The effect of light on membrane potential, apoplastic pH and cell expansion in leaves of Pisum sativum L. var. Argenteum.

Abstract.

The connection between three light responses of green leaf cells-membrane potential (V_m), H⁺ net efflux and growth, was analyzed. Illumination of mesophyll cells in leaves from Argenteum peas caused two rapid responses: (i) a de- and repolarization of V_m and (ii) an alkalinization of the apoplast. The rapid responses were completely eliminated by the photosynthetic inhibitor 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) but not affected by ortho-vanadate, an inhibitor of the plasma membrane (PM) H⁺-ATPase. The rapid changes were followed by a set of delayed responses: (i) a slow, gradual hyperpolarization of V_m, (ii) a gradual acidification of the mesophyll apoplast and (iii) an increased rate of elongation. These three light responses persisted under DCMU but were completely eliminated by vanadate. The data show that the delayed (in contrast to the rapid) responses were due to a stimulation of PM H⁺ pumps which occurred independently of non-cyclic photosynthetic electron transport and the “dark” processes depending on it. When the rapid responses were blocked by DCMU, light-induced acidification, hyperpolarization of the membrane potential and growth proceeded simultaneously. A shared (4-min) lag phase indicated slower signal processing in mesophyll than in epidermal cells where light stimulation of PM H⁺ pumps was rapid.