Does calcium ameliorate the negative effect of NaCl on melon root water transport by regulating aquaporin activity?

The hydraulic conductance (L-0) of detached, exuding root systems from melon (Cucumis melo cv. Amarillo oro) was measured. All plants received a half-strength Hoagland nutrient solution, and plants stressed either solely with NaCl (50 mM) or with NaCl (50 mM) following treatment (2 d) with CaCl2 (10 mM) were compared with controls and CaCl2-treated (10 mM) plants. The L-0 of NaCl-treated plants was markedly decreased when compared to control and CaCl2-treated plants, but the decrease was smaller when NaCl was added to plants previously treated with CaCl2. A similar effect was observed when the flux of Ca2+ into the xylem and the Ca2+ concentration in the plasma membrane of the root cells were determined. In control, CaCl2- and NaCl + CaCl2-treated plants, HgCl2 treatment (50 mu M) caused a sharp decline in L-0 to values similar to those of NaCl-stressed roots, but L-0 was restored by treatment with 5 mM DTT. However, in NaCl roots only a slight effect of Hg2+ and DTT were observed. The effect of all treatments on L-0 was similar to that on osmotic water permeability (P-t) of individual protoplasts isolated from roots. The results suggest that NaCl decreased the passage of water through the membrane and roots by reducing the activity of Hg-sensitive water channels. The ameliorative effect of Ca2+ on NaCl stress could be related to water-channel function.