Contributions of the Capsule Wall and Bracts to the Developing Cotton Fruit 1

Simultaneous exposure of sympodial leaf and fruit of cotton ( Gossypium hirsutum L.) to 14 CO 2 revealed that the bracts were 28% as efficient as leaves for CO 2 uptake on a unit‐weight basis, or 20% on an area basis, while the capsule wall was less than 1% as effective as the leaf. Subsequent translocation to the fruit was much more rapid from the leaf than from the bracts. The capsule wall, on the other hand, served as a sink for photosynthate accumulation. Stomatal density was not limiting the uptake capacity of the bracts, since there were more stomata on the outside surface of the bracts (11,325/cm 2 ) than on the lower surface of the leaf (7,187/cm, 2 ). Chlorophyll concentration was three times greater in the leaf than in the bract (2.28 mg, g fresh weight‐ 1 compared to 0.83). Similarly, the comparatively low rate of CO 2 fixation by the capsule wall was not explained by the stomatal frequency on the capsule wall (4407/cm 2 ).