P and WUE_s-P were influenced by the canopy structure,in addition to environmental changes.?4?In the simulation of stomatal conductance(g_sw)with coupled photosynthesis-stomatal model,the influence of soil water conditions should be considered.For mesophyll conductance?g_m?simulation,the potential mesophyll conductance(g_m,p)and SWC dependent model,failed to reflect the physiological and environmental?C_a?effects on g_m;in contrast,the modified coupled photosynthesis-mesophyll model provided good agreement between simulated and observed g_m.Considering the proportion of instantaneous respiration and"unproductive"water loss,the model for calculating WUE_i-L was scaling up to whole-plant level for modelling WUE_i-P,and accounting for SWC in the photosynthesis-stomatal coupling model outperformed than that with exclusion of SWC.Furthermore,the effects of g_m on estimating WUE_s-P should be taken into account;in this situation,introducing the coupled photosynthesis-stomatal and photosynthesis-mesophyll models into the whole-plant model for WUE_s,P effectively captured its response pattern to C_a and SWC conditions.?5?The P_n,P and E_P showed a significant relationship,and their ratio WUE_i-P reached maximum?threshold?of 28.50 mmol·mol^-1 at C₈₀₀×50%-60%FC.The significant correlation between?P_n,P and?E_Pwas also observed,leading to the WUE_s-P reached maximum?threshold?of 24.35 mmol·mol^-1 at C₈₀₀×50%-60%FC.Overall,whole-plant water processes were significantly influenced by SWC,but not by C_a.As for the whole-plant carbon process and water-carbon coupling,both SWC and C_a changes exerted significant influences on them,and elevated C_a is beneficial to alleviate the drought stress.Furthermore,this study confirmed that accurate estimation of WUE_s,Prequires an improved predictive accuracy of g_sw and g_m.These results have important implications for predicting how plants respond to climate change.