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

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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 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.