WATER-USE AND GROWTH OF COTTON IN RESPONSE TO ELEVATED CO2 IN WET ANDDRYING SOIL

Cotton (Gossypium hirsutum cv. Sicala 34) was grown at 352 ('low CO2') or 710 ('high CO2') mu L L(-1) atmospheric CO2 in continuously wet so il, or in drying soil, or in drying soil re-wetted after plant wilting . In wet soil, the approximately 15% reduction in transpiration per un it leaf area owing to high CO2 was only half that for other species, w hereas effects on growth and leaf area were relatively larger. Consequ ently, water use per plant was 45-50% higher for high CO2 plants in co ntrast 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 c ycle 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 transpirati on rate of high CO2 plants to fall by up to 60% as the soil dried rela tive to plants drying at low CO2. After re-wetting the dry soil, the r eduction in transpiration rate at high CO2 returned within hours to th e value of 15% seen in wet soil. The results were inconsistent with th e idea that water deficits increase the sensitivity of stomatal apertu re to CO2. Other consequences of drier soil under high CO2 compared wi th low CO2 were: (a) unlike in many other species, in cotton, the rela tive growth enhancement by high CO2 is not higher under drying soil co mpared with wet soil owing to the opposite effect on soil water conten t; 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 confou nding of indirect effects of soil water with the direct CO2 effects ma y explain the wide variability of literature reports about CO2 effects on stomatal conductance and water use.