Photosynthetic capacity and carbon contribution of leaves and bracts to developing floral buds in cotton

During ontogeny of Gossypium hirsutum L. floral buds (squares), increases i n area and dry mass (DM) of floral bracts and the subtending sympodial leaf followed a sigmoid growth curve with increasing square age. The maximum gr owth rates of the bract area and bract DM occurred between 15 and 20 d afte r square first appearance (3 mm in diameter). Net photosynthetic rate (P-N) of the sympodial leaf at first fruiting branch position of main-stem node 10 reached a maximum when the subtended square developed into a white flowe r. Floral bracts had much lower P-N and higher dark respiration than the su btending leaf. The amount of (CO2)-C-14 fixation by the bracts of a 20-d-ol d square was only 22% of the subtending leaf, but 56% of C-14-assimilate in the floral bud was accumulated from the bracts, 27% from the subtending le af, and only 17% from the main-stem leaf at 6 h after C-14 feeding these so urces. Hence floral bracts play an important role in the carbon supply of d eveloping cotton squares.