The regulation of potassium influx into intact roots of barley by internal potassium levels

Abstract

Isotherms were obtained for the influx of K⁺ into roots of intact barley (Hordeum vulgare (L.) cv. Conquest) plants which had previously been grown in either 0.5 mM CaSO₄ solution or 0.5 mM CaSO₄ plus 5 mM KGl solution. K_m and V_max values obtained from these influx data were 0.03 ± 0.003 mM and 10.8 μmol g⁻¹ h⁻¹ respectively for the low-K⁺ plants and 0.1 ± 0.01 mM and 1.74 ± 0.08 μmol g⁻¹ h⁻¹ for the high-K⁺ plants.When the K⁺ content of low-K⁺ plants was increased, by pretreating the roots of intact plants in 10 mM KCl plus 0.5 mM CaSO₄ solution, K⁺ influx values, as determined from 0.1 mM KCl plus 0.5 mM CaCl₂ solutions, declined continuously during the 12 h of pretreatment. Hill plots of this data revealed a consistent biphasic response to internal K⁺. Below 50 μmol K⁺ g⁻¹ the Hill coefficient was close to unity. Above 50 μmol g⁻¹ the Hill coefficient increased to values close to 3. The time, or root K⁺ concentration, at which this change in Hill coefficient occurred corresponded to the first indications of net transfer of K⁺ to the shoot.