Effect of cations on IAA-induced proton excretion in the xylem of Vigna unguiculata

Perfusion of IAA through the xylem of a hypocotyl segment of Vigna unguiculata L. cv. Otsubu hyperpolarized the boundary membrane between the xylem and the symplast followed by acidification of the xylem exudate. K⁺, Ca²⁺ or Mg²⁺ enhanced IAA-induced acidification of the xylem exudate. Cation-induced acidification in IAA-treated segment was observed even under anoxia. However, Na⁺ had only a small effect on the IAA-induced acidification. Thus, the acidification enhanced by cations may not be due to the stimulation of a plasmalemma proton pump but originates in the cation/proton exchange on the cell wall. Apparently, IAA stimulates the electrogenic proton-pump at the plasmalemma that excretes protons to the cell wall apoplast. The cell wall acts as a buffer, however, to hide the proton-excretion unless the protons are exchanged with other cations. This may be the reason why IAA-induced acidification of the bathing medium was not always observed when tested on several plant tissues.