Soybean Leaf Water Potential Responses to Carbon Dioxide and Drought

Rising CO 2 can have direct effects on crop water relations and indirect effects on water available for growth. We studied the effects of elevated CO 2 and drought on leaf water relations of soybean [Glycine max (L.) Merr. cv. Bragg] and considered the hypothesis of osmotic adjustment mediated by increased photosynthesis (Hypothesis 1) vs. the hypothesis of water conservation mediated by decreased stomatal conductance (Hypothesis 2) to explain improved water relations of plants growing under elevated CO 2 In Exp. 1, soybean was grown at 330, 450, 660, and 800 μmol mol -1 CO 2 in sunlit, closed-circulation, controlled-environment chambers under well-watered conditions. Leaf total water potential (WP), osmotic potential (OP), and turgor potential (TP) were measured at midday during V4 to R6 stages of development. In Exp. 2 (well-watered, R1-R3) and Exp. 3 (13-d drying cycle, R6 seed filling), soybean was grown at 330 and 660 μmol mol -1 CO 2 , and WP, OP, and TP were measured five times per day on sunlit and shaded leaves. In Exp. 3, stomatal conductance (g s ) and transpiration rate (TR) of leaves were also measured. Experiments 1 and 2 showed that elevated CO 2 increased TP and decreased OP, but did not affect leaf WP, thus favoring Hypothesis 1. In Exp. 3, leaf WP was higher in elevated than ambient CO 2 . Diurnal TP was higher in elevated than ambient CO 2 at the beginning of drought, and was maintained longer each day as drought progressed. At the end of drought, TP and WP was higher in elevated than ambient CO 2 . Elevated CO 2 leaves had lower TR because of lower g s than ambient CO 2 counterparts. Thus, Exp. 3 supported Hypothesis 2, that both stressed and nonstressed plants in elevated CO 2 have a better water status (e.g., higher TP) than plants in ambient CO 2 due to water conservation mediated by decreased g s . Remobilization of leaf nutrients during seed filling may limit the capability for osmotic adjustment. Regardless of the mechanisms, growth of plants in elevated CO 2 should be less affected by drought than plants in ambient CO 2 .