Effect of Cold Osmotic Shock on K^+ Efflux from Nicotiana tabacum L. Leaf Discs Induced by Abscisic Acid and Ionophores

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Cold osmotic shock, i.e., transfer of tobacco (Nicotiana tabacum L. cv. Samsun NN) leaf discs from 0.5 M mannitol at 25℃ to distilled water at 2℃ appears to modify the events selectively at the plasma membrane of tobacco leaf cells. Especially, the efflux of K^+ and ion leakage was markedly increased by cold osmotic shock, which is dependent on both osmotic pressure and the temperature of the solution bathing the discs; i.e., K^+ efflux was increased by raising the osmotic pressure of mannitol and further intensified by subsequently lowering the temperature. The shock effects are temporary and disappear 2 to 3 hr after the treatment. The efflux of K^+ from the discs into the external medium (1 mM MES-Tris, pH 6.0) was stimulated by abscisic acid (ABA) and gramicidin S, slightly stimulated by nigericin and slightly inhibited by kinetin. Valinomycin was insensitive to K^+ efflux. ABA-induced stimulation of K^+ efflux was inhibited by cold osmotic shock, but not the gramicidin-induced stimulation. The ABA-induced stimulation was partly inhibited by kinetin, whereas the gramicidin-induced one was not. The stimulatory effect on K^+ efflux was additive when ABA and ionophore were added together to the external medium. These results indicate that ABA acts at the plasma membrane that is selectively modified by cold osmotic shock. The stimulation of K^+ efflux induced by ABA and ionophore occurs via different processes.
日本植物生理学会の論文