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 s elf-referencing (vibrating) ion-selective probe technique. With either APW 7 (artificial pond water, pH 7) or APW 7 plus 200 mM mannitol/sorbitol, a s teady state was reached in which growth rates were the same (0.75 and 0.82 mu m min(-1)) as were the net fluxes of calcium (inward), potassium (outwar d), and chloride (inward). The outward Aux of protons was about 20-fold lar ger in APW 7 plus 200 mdl mannitol/sorbitol compared to APW 7 (P = 0.028). Significant changes in ionic fluxes within 5 min of osmotic changes were ob served for hyperosmotic treatment: a 7.5-fold increase in the inward calciu m 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 pr ocess of osmotic adjustment (and not a consequence of binding/release from the cell wall), the magnitude of the flux was considerably less than the po tassium efflux. Unlike the situation in other organisms where volume regula tion explicitly relies upon modulation of ionic fluxes (especially sodium, potassium and chloride), the root hairs may rely on other mechanisms of osm otic regulation, possibly coupled to differences in proton efflux under ste ady state growth conditions. The root hairs do exhibit an osmotic-induced e lectrical signal (R. R. Lew. 1996. Plant Physiol. 112. 1089-1100) which may be a component of initial signal transduction controlling osmotic regulati on.