Evaluation of Arsenate- and Vanadate-Associated Changes of Electrical Membrane Potential and Phosphate Transport inLemna gibbaG1

Interference of arsenate and vanadate with phosphate uptake in Lemna gibba L. was studied by measuring voltage changes and (³²P)phosphate uptake. Arsenate proved to be competitive with the high- and low-affinity phosphate uptake system. It induced transient membrane potential changes of up to 120 mV which were similar to those induced by phosphate and indicated a cotransport mechanism with at least 2H+/H₂As$O4-$. The amplitude of the transient arsenate-induced membrane depolarization was strongly influenced by phosphate starvation. A permanent membrane depolarization to the diffusion potential was achieved within 2 to 6 h in P-starved plants. Thus, arsenate is indeed a strongly competitive physiological analogue of phosphate in higher plants. Vanadate was easily transported into L. gibba as concluded from transient E_m changes of up to 110 mV. Vanadate interfered only slightly and non-specifically with the two phosphate transport mechanisms. Like phosphate, vanadate uptake seems to be an H+-cotransport mechanism, both with similar optima at pH 6.0. Unlike phosphate uptake, vanadate-linked membrane depolarization was not affected by high intracellular phosphate concentrations. P-starvation did not enhance the weak long-term effect on E_m. Hence, vanadate, in contrast to arsenate, is not regarded as a physiological phosphate analogue. The distinct and rapid vanadate-induced and permanent membrane depolarization of Avena sativa, Triticum aestivum and Glycine max leaves was not seen in Lemna nor in leaves of Gossypium hirsutum and Nicotiana tabacum. Plasmalemma-enriched preparations of L. gibba revealed, however, a high vanadate-sensitive ATPase activity (87%). As a possible explanation for these differences it is suggested that the latter plant species have cytosolic vanadate-detoxifying properties, i.e. they can reduce vanadate to vanadyl ions, in contrast to the former group of plant species.