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 pho sphate stress. The most severely stressed plants absorbed phosphate at twice the rate for plants grown in adequate phosphate. Characteristic s of phosphate uptake, membrane electrical potentials (Delta E(vo)), a nd net efflux of protons (phi H-net(+)) were measured. Addition of pho sphate caused an immediate but transient depolarization of Delta E(vo) in phosphate-deficient but not phosphate-adequate plants which sugges ts a difference between plants of different phosphate status in the st oichiometry of the H+/H2PO4- symport. On addition of phosphate, phi H- net(+) increased in phosphate-deficient plants but decreased in phosph ate-adequate plants. The effects of N-ethylmaleimide and vanadate on p hosphate 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-ne t(+) were similar for phosphorus-defficient and phosphorus-adequate pl ants. These results indicate that there are qualitative differences in the dominant mechanisms of phosphate absorption between different lev els of phosphate stress.