Immediate and steady state extracellular ionic fluxes of growing Arabidopsis thaliana root hairs under hyperosmotic and hypoosmotic conditions

Osmotic‐induced shifts in extracellular ionic fluxes at the apex of growing Arabidopsis thaliana L. root hairs were examined using the extracellular self‐referencing (vibrating) ion‐selective probe technique. With either APW 7 (artificial pond water, pH 7) or APW 7 plus 200 m M mannitol/sorbitol, a steady state was reached in which growth rates were the same (0.75 and 0.82 µm min⁻¹) as were the net fluxes of calcium (inward), potassium (outward), and chloride (inward). The outward flux of protons was about 20‐fold larger in APW 7 plus 200 m M mannitol/sorbitol compared to APW 7 ( P = 0.028). Significant changes in ionic fluxes within 5 min of osmotic changes were observed for hyperosmotic treatment: a 7.5‐fold increase in the inward calcium flux ( P = 0.041) and a 1.7‐fold increase in the outward potassium flux ( P = 0.007). Although the change in the calcium flux was consistent with a process of osmotic adjustment (and not a consequence of binding/release from the cell wall), the magnitude of the flux was considerably less than the potassium efflux. Unlike the situation in other organisms where volume regulation explicitly relies upon modulation of ionic fluxes (especially sodium, potassium and chloride), the root hairs may rely on other mechanisms of osmotic regulation, possibly coupled to differences in proton efflux under steady state growth conditions. The root hairs do exhibit an osmotic‐induced electrical signal (R. R. Lew. 1996. Plant Physiol. 112: 1089–1100) which may be a component of initial signal transduction controlling osmotic regulation.