The global increase in concentration of atmospheric CO₂ is expected to result in a doubling of current CO₂ levels some time during the next century. The primary direct effect of elevated CO₂ concentrations on plants with the C(3) carbon fixation pathway is an increase in net photosynthesis possibly resulting in enhanced growth and yield. The objective of this investigation was to determine the photosynthetic and growth response of cotton to long term exposure to increased levels of CO₂ over a range-of temperatures.

Cotton (Gossypium hirsutum L.) plants were grown in daylit plant growth chambers with temperature and CO₂ as variables controlled during the experiment. The average temperatures during the experiment were maintained at 17.8, 18.7, 22.7, 26.6 and 30.6 C with CO₂, levels of 350 and 700 ppm at each temperature. Plant height, number of nodes, leaf area and fruiting were measured throughout the experiment and dry weight measurements were obtained on three occasions. Canopy photosynthesis and respiration rates were measured throughout the experiment using a conductimetric analyzer, an infrared gas analyzer and a dedicated computer.

Plant height, number of main stem nodes and area individual leaves were all significantly affected by temperature and CO₂ levels. The optimum temperature for growth and developmental processes was found to be 26.6 C, The higher CO₂ concentration increased the area of individual leaves and the effect was more pronounced at higher temperatures. The duration of individual leaf growth decreased with increase in temperature. At the higher CO₂ level enhanced leaf growth rates were observed. The leaf expansion rates increased up to 6 to 12 days after leaf unfolding depending on temperature then rapidly declined at both the CO₂ levels tested. Increases in CO₂ levels and temperature increased photosynthesis throughout the experiment and the effect of high CO₂ on photosynthesis was more pronounced at higher temperatures. This increase in photosynthesis resulted in increased dry matter accumulation.