Osmotic sensitivity of Ca²⁺ and H⁺ transporters in corn roots: Effect on fluxes and their oscillations in the elongation region

Seedling roots of cam were treated with different concentrations of mannitol-containing solution for 1 to 1.5 hr, and net fluxes of Ca²⁺ and H⁺ were measured in the elongation region. H⁺ fluxes were much more sensitive to osmotic pressure than were Ca²⁺ fluxes. Oscillations of 7-min period in H⁺ flux, normally observed in the control, were almost fully suppressed at high osmotic concentrations. Net H⁺ flux was shifted from average efflux of 25 ± 3 nmol m^-2 sec^-1 to average influx of 10 ± 5 nmol m^-2 sec^-1 after the incubation in 100 mM mannitol. The larger the osmotic concentration, the larger was the H⁺ influx. This flux caused the unbuffered solution of pH 4.85 to change to pH 5.3 after mannitol application, It appears that the osmoticum suppresses oscillatory H⁺ extrusion at the plasma membrane. Discrete Fourier Transforms of the H⁺ flux data showed that, apart from suppression of the 7-min oscillations in H⁺ flux, mannitol also promoted the appearance of faster 2-min oscillations. Ca²⁺ influx slightly increased after mannitol treatment. In addition the 7-min oscillatory component of Ca²⁺ flux remained apparent thereby showing independence of H⁺ flux.