Irrigation and elevated carbon dioxide effects on whole canopy photosynthesis and water use efficiency in cotton (Gossypium hirsutum L.)

The study compared the effects of water stress under ambient (350 μmol mol^-1) and elevated (700 μmol mol^-1) carbon dioxide concentrations on canopy carbon assimilation rates, soil water use, and water use efficiency (WUE) of cotton plants on a daily basis. This research was carried out in four sun-lit SPAR (Soil Plant Atmosphere Research) units with soil bins located at Beltsville, Maryland, USA. Whole canopy net photosynthetic rates were recorded at 5-min intervals, and soil water content was measured hourly using time-domain reflectometry. Pre-dawn and mid-day leaf water potentials were measured during the water stress and the recovery periods. The soil water use rates of plants grown under elevated CO₂ were significantly lower than those grown under ambient levels for both water-stressed and well-watered plants. The WUE of the ambient treatment did not begin to increase until most of the available water was depleted from the soil. The elevated CO₂ non-irrigated treatment was able to maintain carbon assimilation rates similar to that of the well-watered treatment while decreasing water uptake during the first 10 days of the drying period. Both water use and carbon assimilation rates decreased in the ambient non-irrigated treatment, shortly after withholding water. Plants grown at 350 μmol mol^-1 CO₂ concentration depleted more water from the deepest part of the profile (0.35-0.85 m) than did the plants from the elevated CO₂ treatment.