Quantitative Analysis of Photosynthate Unloading in Developing Seeds of Phaseolus vulgaris L. : I. The Use of Steady-State Labeling

The pathway and kinetics of photosynthate unloading in developing seeds of bean (Phaseolus vulgaris L.) were investigated using steady-state labeling with ¹⁴CO₂. The continuous assimilation of ¹⁴CO₂ at constant specific activity produced stable tracer fluxes that facilitated straightforward analyses of photosynthate import and unloading in developing seeds. The kinetics of tracer equilibration within intact seeds were compatible with a symplastic route of photosynthate unloading in the seed coat. The import and partitioning of tracer within seeds were partially disrupted by the surgical excision of the distal halves of seeds as practiced during the preparation of “empty” seed coats for perfusion.