Characteristics of Na+ uptake in sugar beet (Beta vulgaris L.) seedlings under mild salt conditions

Salinity is one of the major environmental factors, which limits crop productivity worldwide. To investigate sodium (Na+) uptake pathways in sugar beet (Beta vulgaris L.) under mild salt conditions, in the present work, Na+ and potassium (K+) accumulation, Na+/K+ ratio, and Na+ and K+ net uptake rate in plants exposed to various concentrations of NaCl (0-50 mM) were analyzed in the absence or presence of KCl (10 and 50 mM) and K+ channel inhibitors Tetraethylammonium-Cl (TEA(+), 5 and 10 mM), CsCl (Cs+, 3 and 6 mM) and BaCl2 (Ba2+, 5 and 10 mM). The results showed that high concentration (50 mM) of KCl significantly reduced Na+/K+ ratios in shoot and root of sugar beet in the absence or presence of NaCl. 10 or 50 mM KCl also decreased Na+ net uptake rate, or had no effects on it at 5, 10, and 50 mM NaCl, while enhanced K+ net uptake rate with external NaCl concentration at 5 and 25 mM. It seemed that high external K+ levels could maintain lower Na+/K+ ratio in sugar beet by enhancing K+ uptake and restricting Na+ uptake. Both 5 and 10 mM TEA(+), which are considered to be a blocker of K+ channels, had no significant effects on net uptake rates of Na+ and K+ in sugar beet in the absence or presence of NaCl. However, 3 or 6 mM Cs+, which is also known to be an inhibitor of the K+ inward-rectifying channel (AKT1), led to significant reduction of K+ net uptake rate but did not affect Na+ net uptake rate in the presence of NaCl. 5 or 10 mM Ba2+, which is known as another blocker of K+ channel and transporter (HKT), not only reduced Na+ net uptake rate but also decreased K+ net uptake rate (except at 25 mM NaCl) in sugar beet at 5-50 mM NaCl. It is clear that Na+ uptake in sugar beet is very sensitive to Ba2+ but insensitive to TEA(+) or Cs+, and that K+ uptake is sensitive to Cs+ or Ba2+, whereas it is insensitive to TEA(+). We proposed that the AKT1 may mediate K+ uptake and HKT1 may mediate Na+ uptake in sugar beet at 5-50 mM NaCl.