The plant growth regulator PGR-IV was evaluated in whole-plant test chambers for its effects on the carbon and water economy of cotton plants grown at 20 and 30 øC. PGR-IV applications increased rates of carbon uptake, carbon losses, and net carbon gains at 20 øC, but no effects were detected at 30 øC. The increases in carbon uptake were proportionally lower than the increases in carbon losses through respiration and consequently treated plants showed lower values of carbon use efficiency at 20 C. PGR-IV also induced reductions in internode length and consequently in plant height. At 30 øC PGR-IV applications induced increments in the rate of transpiration and reductions in water use efficiency, 22 days after the first application.