PHOSPHATE-UPTAKE, PROTON EXTRUSION AND MEMBRANE ELECTROPOTENTIALS OF PHOSPHORUS-DEFICIENT TRIFOLIUM-REPENS L

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 (Delta E(vo)), and net efflux of protons (phi H-net(+)) were measured. Addition of phosphate caused an immediate but transient depolarization of Delta E(vo) in phosphate-deficient but not phosphate-adequate plants which suggests a difference between plants of different phosphate status in the stoichiometry of the H+/H2PO4- symport. On addition of phosphate, phi H-net(+) increased in phosphate-deficient plants but decreased in phosphate-adequate plants. The effects of N-ethylmaleimide and vanadate on phosphate absorption and phi H-net(+) were also found to depend on the phosphorus status of the plants. However the effects of pH on phi H-net(+) were similar for phosphorus-defficient 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.