Fast water uptake and outflow and electrophysiological responses of roots induced by changes in salinity, osmotic pressure, and pH of external solution

Abstract

Fast reversible water losses by roots of intact seedlings of tomato (Lycopersicon esculentum L.) and sunflower (Helianthus annuus L.) in response to changes in composition and concentration of external medium were studied with a highly sensitive fast-response gravimetric method. The dehydrating effect of solutes, applied at low and moderate concentrations (5–100 mM), increased in the following row: ethanol < glycerol < sucrose ≪ neutral salts < base < acid. At concentrations below 10 mM, nonelectrolytes did not cause significant water losses from roots. Neutral salts had a characteristic gradual effect in a wide range of concentrations (0.3–500 mM NaCl). Amplitudes of gravimetric responses to treatments with bases and acids were 1.5–2 times higher (NaOH) and more than 3 times higher (HCl) than the response to equimolar concentration (5 mM) of a neutral salt. In all cases the water loss from roots was fast, reversible, and well reproducible. The presence of electrical charge (ions vs. neutral molecules) was crucial for the strength of the solute effect, especially at low concentrations. In parallel experiments with tomato seedlings, fast kinetics of electric potential difference between the root and the hypocotyle (electrophysiological response) was measured after a change in the composition and concentration of external solution. Possible mechanisms of observed phenomena are discussed.