Coordination of stomatal control and stem water storage on plant water use in desert riparian trees

Abstract

Populus euphratica Oliv. is a relatively ‘conservative’ water user even when growing in favourable water conditions, but the mechanistic understanding of this has received little attention. We undertook an experiment to determine trees water use by measuring stem and root sap flow (Fs), variation in stem diameter (Ds), leaf stomatal gas-exchange (e.g. conductance, gs and transpiration, Tr) and water potential $$\left( {{\psi _{\text{L}}}} \right)$$ during the growing season for P. euphratica. There was a hysteretic ‘apparent feedforward’ of stomatal response to increasing VPD. Mean of gs was not significantly different among months. $${\psi _{\text{L}}}$$ was negatively related to gs and Tr, but in contrast, stem Fs was positively associated to Tr but not to gs. There was no lag in the daily onset and cessation of Fs between the bottom and top of the trunk, possibly due to the short distance between measurement points (about 2 m), however, the lag time in Fs between the bottom of the trunk and roots, approximately 30 min, suggested that stored water was withdrawn first from the trunk and subsequently the roots. Daily contraction of Ds (− Ds) increased with increasing Fs during both day and night, and expansion of Ds (+ Ds) showed a logarithmic rise to a maximum with increasing Fs during the day. Day Fs and − Ds were also logarithmic with respect to VPD, with a correlation coefficient equal to 0.51 and 0.50, respectively (P < 0.001). This suggests that water use of P. euphratica was determined by the stomatal control and stem water storage together, which has great significance for the species buffers xylem water deficit, maintaining high leaf production, and water use efficiency.