Cellular and subcellular volumes in mature leaves of potato (Solanum tuberosum cv. Désirée) were determined stereologically from light and electron micrographs. Leaves of ten‐week‐old plants with a total leaf volume of 623 μL per mg chlorophyll (Chl) 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 Chl) of 323 μL (76 %) vacuole, 35 μL (8 %) chloroplast stroma, 22 μL (5 %) cytosol plus nucleus and peroxisomes, and 4μL, (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‐l,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 for spinach and barley leaves, we conclude that it may be characteristic for mesophyll cells in general.

In potato plants (Solanum tuberosum), a chimeric yeast-derived invertase gene fused to a 35S cauliflower mosaic virus promoter has been expressed. The protein was targeted to the cell wall by using the signal peptide of proteinase inhibitor 11 fused to the amino terminus of the yeast invertase. The transformed plants had crinkled leaves, showed a reduced growth rate, and produced fewer tubers. Although in the apoplast of the leaves of the transformed plants the content of glucose and fructose rose by a factor of 20, and that of sucrose declined 20-fold, 98% of the carbohydrate in the phloem sap consisted of sucrose, demonstrating the strong specificity of phloem loading. In the leaf cells of the transformed plants, glucose, fructose, and amino acids, especially proline, were accumulated. Consequently, the osmolality of the cell sap rose from 250 to 350 mosmol/kg. Our results show that the observed 75% decrease of photosynthesis is not caused by a feedback regulation of sucrose synthesis and is accompanied by an increase in the osmotic pressure in the leaf cells. In the transformed plants, not only the amino acid to sucrose ratio in the phloem sap, but also the amino acid and protein contents in the tubers were found to be elevated. In the tubers of the transformed plants, the protein to starch ratio increased.In most plants, sucrose affect the photosynthesis metabolism of young leaves. Due to the invertase activity, the leaf contents of glucose and fructose were found to increase with increasing leaf age, accompanied by an increased discoloration and a reduced photosynthesis (21). In the transformed tobacco (21) and tomato plants (5), the growth was inhibited, and the leaves developed a curly structure and bleached regions with increasing age. Qualitative studies with transgenic tomato plants indicated that the starch contained in the leaves of these plants did not disappear during the night, in contrast to wild-type plants, in which most of the leaf starch is consumed during the night. In the transformed tobacco plants, acid invertase was highly expressed in all leaves, but its activity increased with leaf age and was especially high in the bleached sectors of the leaves (20,21). Compared to the wild type, the level of starch in the green sectors was found to be increased at the end of the illumination period. The contents of glucose, fructose, and sucrose, as measured after 6 or 12 h of illumination, however, were in the green regions of the leaves of transformed plants not markedly different from those in the wild type. Moreover, the rates of photosynthesis in the green sectors of the leaves of transformed plants, as

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