Interest and concern for the Earth's changing atmosphere has increased dramatically in recent years. Future increases in atmospheric CO₂ concentration will directly affect physiological processes and growth rates of plants. Indirect climatic effects such as global warming, changing precipitation patterns, increasing cloud cover, and frequency of weather extremes may have a greater impact than the direct CO₂-induced changes on plant processes. We conducted several experiments in naturally-lit, temperature,-and CO₂-controlled chambers and measured cotton crop responses to the direct and interactive effects of CO₂ and water deficits.

Absolute differences in photosynthetic rates at different CO₂ concentrations became progressively greater with time as plants entered reproductive phases of growth. The photosynthetic response to CO₂ increased up to about 700 to 900 µL L^-1 CO₂. Plants grown in high CO₂ atmospheres produced more photosynthesis in water stressed conditions than plants grown in ambient CO₂ atmospheres. Plants grown in high CO₂ transpired less at nearly optimum water potentials than plants grown in low CO₂, but transpiration rates were not different when leaf water potentials decreased below -2.0 MPa. Photosynthetic process was less sensitive to water deficit conditions compared to stem or leaf expansion.

We found plants grown in high-CO₂ environments produced more vegetative branches and more secondary fruiting branches than plants grown in ambient CO₂ environments. The weight per unit length of lower mainstem internodes of plants grown in high-CO₂ also became greater. Boll growth rate was less sensitive to increased CO₂ and water deficit conditions. A combination of factors such as additional branches and less abscission of flowers and flower buds coupled with increased photosynthesis and enhanced leaf area per plant caused more fruit to be produced by cotton plants growing in enriched CO₂ environments at optimum water or water deficit conditions. Our results suggest that higher water-use efficiency together with increased photosynthetic capacity should enhance the productivity and economic usefulness of cotton as atmospheric concentration increase in future.