Rates of Sugar Uptake by Guard Cell Protoplasts ofPisum sativumL. Related to the Solute Requirement for Stomatal Opening

We wished to determine whether the capacity of the sugar uptake mechanisms of guard cells of the Argenteum mutant of pea (Pisum sativum L.) sufficed to support a concurrent stomatal opening movement. Sugar uptake by guard cell protoplasts was determined by silicone-oil-filtering centrifugation. The protoplasts took up [14C]glucose, [14C]fructose, and [14C]sucrose (Suc), apparently in symport with protons. Mannose, galactose, and fructose competed with Glc for transport by a presumed hexose carrier. The uptake of Glc saturated with aK  m of 0.12 mm and aV  max of 19 fmol cell−1h−1. At external concentrations <1 mm, the uptake of Suc was slower than that of Glc. It exhibited a saturating component with a K  m varying between 0.25 and 0.8 mm and a V  max between 1 and 10 fmol cell−1 h−1, and at external concentrations >1 mm, a non-saturating component. At apoplastic sugar concentrations below 4 mm, sugar import was estimated to be mainly in the form of hexoses and too slow to support a simultaneous stomatal opening movement. If, however, during times of high photosynthesis and transpiration, the apoplastic Suc concentration rose and entered the range of non-saturating import, absorbed Suc could replace potassium malate as the osmoticum for the maintenance of stomatal opening.