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Soybean Leaf Gas-Exchange Responses to Carbon Dioxide and Water Stress

USDA-ARS, Agronomy Dep., Univ. of Florida, Gainesville, FL 32611-0840
Botany Div., Ctr. de Pesquisas do Cacau, Diviso de Botanica, Caixa Postal 7, Itabuna, Bahia, Brazil

J. W. Mishoe and J. W. Jones

Agric. Engineering Dep., Univ. of Florida, Gainesville, FL 32611-0570

^* Corresponding author.

As global carbon dioxide concentrations rise, we need to understand the combination of direct effects of this gas and the anticipated effects of climate change, including drought, on physiology and growth of all crops. Effects of CO₂ on plants begin at the leaf level; our objectives, therefore, were to determine interrelationships among factors governing gas exchange responses of soybean [Glycine max (L.) Merr.] leaves to elevated CO₂ and water stress. Photosynthetic CO₂ assimilation and transpiration rates were measured in cuvettes on leaflets of soybean (cv. Bragg) grown in controlled-environment chambers at 330 and 660 µmol CO₂ mol^–1 air. Leaflets at high CO², either water-stressed or wellwatered, had higher photosynthetic and lower transpiration rates, and therefore higher water-use efficiencies (WUE), than those at control CO₂ levels. As irrigation was withheld during an ll-d period, WUE decreased about 30 to 50% with respect to the well-watered treatments. Midday leaf temperature and leaf-to-air vapor pressure gradient levels increased as the water stress progressed. For water stress treatments, midday leaf conductance (G_tw) was generally higher and residual internal conductance (G_r) was generally lower in low than in high CO₂. Ratios of midday G_r/G_tc were nearly constant throughout the period in both the stressed and the well-watered treatments. The ratios of intercellular C_i, to ambient C_a, CO₂ concentration (i.e., C_i/C_a) during the water stress period remained similar to the respective nonstressed treatments within each CO₂ level. These findings support the concept that leaf conductances are governed by CO₂ assimilation rates under water-stressed as well as unstressed conditions.

Received for publication February 26, 1993.

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