Water Use and Growth of Cotton in Response to Elevated CO₂ in Wet and Drying Soil

Abstract

Cotton (Gossypium hirsutum cv. Sicala 34) was grown at 352 ('low CO₂') or 710 ('high CO₂') μL L^-1 atmospheric CO₂ in continuously wet soil, or in drying soil, or in drying soil re-wetted after plant wilting. In wet soil, the approximately 15% reduction in transpiration per unit leaf area owing to high CO₂ was only half that for other species, whereas effects on growth and leaf area were relatively larger. Consequently, water use per plant was 45-50% higher for high CO₂ plants in contrast to other species for which the rate of water use is either the same or lower in high CO₂. Greater plant water use early in a drying cycle caused the soil to dry faster under high CO₂ than under low CO₂. The addition of the consequential greater water stress at high CO₂ in drying soil to the direct CO₂ effect on stomata caused the transpiration rate of high CO₂ plants to fall by up to 60% as the soil dried relative to plants drying at low CO₂. After re-wetting the dry soil, the reduction in transpiration rate at high CO₂ returned within hours to the value of 15% seen in wet soil. The results were inconsistent with the idea that water deficits increase the sensitivity of stomatal aperture to CO₂. Other consequences of drier soil under high CO₂ compared with low CO₂ were: (a) unlike in many other species, in cotton, the relative growth enhancement by high CO₂ is not higher under drying soil compared with wet soil owing to the opposite effect on soil water content; and (b) the increased water-use efficiency in drying soil relative to wet soil was greater in high CO₂ plants than in low CO₂. The confounding of indirect effects of soil water with the direct CO₂ effects may explain the wide variability of literature reports about CO₂ effects on stomatal conductance and water use.