Osmotic adjustment and energy limitations to plant growth in saline soil.

Plant roots must exclude almost all of the Na and Cl in saline soil while taking up water, otherwise these ions would build up to high concentrations in leaves. Plants evaporate c. 50 times more water than they retain, so 98% exclusion would result in shoot NaCl concentrations equal to that of the external medium. Taking up just 2% of the NaCl allows a plant to osmotically adjust the Na and Cl in vacuoles, while organic solutes provide the balancing osmotic pressure in the cytoplasm. We quantify the costs of this exclusion by roots, the regulation of Na and Cl transport through the plant, and the costs of osmotic adjustment with organic solutes in roots.