Sodium enhances nitrate uptake in Swiss chard (Beta vulgaris var. cicla L.)

Swiss chard (Beta vulgaris var. cicla L.) is a halophilic plant, which has increased growth under moderate salinity. The stimulation of total nitrogen (N) concentration was also observed with the increase of sodium chloride (NaCl) levels in previous studies. The purpose of this study was to investigate whether the stimulation of N uptake is the result of growth stimulation or the direct effect of NaCl. We compared the effects of NaCl on nitrate (NO3-) uptake and nitrate reductase (NR) activity in halophilic Swiss chard with those in salinity-tolerant barley (Hordeum vulgare L.). Both species were grown hydroponically at NaCl concentrations ranging from 0 to 100mmolL(-1). Leaf dry weight of both species was not significantly affected by the treatments. In barley extracted leaf sap, the NO3- concentration was positively correlated with the potassium ion (K+) concentration, but negatively correlated with the sodium ion (Na+) concentration. In contrast, in Swiss chard extracted leaf sap, the Na+ and NO3- concentrations were strongly and positively correlated. High salinity enhanced NR activity in leaves of Swiss chard, but decreased that in barley. Since NO3- is a substrate of NR, increased NO3- uptake may explain the increased NR activity. We found that the effect of NaCl on NO3- uptake in halophilic Swiss chard and salinity-tolerant barley was completely different. We concluded that the NO3- uptake was enhanced directly by Na+ in Swiss chard.