ELEVATED LEVELS OF BOTH SUCROSE-PHOSPHATE SYNTHASE AND SUCROSE SYNTHASE IN VICIA GUARD-CELLS INDICATE CELL-SPECIFIC CARBOHYDRATE INTERCONVERSIONS

A long series of reports correlate larger stomatal aperture size with elevated concentration of sucrose (Suc) in guard cells. To assess the role and autonomy of guard cells with respect to these changes, we have determined quantitatively the cellular distribution of the synthetic enzyme, Suc-phosphate synthase (SPS) and the degradative enzyme Suc synthase (SS) in Vicia leaflet. As expected for Suc-exporting cells, the photosynthetic parenchyma had a high SPS:SS ratio of approximately 45. Also as expected, in epidermal cells, which had only few and rudimentary plastids, the SPS:SS ratio was low (0.4). Of all cells and tissues measured, those that had the highest specific activity of SPS (about 4.8 mumol mg-1 of protein h-1) were guard cells. Guard cells also had a very high relative specific activity of SS.