Preferential Accumulation by Mesophyll Cells at Low and by Veins at High Exogenous Amino Acid and Sugar Concentrations inCommelina benghalensisL. Leaves
The contribution to solute uptake by mesophyll cells and veins in leaf discs, was assessed through a study of uptake in relation to concentration for 14C-labelled substrates (sucrose, glucose, arginine, proline, valine and α-aminoisobutyric acid) using isolated mesophyll cells and stripped leaf discs of Commelina benghalensis L. Uptake per unit fresh weight was higher in mesophyll cells than in discs at low substrate concentrations (lower than about 0·5 mol m−3). At higher concentrations, uptake by discs exceeded that by mesophyll cells except for glucose uptake which was higher in mesophyll cells over the whole concentration range. The profiles of uptake versus concentration displayed biphasic kinetics in mesophyll cells and discs. Comparison of the uptake characteristics obtained by iterative fitting confirmed that the high-affinity systems of uptake prevail in the mesophyll cells, whereas the low-affinity systems are dominant in the veins. The results provide good evidence that, supplementary to direct vein loading, a pathway via the mesophyll contributes strongly to the photosynthate loading by veins in stripped discs.