Rates of sugar uptake by stomatal guard cell protoplasts

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

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In Plant Physiol 121: 647–656 – doi: 10.1104/pp.121.2.647 – 

http://www.plantphysiol.org/content/121/2/647

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 [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.