[The influx of K(+) ions in leaves of Elodea densa, dependence on light, potassium concentration, and temperature]

Abstract

1. The influx of potassium ions in leaves of Elodea densa during short periods of time was measured using (42)K and (86)Rb as tracers. The K(+) influx was linear with time (Fig. 1) without a contribution by Donnan adsorption even in 1 min experiments. 2. Light increased the K(+) influx in air by a factor of up to 30-50 compared to dark/air. Light-induction of the K(+) influx is similar to the light-induction of photosynthesis except for the initial O2 outburst. The half-time of induction, however, is somewhat larger for K(+) influx than for photosynthesis (Fig.2). 3. The isotherms of K(+) influx exhibit the dual mechanism documented for many other species (Figs. 3 and 4). 4. Similar dual isotherms of K(+) influx are obtained in dark/air, light/air, and light/N2, suggesting similar transport mechanisms in light and dark (Figs. 3 and 4). 5. Using (86)Rb as a tracer for K(+), lower values of influx are obtained than with (42)K, the preference for (42)K being higher at low concentrations (Figs. 5,6). However, the light-stimulation (Fig. 5) and the effect of inhibitors on K(+) influx (Table 4) are also found with (86)Rb, indicating that it may be used for such measurements. 6. A change of temperature results in a dual Arrhenius plot (Fig. 7) of K(+) influx with two different apparent activation energies in the light as well as in the dark. The values of E app in the range of strong dependence on temperature are almost equal in light and dark. 7. The causes of the increased K(+) influx in the light are discussed. The influx is inhibited by uncoupling agents and inhibitors of the energy transfer (Table 3) suggesting a dependence on ATP production. On the basis of the carrier concept and using the equations of coenzyme kinetics, a change of the apparent K m (') and V max (') values caused by light can be predicted in the direction found experimentally (Fig. 8). However, the necessary rise of ATP concentration in the light is higher than can be anticipated in vivo. The increase of K(+) influx in the light is therefore attributed additionally to a) a hyperpolarization of the vacuolar potential in the light and b) a possible increase of the K(+) permeability in the light; further there may be c) a K(+) influx linked to ATP at a higher stoichiometry than 1/1 and/or d) an influx coupled to the light-stimulated Cl(-) influx.