Water relations and gas exchange in poplar and willow under water stress and elevated atmospheric CO2

Predictions of shifts in rainfall patterns as atmospheric CO2] increases could impact the growth of fast growing trees such as Populus spp. and Salix spp. and the interaction between elevated CO2 and water stress in these species is unknown. The objectives of this study were to characterize the responses to elevated CO2 and water stress in these two species, and to determine if elevated CO2 mitigated drought stress effects. Gas exchange, water potential components, whole plant transpiration and growth response to soil drying and recovery were assessed in hybrid poplar (clone 53-246) and willow (Salix sagitta) rooted cuttings growing in either ambient (350 &mgr;mol mol-1) or elevated (700 &mgr;mol mol-1) atmospheric CO2 concentration (CO2]). Predawn water potential decreased with increasing water stress while midday water potentials remained unchanged (isohydric response). Turgor potentials at both predawn and midday increased in elevated CO2], indicative of osmotic adjustment. Gas exchange was reduced by water stress while elevated CO2] increased photosynthetic rates, reduced leaf conductance and nearly doubled instantaneous transpiration efficiency in both species. Dark respiration decreased in elevated CO2] and water stress reduced Rd in the trees growing in ambient CO2]. Willow had 56% lower whole plant hydraulic conductivity than poplar, and showed a 14% increase in elevated CO2] while poplar was unresponsive. The physiological responses exhibited by poplar and willow to elevated CO2] and water stress, singly, suggest that these species respond like other tree species. The interaction of CO2] and water stress suggests that elevated CO2] did mitigate the effects of water stress in willow, but not in poplar.