Influx and efflux of p in roots of intact maize plants : double-labeling with p and p.

Rates of P influx and efflux were determined in whole plants at ambient P concentrations comparable to those found in soil solutions. Maize (Zea mays L. var NC+59) seedlings were trimmed (endosperm and adventitious roots removed) and grown in a greenhouse in solution cultures at P concentrations of approximately 0.4 and 1.8 micromolar. Roots of intact plants previously exposed to (32)P-labeled solutions at 0.2 and 2.0 micromolar P for 48 hours were rinsed 10 minutes in P-free solution and exposed to (33)P solutions at 0.2 and 2.0 micromolar for 10 minutes. Net depletion of (33)P from and appearance of (32)P in the ambient solution were used to measure influx and efflux. The ration of (32)P efflux to (33)P influx was about 0.68 at 0.2 micromolar and 0.08 at 2.0 micromolar. When plants were allowed to deplete P from solutions, the P concentration in the medium dropped to about 0.15 micromolar within 24 hours and 0.05 micromolar within 60 hours. Results indicate that P efflux is a substantial component of net P accumulation at P concentrations normally found in soil solutions.