CANOPY PHOTOSYNTHESIS AND TRANSPIRATION OF FIELD-GROWN COTTON EXPOSED TO FREE-AIR CO2 ENRICHMENT (FACE) AND DIFFERENTIAL IRRIGATION

Growth, yield and leaf photosynthetic rates of cotton (Gossypium hirsutum L. ) all respond strongly to CO2 enrichment, but the gas exchange of whole cotton canopies grown under elevated CO2 has not been investigated. We compared the effects of CO2 enrichment on both single-leaf and whole-canopy photosynthetic rates in cotton. We also determined whole-canopy photosynthetic and transpiration rates in cotton in response to CO2 enrichment and differential irrigation. Field-grown cotton was exposed to either 550 mumol mol-1 of CO2 using the free-air carbon dioxide enrichment (FACE) system or to 370 mumol mol-1 in control plots. In the second year of the experiment, half of each plot received reduced levels of irrigation. Rates of photosynthesis and stomatal conductance of single leaves were determined using a portable photosynthesis system and a portable steady-state porometer, respectively. Rates of whole-canopy photosynthesis and transpiration were determined using a custom-built chamber (about 1 m x 1 m). Midday net photosynthesis rates of both leaves and canopies were 19-41% higher in the CO2-enriched plots than in control plots. The CO2 effect on leaf photosynthesis was greatest in July, whereas the CO2 effect on canopy photosynthesis was greatest in June and decreased thereafter as mutual shading of leaves and the amount of non-photosynthetic biomass increased. Midday stomatal conductance values of leaves were 13-44% greater in control plants than in CO2-enriched plants. Except for late in the second season, canopy transpiration rates were not affected by the CO2 treatment because the decrease in stomatal conductance was offset by an increase in plant size. Differential irrigation led to no significant differences in either canopy photosynthesis or transpiration, possibly because differential irrigation was applied only during the second half of the season. It appears that cotton crops grown in a future, higher-CO2 climate may have increased photosynthetic rates, but water requirements may not be reduced.