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 t he kinetics of H+, Ca2+, K+, and Cl- fluxes and the changes in their concen trations caused by illumination near the mesophyll and attached epidermis o f bean (Vicia faba L.). These flux measurements were related to light-induc ed changes in the plasma membrane potential. The influx of Ca2+ was the mai n depolarizing agent in electrical responses to light in the mesophyll. Cha nges in the net fluxes of H+, K+, and Cl- occurred only after a significant delay of about 2 min, whereas light-stimulated influx of Ca2+ 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 ep idermal 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 initi al alkalinization of the medium as a result of CO2 uptake by photosynthesiz ing 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 i on fluxes but not for CO2 diffusion into the leaf.