Vacuoles of mesophyll cells as a transient reservoir for assimilates

We studied the efflux of radioactive photosynthetic products from the central vacuole into the cytosol of protoplasts isolated from the mesophyll tissue of the sugar beet (Beta vulgaris L.) after their darkening and subsequent cessation of photosynthesis. Among the products accumulated in the vacuole were the C-14-labelled sugars malate and alanine, small amounts of citric, glutamic, and aspartic acids, and some other amino acids. During the initial 20-30 min of darkness, there was no substantial utilization of photoassimilates accumulated in the vacuole during the preceding light period. An efflux of assimilates occurred later, after 30-40 min of darkness. A decrease in the vacuolar C-14-sucrose occurred not only due to its exit into the cytosol but also because of its conversion into C-14-monosaccharides by the vacuolar invertase. In fact, this decrease in the sucrose content correlated well with the accumulation of monosaccharides. Immediately after photosynthesis ceased, the chloroplastic C-14-starch was utilized for the maintenance of cytoplasmic metabolism. After 30-min darkness, the content of starch in the chloroplasts decreased by several times. We believe that the vacuoles of sugar-beet mesophyll cells are transient reservoirs for assimilates and the products of their conversion (glucose and fructose), which can rapidly leave the vacuole to maintain homeostasis in the cytosol under varying environmental conditions.