title: Rates of sugar uptake by guard cell protoplasts of pisum sativum L. Related To the solute requirement for stomatal opening
abstract: 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 [(14)C]glucose, [(14)C]fructose, and [(14)C]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 a K(m) of 0.12 mM and a V(max) of 19 fmol cell(-1) h(-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.
authors.affiliation: Albrecht-von-Haller-Institut fur Pflanzenwissenschaften, Universitat Gottingen, Untere Karspule 2, 37073 Gottingen, Germany.
authors.initials: G
authors.lastname: Ritte
authors.initials: J
authors.lastname: Rosenfeld
authors.initials: K
authors.lastname: Rohrig
authors.initials: K
authors.lastname: Raschke