Coordination between liquid and gas phase conductance enables survival during water stress in an isohydrodynamic Eucalyptus cultivar

Abstract The regulation on transpiration and water status is critical for tree survival during drought. Eucalyptus is introduced and widely planted in southern China for timber production. Eucalyptus plantations are expanding into rugged terrains with shallow soils and soil water capacitance. With the ongoing decline in precipitation and the typical monsoon climate in this region, the trees suffer regular seasonal drought, yet their response mechanisms to drought remain obscure. We examined leaf water status, stomatal and hydraulic behavior, and whole-tree transpiration of a wildly planted eucalyptus cultivar, Eucalyptus urophylla × Eucalyptus grandis, across seasons with different water availability in order to identify its response strategy to drought. During the study period, both predawn (ΨPD) and midday (ΨMD) leaf water potential varied considerably, but the water potential gradient between the roots and leaves (ΔΨS-L), measured as the difference between ΨPD and ΨMD, remained constant, indicating an isohydrodynamic regulation on water status under drought. Despite strong stomatal down regulation on transpiration in response to increasing evaporative demand, leaf water potential declined to levels approaching -2 MPa at midday in the driest months, inducing more than 60% loss of xylem hydraulic conductance. Both stomatal and whole-tree hydraulic conductance linearly declined with increasing drought intensity. Whole-tree hydraulic conductance was exponentially correlated with the sensitivity of stomatal conductance to VPD. These results demonstrated that stomatal and hydraulic regulation on transpiration of eucalyptus was coupled through tight correlation between stomatal response to evaporative demand and whole-tree hydraulic conductance. This coordination between liquid and gas phase conductance found in this study was helpful to keep a balance between allowable maximum transpiration and hydraulic safety of eucalyptus under varying soil and atmosphere moisture conditions.