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Plants of 16 agricultural and horticultural species were grown from seed in spaced pots in two glasshouses, one with normal and one with twice the present atmospheric CO2 concentration. Water use and leaf area development were measured while soil moisture content declined from field capacity to c. 6%. High CO2 increased leaf area in all but two species, the increase varying from 20 to 75%. However, the water loss per plant followed a similar time course to that of plants in control CO2 concentration because of the reduction of daily transpiration rate per unit leaf area (range 4-39%). Cowpea and sunflower plants rewatered after one soil drying cycle showed 9 and 5%, respectively, lower transpiration rate in high CO2 over a subsequent drying cycle than in the first cycle. Averaging across all species and soil moisture contents, transpiration rate was less reduced by high CO2 (21%) than was stomatal conductance (36%) and this is attributed to the increased leaf temperature caused by reduced stomatal conductance.
Morison et al. (Mon,) studied this question.