Whole-plant instantaneous and short-term water-use efficiency in response to soil water content and CO₂ concentration

Aims Water-use efficiency (WUE) is a key parameter to understand plant survival strategies and promote forest management in response to climate change. Here, the whole-plant instantaneous WUE (WUE i-P ) and short-term WUE (WUE s-P ) were investigated in response to soil water content (SWC) and atmosphere CO 2 concentration ( C a ). Methods The WUE i-P was measured using a plant chamber and mini-lysimeters. The WUE s-P was estimated using different isotopic models. These estimates were compared with measured results (WUE s-P,mea ; the ratio of the whole-plant cumulative CO 2 assimilation to water loss). Results Except at severe drought, WUE i-P generally decreased with increasing SWC, but increased with increasing C a . At mild and moderate drought, the percentage increases in WUE i-P by elevating C a from 600 to 800 μmol·mol −1 (23.45%–32.78%) were higher than those from 400 to 600 μmol·mol −1 (9.12%–8.33%). However, the opposite pattern was found under well- and excessive-watered conditions. The variation in WUE s-P,mea in response to C a × SWC was similar to that in WUE i-P . The developed whole-plant isotopic model (i.e. the plant classical model) is based on the hypothesis that the mesophyll conductance ( g m ) should be considered to model whole-plant WUE. This model provided the best fit with WUE s-P,mea compared with previously proposed models (plant linear model, as well as leaf linear and classical model). This indicated that the contribution of g m , whole-plant respiration, and unproductive water loss should be considered when scaling from leaf to whole-plant level. Conclusions These results have implications for mechanisms of plant water and carbon cycles and improve predictive capability for whole-plant WUE from δ 13 C.