Response to short-term inundation with isoosmotic solutions of seawater and sorbitol in a C4 nonhalophyte: evidence for a salt tolerance mechanism.

To examine the importance of Na+ and Cl- to osmotic adjustment in a salt-tolerant ecotype of the C4 nonhalophyte Andropogon glomeratus, plants were watered with sorbitol, a neutral osmoticum, and synthetic seawater, for five days. Gas exchange measurements were made during the course of the watering treatment and during a recovery period following treatment. Leaf osmotic adjustment occurred only in plants watered with seawater, and was associated with an increase in Na+ and Cl- concentrations. Estimates of the molar concentrations indicated these ions could account for 95% of the leaf osmotic adjustment. Net photosynthetic CO2 uptake was less effected during the watering treatment, and photosynthetic recovery was greater following the treatment in plants watered with seawater. Photosynthetic inhibition was related primarily to metabolic factors, including a decrease in carboxylation efficiency. A model is presented for a mechanism promoting tolerance to transient seawater inundation in A. glomeratus.