Preferential Accumulation by Mesophyll Cells at Low and by Veins at High Exogenous Amino Acid and Sugar Concentrations in Commelina benghalensis L. Leaves

The contribution to solute uptake by mesophyll cells and veinsin leaf discs, was assessed through a study of uptake in relationto concentration for ¹⁴C-labelled substrates (sucrose, glucose,arginine, proline, valine and -aminoisobutyric acid) using isolatedmesophyll cells and stripped leaf discs of Commelina benghalensisL. Uptake per unit fresh weight was higher in mesophyll cellsthan in discs at low substrate concentrations (lower than about0·5 mol m^–3). At higher concentrations, uptakeby discs exceeded that by mesophyll cells except for glucoseuptake which was higher in mesophyll cells over the whole concentrationrange. The profiles of uptake versus concentration displayedbiphasic kinetics in mesophyll cells and discs. Comparison ofthe uptake characteristics obtained by iterative fitting confirmedthat the high-affinity systems of uptake prevail in the mesophyllcells, whereas the low-affinity systems are dominant in theveins. The results provide good evidence that, supplementaryto direct vein loading, a pathway via the mesophyll contributesstrongly to the photosynthate loading by veins in stripped discs. Key words: Commelina benghalensis L., amino acid uptake, mesophyll, minor veins, phloem loading, sugar uptake