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

Abstract

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.