Regulation of water channel activity in whole roots and in protoplasts from roots of melon plants grown under saline conditions

Measurements of the hydraulic conductance (L-0) of roots of melon plants (Cucumis melo L.) derived from roots grown under saline conditions were performed to determine the effect of NaCl and which ion, Na+ or Cl-, is involved. Root hydraulic conductance of plants treated with a 50 mM NaCl, 47 mM Na+ or 45 mM Cl- salts mixture was reduced, but the reduction was less when 10 mM CaCl2 was added before the salts, except in the case of the Cl- salt mixture. Only when CaCl2 was applied before NaCl was there an ameliorative effect on L-0 (25.8% increase). Addition of HgCl2 reduced the L-0 of control plants, but the reduction progressively decreased as the NaCl concentration was increased (from 0 to 50 mM). Osmotic water permeability (Pf) values were calculated in root protoplasts treated with 90 mM NaCl. Large reductions were observed with the NaCl treatment (10.38 mu m s(-1) for the control and 3.31 mu m s(-1) for the NaCl treatment). In addition, Pf measurements were carried out for protoplasts treated with 100 mM NaCl plus the phosphatase inhibitor, okadaic acid (5 mu M). The effect of okadaic acid on P-f values before and after NaCl addition was similar (6.61 and 7.01 mu m s(-1), respectively), showing a smaller decrease of P-f than with NaCl alone with respect to control protoplasts. The results showed that the negative effect of NaCl on water channel activity was not due to a high ion concentration effect on channel pores or to the increase in osmotic pressure. We suggest that it was due to a direct action of NaCl on protein regulation.