Does the Ratio of Na+ and K+ Affect the Hydrolytic and Pumping Activity of the Plasma Membrane H+-Atpase from Maize (Zea Mays L.) Shoot?

Salinity limits plant growth and impairs agricultural productivity. In the first phase of salt stress, growth of plants is impaired predominantly by osmotic stress and only slightly by Na+ effects. The aim of our work was to investigate whether Na+ affects both hydrolytic and pumping activity of the plasma membrane H+ ATPase. Sodium salinity leads to a shift of the Na+/K+ ratio resulting in a displacement of K+ in the plant. It was shown that high cytoplasmatic Na+ concentrations and low K+ concentrations have an effect on the H+ ATPase activity of the plasma membrane. Reduced H+-ATPase activities caused a reduction of the acidification of the apoplast. This process limits the cell-wall extensibility and thus reduces growth after salt stress. For this reason, H+-ATPase activity, which is important for cell elongation was measured under the influence of different Na+/K+ ratios. Plasma membrane was isolated from two days old maize shoots using the aqueous polymer two-phase technique. ATPase activity was determined by measuring the release of Pi. The H+-pumping activity was revealed by absorbance quenching of acridine orange. Identical plant shoot material was used for ATPase extraction and effects of Na+ and K+ were tested in vitro. High concentration of 100 mM Na+ decreased the hydrolytic H+-ATPase activity to values of 80%, in comparison to high concentration of K+ (100 mM) without Na+. However, under comparable conditions the H+ pumping activity was decreased to 30%.