Electrical characteristics of stomatal guard cells: The ionic basis of the membrane potential and the consequence of potassium chlorides leakage from microelectrodes

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Abstract
The membrane electrical characteristics of stomatal guard cells in epidermal strips from Vicia faba L. and Commelina communis L. were explored using conventional electrophysiological methods, but with double-barrelled microelectrodes containing dilute electrolyte solutions. When electrodes were filled with the customary 1–3 M KCl solutions, membrane potentials and resistances were low, typically decaying over 2–5 min to near-30 mV and 2 in cells bathed in 0.1 mM KCl and 1 mM Ca2+, pH 7.4. By contrast, cells impaled with electrodes containing 50 or 200 mM K+-acetate gave values of-182±7 mV and 16±2 kω·cm2 (input resistances 0.8–3.1 Gω, n=54). Potentials as high as (-) 282 mV (inside negative) were recorded, and impalement were held for up to 2 h without appreciable decline in either membrane parameter. Comparison of results obtained with several electrolytes indicated that Cl- leakage from the microelectrode was primarily responsible for the decline in potential and resistance recorded with the molar KCl electrolytes. Guard cells loaded with salt from the electrodes also acquired marked potential and conductance responses to external Ca2+, which are tentatively ascribed to a K+ conductance (channel) at the guard cell plasma membrane. Measurements using dilute K+-acetate-filled electrodes revealed, in the guard cells, electrical properties common to plant and fungal cell membranes. The cells showed a high selectivity for K+ over Na+ (permeability ratio PNa/PK=0.006) and a near-Nernstian potential response to external pH over the range 4.5–7.4 (apparent PH/PK=500–600). Little response to external Ca2+ was observed, and the cells were virtually insensitive to CO2. These results are discussed in the context of primary, charge-carrying transport at the guard cell plasma membrane, and with reference to possible mechanisms for K+ transport during stomatal movements. They discount previous notions of Ca2+-and CO2-mediated transport control. It is argued, also, that passive (diffusional) mechanisms are unlikely to contribute to K+ uptake during stomatal opening, despite membrane potentials which, under certain, well-defined conditions, lie negative of the potassium equilibrium potential likely prevailing.