The role of Ca2+, H+, and Cl− ions in generation of variation potential in pumpkin plants

The contributions of Ca 2+ , H + , and Cl − in generation of variation potentials (VP) in 3- to 4-week-old pumpkin ( Cucurbita pepo L., cv. Mozoleevskaya) plants were assessed. During VP generation, transient alkalinization of the medium around the stem was recorded with a potentiometric method. The pH changes were kinetically similar to the electric potential changes and were apparently due to temporal suppression of the plasma-membrane electrogenic H + pump. These data and the observed inhibition of VP in the stem zone treated locally with a metabolic inhibitor (NaN 3 ) indicate that the VP generation is related to the reversible suppression of the H + -pump. The anion channel blocker (ethacrynic acid) decelerated significantly the front slope of VP and reduced the VP amplitude. A short-term increase in external Cl − concentration around the stem was observed during potential transients representing the VP front slope and the pulses integrated into VP. The removal of Ca 2+ from extracellular medium inhibited the VP generation. It is proposed that Ca 2+ plays a role in activation of anion channels and in the H + -pump inactivation. The VP generation is probably determined by a complex mechanism, with contributions from passive ion fluxes (Ca 2+ , Cl − ) moving along the electrochemical gradients and from changes in the electrogenic pump activity.