Sodium, but not potassium, enhances root to leaf nitrate translocation in Swiss chard (Beta vulgaris var. cicla L.)

• We investigated the effects of Na+ or K+ on NO3− uptake in Swiss chard.
• Leaf sap and xylem sap NO3− was accumulated more in Na than in K treatment.
• K+ enhanced Cl– translocation from root to leaf.
• Na+enhances NO3− uptake, which relates to the root to leaf xylem translocation.

Salinity is known to reduce NO3− uptake in many plants. Our previous study found that the increasing NaCl concentrations in media enhanced NO3− uptake in leaves. Here, we investigated whether nitrate (NO3−) uptake by Swiss chard (Beta vulgaris var. cicla L.) was affected more by Na+ than by K+, and elucidated which NO3− uptake processes were related to the application of these ions in the culture medium. Plants were grown hydroponically and subjected to 5 mM or 50 mM of either K or Na treatments for 7 days: K5 (K 5, Na 0), K50 (K 50, Na 0), Na5 (K 0, Na 5), and Na50 (K 0, Na 50). The leaf NO3− concentration was significantly enhanced in the Na treatments compared with the K treatments. The ratio of leaf to root NO3− concentrations in K5 and Na5 were 0.46 and 1.59, respectively, which indicates that NO3− is translocated to the leaves in the Na treatments. The NO3− concentration in the xylem sap, collected 6 h after the start of the treatments, was significantly higher in Na50 than in the other treatments. These are the opposite of the results for Arabidopsis  , which accumulates NO3− in its roots to increase its salinity tolerance. Our results suggest that Na but not K enhances NO3− uptake and transport to the leaves in Swiss chard and that its function may relate to xylem translocation of NO3−.