The major storage function of cotyledons of Acer negundo, A. platanoides and A. pseudoplatanus is completed shortly after seedling emergence. Cotyledon photosynthates are essential for the maintenance of leaf production and seedling dry weight increase during the first week after emergence. However, cotyledon photosynthesis is not essential for seedling survival. Initially, much of the photosynthate exported from the cotyledons moves into the developing first leaf pair. This is most marked in A. platanoides and A. pseudoplatanus. Later, as the requirement for cotyledon photosynthate declines, less is exported from the cotyledon, mostly to the hypo-cotyl and roots. In A. negundo successive pairs of leaves are produced before the preceding pair have fully expanded and the cotyledons supply photosynthates to the latter in contrast to A. platanoides and A. negundo in which they are presumably supplied by the first leaf pair.
Substantial losses of radioactivity occur from the seedlings after 14CO2 application to cotyledons of A. pseudoplatanus, mainly during the first 48 h; this coincides with the maximum rate of radiocarbon export from the cotyledon. It is suggested that these losses are due to respiration and that they probably take place from the whole seedling, not merely from the treated organ.