Effects of water vapor pressure difference on leaf gas exchange in potato and sorghum at ambient and elevated carbon dioxide under field conditions

High leaf to air water vapor pressure differences often substantially reduce rates of assimilation of carbon dioxide, especially in C-3 species. Rising concentrations of carbon dioxide [CO2] in the atmosphere could reduce the sensitivity of assimilation rate to partial stomatal closure caused by high vapor pressure difference by a variety of mechanisms. However, field data addressing this question are scarce. In this study, we examined day-to-day variation in midday gas exchange rates of upper canopy leaves of potato and sorghum grown at the current ambient [CO2] and ambient+ 350 mumol mol(-1) [CO2] in field plots. Stomatal conductance and assimilation rate were negatively correlated with vapor pressure difference across days. Assimilation rate was not less sensitive to vapor pressure difference at elevated than at ambient [CO2] in either species. For both potato and sorghum short-term increases in vapor pressure difference for individual leaves produced significantly smaller responses of leaf gas exchange than did the day-to-day variation in vapor pressure difference, again with no reduced sensitivity at elevated [CO2]. The smaller response of gas exchange to short-term manipulations of vapor pressure difference than to day-to-day variation may indicate that much of the response to high vapor pressure difference apparent in the day-to-day variation resulted from leaf water deficits caused by exposure of the whole canopy to high vapor pressure difference, rather than from direct effects of high water vapor pressure difference. The lack of a [CO2] effect on the sensitivity of assimilation rate to vapor pressure difference, and the substantial sensitivity of assimilation rate to vapor pressure difference in the C-4 species both resulted from reductions in assimilation at a given internal [CO2] at high vapor pressure difference. An implication of these results is that that high leaf to air water vapor pressure difference may continue to be a major limitation to assimilation rates in C-3 and C-4 crop species even at twice the current concentration of carbon dioxide in the atmosphere. (C) 2003 Elsevier Science B.V. All rights reserved.