Light-Induced Changes in Hydrogen, Calcium, Potassium, and Chloride Ion Fluxes and Concentrations from the Mesophyll and Epidermal Tissues of Bean Leaves. Understanding the Ionic Basis of Light-Induced Bioelectrogenesis

Noninvasive, ion-selective vibrating microelectrodes were used to measure the kinetics of H(+), Ca(2+), K(+), and Cl(−) fluxes and the changes in their concentrations caused by illumination near the mesophyll and attached epidermis of bean (Vicia faba L.). These flux measurements were related to light-induced changes in the plasma membrane potential. The influx of Ca(2+) was the main depolarizing agent in electrical responses to light in the mesophyll. Changes in the net fluxes of H(+), K(+), and Cl(−) occurred only after a significant delay of about 2 min, whereas light-stimulated influx of Ca(2+) began within the time resolution of our measurements (5 s). In the absence of H(+) flux, light caused an initial quick rise of external pH near the mesophyll and epidermal tissues. In the mesophyll this fast alkalinization was followed by slower, oscillatory pH changes (5–15 min); in the epidermis the external pH increased steadily and reached a plateau 3 min later. We explain the initial alkalinization of the medium as a result of CO(2) uptake by photosynthesizing tissue, whereas activation of the plasma membrane H(+) pump occurred 1.5 to 2 min later. The epidermal layer seems to be a substantial barrier for ion fluxes but not for CO(2) diffusion into the leaf.