Pathway of Carbon Transport Within Developing Ovules of Phaseolus vulgaris L

14C-labelled photosynthates, on reaching developing ovules of Phaseolus vulgaris plants, were transported rapidly and evenly throughout the highly vascularized seed coats. Apoplastic transfer of 14C-labelled photosynthates from the seed coats to the developing embryos occurred over the entire inner surface of the seed coats. Further lateral transfer of the photosynthate through the cotyledons exhibited characteristics consistent with a diffusional process. The 14C-labelled photosynthates transported from the seed coat to cotyledon symplasts were principally composed of sucrose. The free space of the cotyledons was fully permeated by photosynthates at concentrations in the range of 100-200 mM sucrose equivalents. This pool of photosynthates was continuously maintained by transfer from the seed coats. Furthermore, initial rates of depletion of the free-space pool of photosynthates, following removal of the seed coats, accounted for cotyledon growth rates. These observations are consistent with photosynthate movement being largely restricted to the apoplast of the cotyledons.