PHOTOSYNTHESIS, TRANSPIRATION, AND WATER-USE EFFICIENCY OF COTTON LEAVES AND FRUIT

The contribution of plant organs other than leaves, particularly fruit, to the photosynthetic and water economy of crops has received little attention. Therefore, field studies were conducted to examine the gas-exchange activities of cotton (Gossypium hirsutum L.) leaves, bracts, and the capsule wall of developing fruit with respect tophotosynthetic rate (P(N)), transpiration rate (E), and water-use efficiency (WUE). Subtending leaves at main-stem nodes 8 and 10 achieved a maximum P(N) of 0.82 mg(CO2) m-2 s-1 compared to 0.12 Mg(CO2) m-2 s-1 for the bracts. E of leaves was 260 mg(H2O) m-2 s-1 at 10 d after anthesis, followed by that of the bracts and the capsule wall where E averaged 25 and 11 %, respectively. P(N) and E were strongly correlated with leaf, bract, and capsule wall stomatal conductance (g(s)), with low conductances limiting the. gas-exchange capacity of bracts and capsule walls. Organ-specific differences in g(s) and P(N) contributed to a 3-fold reduction in WUE for the bracts compared to leaf. These differences in WUE were reflected in a more negative carbon isotope composition (delta-C-13) for the bracts. Estimates of daily fruit WUE increased from 22 g (dry mass produced) kg-1(H2O lost) at 6 d after anthesis to over 140 g(dry mass) kg-1(H2O) during peak dry matter accumulation. Seasonal water loss from an individual fruit exceeded 100 g H2O during ontogeny.Simulated carbon acquisition and water loss for whole-plant canopies indicated that bracts and capsule walls contributed less than 5 % to the carbon and water economy of cotton.