The documented increase in global atmospheric CO₂ concentration and the corresponding potential rise in the mean global surface air temperature, due to increase in "greenhouse" gases, will have major effects on plant growth rates and their reproductive potential. This study was conducted to determine the effects and interactions of CO₂ concentration and air temperature on growth and development of Pima cotton (Gossypium barbadense L., cv. S-6). Plants were grown in naturally lit, controlled-environment chambers from seed to 64 days after emergence (DAE) under optimum water and nutrient conditions. Day/night temperatures of 20/12, 25/17, 30/22, 35/27 and 40/32 C were maintained in CO₂ concentrations of 350 and 700 µL L^-1.

Plant height, number of main-stem nodes and main-stem leaf area were measured at weekly intervals. Dry weight measurements of various plant components and leaf areas were obtained at 16, 26 and 64 DAE. Squares and flowers were tagged as they appeared throughout the experiment.

Plant height increased significantly with increased air temperatures up to 35/27 OC and decreased at 40/32 C. Carbon dioxide-enrichment accelerated plant growth rates up to 30/22 C. Average stem elongation rate increased about 300% from 20/12 OC to 35/27 C, but declined over 14% at 40/32 C compared to the 35/27 C in the 350 µL L^-1 CO₂, grown plants. Doubling of CO₂ induced a 13 to 17% increase in stem elongation rate it 30/22 C and below, unchanged at 35/27 C and a 13% decrease at 40/32 C. Node additions to the main-stem were more strongly influenced by air temperature than CO₂ treatments. Number of nodes per plant were 10, 17, 21, 26, and 28 in plants grown at 20/12, 25/17, 30/22, 35/27 and 40/32 C, respectively, at 64 DAE.

Fully expanded leaves were larger in CO₂-enriched plants at each leaf position across all temperature regimes. At the final harvest, total leaf area was 26% higher in CO₂-enriched environment compared with plants grown in 350 µL L^-1 treatments across all temperatures.

At each air temperature treatment, biomass increased with CO₂-enrichment. For both CO₂ treatments, the biomass was lowest at 20/12 C, increased across air temperatures up to 35/27 C and then declined at 40/32 C. Total weight at 64 DAE was 46, 39, 45, 28 and 15% higher in CO₂-enriched environment compared with the 350 AL L-1 treatments at 20/12, 25/17, 30/22, 35/27 and 40/32 C, respectively. Stem, leaf and root weights were consistently higher at 700 µL L^-1; although the response was lower at the two higher temperatures. Boll and square weights increased with an increase in air temperature and CO₂ treatments up to 30/22 C. At 35/27 C, however, the boll and square weights were only 25% as much in the in 700 µL L^-1 CO₂ grown plants as in the 350 µL L^-1 treatment. Plants grown at 40/32 C failed to produce either fruiting branches or reproductive organs in both the CO₂ treatments.

Days to first square and flower were more strongly influenced by air temperature than CO₂ treatments. Reproductive development began earlier in the 30/22 C than at either the lower or higher temperatures.

These results demonstrate that the response of Pima cotton to the direct and interactive effects of long-term exposure to different temperature and CO₂ treatments is indeed complex and varied. The observed decrease in reproductive mass at 35/27 C, and the failure of reproductive growth at 40/32 C in both the CO₂ treatments indicate a need to select cotton cultivar with higher temperature optimums. Considering the current rise in "greenhouse" gases and the corresponding rise in mean global surface air temperatures, it is imperative that plant breeders respond to this challenge.