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

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 (Gtw) was generally higher and residual internal conductance (Gr) was generally lower in low than in high CO2. Ratios of midday Gr/Gtc were nearly constant throughout the period in both the stressed and the well‐watered treatments. The ratios of intercellular Ci, to ambient Ca, CO2 concentration (i.e., Ci/Ca) 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.