ABSTRACT

The increase in concentration of global atmospheric carbon dioxide, [CO₂], is expected to result in a CO₂ level of 600 ìL L^-1 some time in the next century. The continuing increases in concentration of CO₂ and other greenhouse gases are predicted to raise atmospheric air temperature from 3 to 6EC. The primary effect of elevated [CO₂] on well watered plants with the C₃ carbon fixation pathway is an increase in net photosynthesis. An increase in [CO₂] tends to partially close stomata in plant leaves, resulting in reduced transpiration per unit leaf area and increased tissue temperatures. This direct effect of CO₂ on canopy temperature, in addition to a 3 to 6EC rise in global surface air temperature predicted by atmospheric general circulation models, may have a significant effect on agricultural crops. The beneficial effects of increased Co, an growth and productivity of crops have been well documented. The effects of long-term elevated and temperature and their interactions on productivity of crop plants are poorly understood. The objective of this study was to evaluate the interactive effects of temperature and CO₂ on cotton photosynthesis, respiration, transpiration, water use efficiency, and drymatter accumulation.

A study was conducted in which cotton cv. Deltapine 50 (DPL 50) was grown in plant growth chambers with CO₂ and temperature controlled. Cotton seeds were planted in these naturally lighted chambers, and the chambers were all maintained at 28/23EC (day/night) temperature during seedling emergence and until 14 DAE. At 15 DAE, temperature and CO₂ treatments were imposed, and air temperatures in the chambers were maintained at 20/12, 25/17, 30/22, and 35/27EC. The CO₂ concentrations were maintained at 350 and 700 ìL L^-1 for each temperature. The data on photosynthesis, respiration, transpiration, and water use efficiency were collected throughout the experiment using a computerized control system. The data on drymatter accumulation were recorded at the time of destructive sampling.

Photosynthesis increased at high [CO₂] at all temperatures, with the highest increase at the optimum temperature regime (30/22EC) . Increased [CO₂] decreased transpiration rate and increased water use efficiency at all temperatures. This increase in water use efficiency at high [CO₂] was due to a small decrease in transpiration and significant increase in photosynthesis. It was also observed that the water use efficiency was highest at sunrise and decreased during the day, with the lowest efficiency recorded at sunset. Photosynthetic rates and drymatter accumulation increased at high [CO_2], with the highest increment in drymatter due to high [CO₂] at the optimum temperature (30/22EC). There was no significant effect of [CO₂] on root/shoot ratio except at 20/12EC.