Rates of sugar uptake by guard cell protoplasts of Pisum sativum L. – Related to the solute requirement for stomatal opening
by Ritte G., Rosenfeld J., Rohrig K., Raschke K. (1999)
Gerhard Ritte, Johanna Rosenfeld, Kerstin Rohrig, Klaus Raschke,
Albrecht-von-Haller-Institut für Pflanzenwissenschaften, Universität Göttingen, Untere Karspüle 2, 37073 Göttingen, Germany
In Plant Physiol 121: 647–656 – doi: 10.1104/pp.121.2.647 –
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 aKm of 0.12 mm and aVmax 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 Km varying between 0.25 and 0.8 mm and a Vmax 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.