DECREASED HYDRAULIC CONDUCTANCE IN PLANTS AT ELEVATED CARBON-DIOXIDE

Previous work indicated that long-term exposure to elevated carbon dio xide levels can reduce hydraulic conductance in some species, but the basis of the response was not determined, In this study, hydraulic con ductance was measured at concentrations of both 350 and 700 cm(3) m(-3 ) carbon dioxide for plants grown at both concentrations, to determine the reversibility of the response, In Zea mays and Amaranthus hypocho ndriacus, exposure to the higher carbon dioxide concentration for seve ral hours reduced whole-plant transpiration rate by 22-40%, without an y consistent change in leaf water potential, indicating reversible red uctions in hydraulic conductance at elevated carbon dioxide levels, Hy draulic conductance in these species grown at both carbon dioxide conc entrations responded similarly to measurement concentration of carbon dioxide, indicating that the response was reversible, In Glycine max, which in earlier work had shown a long-term decrease in hydraulic cond uctance at elevated carbon dioxide levels, and in Abutilon theophrasti , no short-term changes in hydraulic conductance with measurement conc entration of carbon dioxide were found, despite lower transpiration ra tes at elevated carbon dioxide, In G. max and Medicago sativa, growth at high dew-point temperature reduced transpiration rate and decreased hydraulic conductance, The results indicate that both reversible and irreversible decreases in hydraulic conductance can occur at elevated carbon dioxide concentrations, and that both could be responses to red uced transpiration rate, rather than to carbon dioxide concentration i tself.