Water relations and xylem transport of nutrients in pepper plants grown under two different salts stress regimes

Two iso-osmotic concentrations of NaCl and Na2SO4 were used for discriminating between the effects of specific ion toxicities of salt stress on pepper plants (Capsicum annuum L.) grown in hydroponic conditions, in a controlled-environment greenhouse. The two salts were applied to plants at different electrical conductivities, and leaf water relations, osmotic adjustment and root hydraulic conductance were measured. Leaf water potential (Psi(w)), leaf osmotic potential (Psi(o)) and leaf turgor potential (Psi(p)) decreased significantly when EC increased, but the decrease was less for NaCl- than for Na2SO4-treated plants. The reduction in stomatal conductance was higher for NaCl- treated plants. There were no differences in the effect of both treatments on the osmotic adjustment, and a reduction in root hydraulic conductance and the flux of solutes into the xylem was observed, except for the saline ions (Na+, Cl- and SO42-). Therefore, pepper growth decreased with increasing salinity because the plants were unable to adjust osmotically or because of the toxic effects of Cl-, SO42- and/or Na+. However, turgor of NaCl- treated plants was maintained at low EC (3 and 4 dS m(-1)) probably due to the maintenance of water transport into the plant (decrease of stomatal conductance), which, together with the lower concentration of Na+ in the plant tissues compared with the Na2SO4 treatment, could be the cause of the smaller decrease in growth.