Proton extrusion by wheat roots exhibiting severe aluminum toxicity symptoms

The mechanisms of Al rhizotoxicity are not known, but disruption of membrane function has been a persistent hypothesis. The objective of this study was to establish whether cells of Al-cultured wheat roots (Triticum aestivum L. cv Tyler) exhibiting severe Al toxicity symptoms were capable of vigorous proton extrusion. The membrane electrical potential difference (E(m)) was measured in individual cells throughout the first centimeter of root tips during perfusion with Al solutions similar to or more concentrated than those of the culture medium. For both Al-cultured and control roots the resting E(m) was -100 millivolts, and 1 millimolar acetic acid induced cyanide-sensitive hyperpolarizations to -180 millivolts at a maximum rate of -30 millivolts per minute. Al, like Ca(2+), enhanced the negativity of the E(m) of cells already treated with acetic acid. Both acetic acid and fusicoccin stimulated net proton extrusion from Al-cultured and control roots, both of which also extruded protons in the absence of these stimulants. These results demonstrate that wheat roots exhibiting severe Al toxicity symptoms had an undiminished capacity to extrude protons, that the membranes were intact, and that ATP synthesis was sufficient to supply the proton-translocating ATPases.