Ion Fluxes and Phytochrome Protons in Mung Bean Hypocotyl Segments

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Abstract
K+[86Rb+] uptake by P. aureus Roxb. hypocotyl segments cut immediately below the hook is inhibited by the active form of phytochrome (Pfr). Short load-short wash experiments indicate that the inhibition of uptake occurs across the plasmalemma. A maximal inhibition of short-term uptake occurs in 10-50 mM KCl. Low temperature had only a small effect on influx and the inhibition of influx from 50 mM KCl. A consideration of the electrochemical gradient for K+ suggests that passive K+ fluxes may predominate under these conditions. Red light induces small depolarizations of membrane potential in subhook cells. Far red light antagonizes this effect. Pfr inhibits efflux of K+[86Rb+] from subhood segments. This effect is also relatively insensitive to low temperature. This inhibition of efflux may reflect inhibition of a K+-K+ exchange process, or reduced passive permeability of the plasmalemma to K+. In contrast, Pfr enhances short-term uptake of K+ [86Rb+] in apical hypocotyl hook segments of P. aureus Roxb. Short load-short wash experiments indicate that fluxes across the plasmalemma are modified by Pfr. A maximal enhancement of short term influx occurs in 50 mM KCl. Influx and the red light enhancement of influx from 50 mM KCl are relatively insensitive to low temperature. Pfr also enhances efflux of K+[86Rb+] from preloaded apical hook segments. This increased influx may reflect enhancement of a K+-K+ exchange process or increased passive permeability of the plasmalemma to K+.
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