It is well established that photosynthesis will be increased in C(3) plants with increased atmospheric carbon dioxide concentrations under optimum water and nutrient conditions. Few studies address the interactive effects of carbon dioxide and other important environmental factors controlling growth. Our objective is to study the interactive role of atmospheric CO₂ and nitrogen nutrition in cotton. Cotton plants (G. hirsutum L., cv. DES 119) were grown from seed in controlled environment chambers for 49 days after emergence at 30/22 C day/night temperatures and at ambient (350 µL L^-1) or twice ambient (700 µL L^-1) CO₂ levels. The plants were reared in pure sand medium. At initial squaring, five different nitrogen treatments: 0, 1, 2, 6 and 10 mM N were imposed for the remaining period. Nutrients were provided with half strength Hoagland's nutrient solution. In the N-deficient solutions, sulphate and chloride ions were used to replace nitrate ions. All plants were irrigated three times a day. Prior to squaring, plants were fed adequate amounts of nutrients and water. Photosynthesis, transpiration, and stomatal conductance in young expanded leaves were measured at different intervals using Li-Cor 6200 portable photosynthetic system. Leaf chlorophyll, protein, starch, and nitrogen contents were determined from the same leaves used for photosynthetic measurements.

Gas exchange, chlorophyll, protein, and starch measurements as functions of leaf nitrogen indicated the following. Photosynthesis was linearly dependent on leaf nitrogen per unit area in both ambient and twice ambient CO₂ grown plants. Plants grown at elevated CO₂ assimilated about 34% more carbon than plants grown at present-day CO₂ levels. Photosynthetic nitrogen use efficiency was greater in high CO₂-grown plants at all nitrogen levels. Photosynthetic rates, expressed as percent of maximum potential, responded similarly in both CO₂ levels to diminishing foliar nitrogen levels. Leaf protein and chlorophyll contents were directly related to leaf nitrogen. Water-loss and conductance interacted with leaf nitrogen and atmospheric CO₂ concentrations. Conductance was lower in high-CO₂ and high-nitrogen grown plants compared to plants grown at ambient CO₂ grown plants. Leaf starch was negatively correlated with reduced photosynthetic rates and leaf nitrogen in plants grown in both CO₂ environments. Starch accumulation seems to be a consequence of lower growth rates.