SUBCELLULAR VOLUMES AND METABOLITE CONCENTRATIONS IN POTATO (SOLANUM-TUBEROSUM CV DESIREE) LEAVES

Cellular and subcellular volumes in mature leaves of potato (Solanum tuberosum cv. Desiree) were determined stereologically from light and electron micrographs. Leaves of ten-week-old plants with a total leaf volume of 623 mu L per mg chlorophyll (Chi) were found to be composed of 12 % epidermis, 68 % mesophyll, 5 % vascular tissue, 3 % apoplast and 16 % gas space. In the epidermal cells 97 % of the volume was occupied by the vacuole. The mesophyll cells consisted (as expressed per mg Chi) of 323 mu L (76 %) vacuole, 35 mu L (8 %) chloroplast stroma, 22 mu L (5 %) cytosol plus nucleus and peroxisomes, and 4 mu L (1 %) mitochondria. A comparison of these values with subcellular volumes previously determined for spinach and barley leaves, shows that the relative sizes of the subcellular volumes are strikingly similar. Subcellular concentrations of carbohydrates, of the phosphorylated intermediates of carbohydrate metabolism, of malate, and of amino acids have been evaluated from measurements of the corresponding subcellular metabolite contents determined using the non-aqueous fractionation technique. Malate, glucose and fructose were found to accumulate in the vacuole, whereas the concentration of sucrose and amino acids in the cytosol were much higher than in the vacuole. The amino acid concentration in the chloroplast stroma is similar to that in the cytosol. Phosphorylated intermediates of carbohydrate metabolism are confined to the chloroplast stroma and the cytosol, confirming the validity of the fractionation method. Whereas triose phosphates and fructose-1,6-bisphosphate are concentrated in the stroma, the concentrations of hexose monophosphates were highest in the cytosol. Since the subcellular metabolite distribution in potato leaves reported here is very similar to that previously described far spinach and barley leaves, we conclude that it may be characteristic for mesophyll cells in general.