Phosphate Uptake, Proton Extrusion and Membrane Electropotentials of Phosphorus-DeficientTrifolium repensL

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Abstract
Trifolium repens plants were grown to produce a range of levels of phosphate stress. The most severely stressed plants absorbed phosphate at twice the rate for plants grown in adequate phosphate. Characteristics of phosphate uptake, membrane electrical potentials (ΔEvo), and net efflux of protons (φH+net+) were measured. Addition of phosphate caused an immediate but transient depolarization of ΔEvo in phosphate-deficient but not phosphate-adequate plants which suggests a difference between plantsof different phosphate status in the stoichiometry of the H+/H2PO4− symport. On addition of phosphate, φHnet+increased in phosphate-deficient plants but decreased in phosphate-adequate plants. The effectsof N-ethylmaleimide and vanadate on phosphate absorption and φHnet+ were also found to depend on the phosphorus status of the plants. However the effects of pH on φHnet+ were similar for phosphorus-deficient and phosphorus-adequate plants. These results indicate that there are qualitative differences in the dominant mechanisms of phosphate absorption between different levels of phosphate stress.

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