Water relations and osmotic adjustment in Apium graveolens during long-term NaCl stress and subsequent relief

To investigate how the regulation of plant water status contributes to salt tolerance in celery (Apium graveolens L. cv. Istar) we determined growth, leaf gas exchange and water relations in hydroponically-grown plants supplied with nutrient solutions containing 5 (control), 50, 100 and 300 mM NaCl for 4 weeks, and supplied subsequently with the control solution to relieve salt stress. At salinity levels of 50 and 100 mM NaCl there was a moderate, albeit significant, reduction in biomass production. A drastic decrease in both dry weight and leaf area occurred at 300 mM NaCl, but the plants remained viable and showed appreciable photosynthesis and growth. Leaf water potential, turgor pressure, stomatal conductance and plant hydraulic conductance (L-p) decreased during salt exposure. After the first week of stress there was a progressive increase in leaf turgor and L-p. which was associated with substantial recovery of stomatal conductance and photosynthesis in 50 and 100 mM NaCl-treated plants. The analysis of the pressure-volume isotherms showed that osmotic adjustment had occurred under all NaCl treatments. Leaf osmotic adjustment in salt-stressed plants was primarily accomplished by accumulation of Na+ and Cl-. Upon relief of stress, salinized plants rehydrated and regained turgor rapidly, and growth and photosynthesis resumed almost fully. These results indicate that maintenance of a favourable water status contributes substantially to the salt resistance of celery.