Salinization of the soil solution decreases the further accumulation of salt in the root zone of the halophyte Atriplex nummularia Lindl. growing above shallow saline groundwater

Water use by plants in landscapes with shallow saline groundwater may lead to the accumulation of salt in the root zone. We examined the accumulation of Na+ and Cl- around the roots of the halophyte Atriplex nummularia Lindl. and the impacts of this increasing salinity for stomatal conductance, water use and growth. Plants were grown in columns filled with a sand-clay mixture and connected at the bottom to reservoirs containing 20, 200 or 400mM NaCl. At 21d, Na+ and Cl- concentrations in the soil solution were affected by the salinity of the groundwater, height above the water table and the root fresh mass density at various soil depths (P<0.001). However, by day 35, the groundwater salinity and height above the water table remained significant factors, but the root fresh mass density was no longer significant. Regression of data from the 200 and 400mM NaCl treatments showed that the rate of Na+ accumulation in the soil increased until the Na+ concentration reached 250mM within the root zone; subsequent decreases in accumulation were associated with decreases in stomatal conductance. Salinization of the soil solution therefore had a feedback effect on further salinization within the root zone. If halophytes growing above shallow saline groundwater transpire the water but leave the salt behind in the soil, then won't the plants eventually die from salt accumulation around the roots? We grew Atriplex nummularia in columns of soil above groundwater of 20-400mM NaCl. We found that the plants regulated their exposure to root salinity through stomatal closure. Thus, salinization of the soil solution had a feedback effect on further salinization within the root zone.