The influence of monovalent cations upon influx and efflux of Ca 2+ in barley roots

Abstract

The effects of monovalent cations, inhibitors of metabolism dinitrophenol (DNP), carbonyl cyanide- p-trifluoromethoxyphenylhydrazone (FCCP), and KCN and temperature variations upon Ca 2+ fluxes in intact roots of barley ( Hordeum vulgare L. cv. Fergus and Herta) seedlings were investigated. 45Ca 2+ influx was depressed in CaSO 4-grown (low-salt) plants by the presence of NH 4 +, K +, or Na + in the uptake medium. In contrast Ca 2+ influx was slightly increased by Li +. In low-salt roots pretreated with KCN and in roots preloaded with K + (high-K + plants), the presence of K + in the medium had no significant effect on Ca 2+ influx, while in roots preloaded with Na +, the presence of K + in the medium depressed Ca 2+ influx. In absolute terms, Ca 2+ influx was significantly greater in high-salt (both K + or Na + preloaded) than in low-salt roots. Patterns of 45Ca 2+ efflux in the absence and in the presence of K +, NH 4 +, or Li + in the external medium showed that these monovalent cations caused stimulation of 45Ca 2+ efflux both from the cytoplasmic and vacuolar phases. It was noted that these modifications of Ca 2+ fluxes by monovalent cations are transient and characteristic of a transitional stage of cation uptake by low-salt roots. We conclude that, together with stimulated active H + efflux (another characteristic of this transitional stage), modifications of Ca 2+ fluxes during monovalent cation uptake by low-salt roots is a response directed towards the maintenance of electrical neutrality. Determination of net fluxes revealed that the plants were close to Ca 2+ flux equilibrium in the growth medium (0.5 mM CaSO 4). Transfer of these plants to 0.5 mM CaSO 4 + 0.25 mM K 2SO 4 caused a net release of CA 2+ into the external medium.