The Permeability of the Guard Cell Plasma Membrane and Tonoplast

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Abstract
Uptake experiments and efflux compartmental analysis of plant hormones, osmotica and toxins using ‘isolated’ guard cells of Valerianella locusta and guard cell protoplasts (GCP) of Vicia faba were performed in order to study the permeability properties of guard cell plasma membrane and tonoplast. The plasma membrane of guard cells exhibits a higher permeability than plasma membranes of mesophyll cells for most solutes investigated. The permeability coefficients (Ps calculated for the guard cell plasma membranes are also significantly higher than the Ps values for the guard cell tonoplast. This applies also for protonated ABA. We suppose that the high permeability for ABAH could be part of the target cell properties. A Collander analysis demonstrates a linear correlation between Ps, values and the ratio Kr/Mr1,5 for both plasma membrane (r = 0.87) and for the tonoplast (r =0.93). Because of deviations from the observed correlations, the permeation of some solutes (ABA, GA, IAA through the tonoplast; methylamine through the plasma membrane) seems to be facilitated by an additional transport mechanism. The Collander analysis of the plasma membrane of GCP shows very similar results to the analysis of the plasma membrane of ‘isolated’ guard cells, indicating that isolation of protoplasts does not alter the permeability of the guard cell plasma membrane.