Carbon Assimilation by Sycamore Cotyledons during Early Seedling Development

Unexpanded cotyledons of mature sycamore seeds showed activedark fixation of CO₂, but were also capable of appreciable photosyntheticactivity. Ultrastructural studies showed the presence of well-definedplastids, although granal structure was atypical. In fully expanded cotyledons the rate of dark CO₂ fixation wasconsiderably reduced, but photosynthetic activity was much greaterthan that of embryonic material. During expansion, considerablelamellar reorganization occurred within the chloroplasts, resultingin plastids of more conventional appearance. Chlorophyll levelsalso increased during expansion, and the increased rate of photosynthesispresumably resulted from these changes.