Effect of potassium deficiency on the plasma membrane H⁺‐ATPase of the wood ray parenchyma in poplar

The effect of K + deficiency on the plasma membrane (PM) H + ‐ATPase was studied in young stems of poplar plants ( Populus tremula × tremuloides ) grown with low or full‐strength K + supply. Immunological assays using different antibodies were applied to test if K + deficiency affects the amount of immunodetectable PM H + ‐ATPases in the stem tissue. The monoclonal antibody clone 46 E5 B11 revealed an increased abundance of PM H + ‐ATPases under conditions of low K + supply, and immunolabelling experiments showed that this increase was restricted to vessel‐associated cells (VACs) of the wood ray parenchyma. Replacement of the monoclonal antibody by a polyclonal antibody against PM H + ‐ATPase gave a specific immunoreactivity on blots as well as tissue sections too, but the labelling intensity showed no difference between plants with low or full‐strength K + supply. Measurements of extracellular H + concentrations using non‐invasive, H + ‐selective microelectrodes revealed a lowering of the pH at the surface of VACs and an enhancement of net efflux of H + in plants grown with low K + supply. The present results indicate an up‐regulation of specific isoforms of the PM H + ‐ATPase in VACs under K + ‐deficient conditions and suggest a key role for these PM H + ‐ATPases in unloading K + from the xylem stream.