The role of leaves in the supply of photosynthates for reproductive development in cotton (Gossypium hirsutum L.) has been a topic of debate for many years. Significant contributors of assimilate for boll growth include the leaf subtending the boll, the leaf subtending the adjacent fruiting position, and the leaf subtending the sympodial branch. However, the contribution of photosynthates derived from the fruiting structures themselves has received little attention. Therefore, field experiments were conducted to document the photosynthetic and respiratory activity of individual cotton fruits and their subtending leaves during the growing season. Gas-exchange measurements revealed that the bracts, which surround the developing ovary, achieved maximum photosynthetic rates of 2.1 umol m^-2 s^-1 compared to 16.5 umol m^-2 s^-1 for the subtending leaf. In addition, unlike the subtending leaf, the bracts did not show a dramatic decline in photosynthesis with increased age nor were they as sensitive to low light and water-deficit stress conditions. These results suggest that photosynthates derived from the bracts may be of increasing importance to the carboneconomy of bolls under certain environmental conditions.

The capsule wall, which contained significant levels of chlorophyll and nitrogen, and numerous stomates, was only a minor site of (14)CO₂ fixation from the ambient atmosphere. Dark respiration by the developing ovary averaged -18.7 umol m^-2 s^-1 for the first six days after anthesis and gradually declined to -2.7 umol m^-2 s^-1 after 40 days. Diurnal patterns of dark respiration were highly age dependent and closely correlated with stomatal conductance of the capsule wall. Stomata of young fruit were highly responsive to diurnal signals but lost this capacity with increasing age and thereby, restricted both water vapor and CO₂ exchange.

Respiratory loss of CO₂ on a per fruit basis was maximal at -158 umol CO₂ fruit^-1 h^-1 at 20 days after anthesis. The internal CO₂ concentration of 20-day-old fruit varied between 0.45 to 1.52% (v/v) which resulted in a large gradient for CO₂ diffusion from the internal to the external atmosphere. Radio-labeled (14)CO₂ injected into the fruit was rapidly assimilated by the capsule wall in the light, while fiber and seed fixed significant amounts of (14)CO₂ in both the light and dark. Our data indicate that fruiting structures in cotton, although sites of significant CO₂ loss, do serve a vital role in the reassimilation of CO₂ and thereby, function as important sources of assimilate for reproductive development.