Leaf hydraulic conductance in elevated atmospheric CO2

Background/Question/Methods

Global climate change models predict that rising CO2 will alter hydrologic cycles on an ecosystem and global scale.  This could result in more frequent water shortages in some ecosystems.  Leaf hydraulic conductance, K_leaf, is a measure of a leaf’s water transport capacity at a given driving force from transpiration.  K_leaf can vary depending on both structural and biochemical traits of the leaf, allowing acclimation during and after growth.  Because growth at elevated [CO2] decreases stomatal conductance and water use on a leaf area basis, we predicted that K_leaf would acclimate under elevated [CO2] to adjust to the lower water demand.  We tested this hypothesis both in chambers and in the field, employing Free Air Concentration Enrichment.  For chamber plants, whole leaves were collected in the early morning at full hydration, and returned to the lab for measurement.  For field plants, leaves were collected at four time points on eight different days.  K_leaf was measured in the lab with the evaporative flux method, and leaf water potential was measured using thermocouple psychrometry.

Results/Conclusions

Although both field- and chamber- grown leaves had lower stomatal conductance as expected at elevated [CO2], no difference in K_leaf was found for either chamber- or field- grown soybeans in response to elevated [CO2].  These results suggest that K_leaf does not acclimate to high [CO2] in soybean.  Also, the decreased hydraulic demand at elevated [CO2] due to lower g_s coupled with the absence of change in hydraulic conductance could help protect the leaf from desiccation during periods of drought.  Significant changes in K_leaf over the course of the day were observed in both ambient and elevated [CO2] plants.  These changes in K_leaf were similar to diurnal changes in leaf water potential (Ψ_leaf), with both K_leaf and Ψ_leaf being highest in the morning, when leaves should be fully hydrated, and decreasing over the course of the day.  It is likely that cavitation resulting from high midday transpiration is responsible for this loss of conductance.