INFLUENCE OF PHOSPHATE STATUS ON PHOSPHATE-UPTAKE KINETICS OF MAIZE (ZEA - MAYS) AND SOYBEAN (GLYCINE-MAX)

To obtain plants of different P status, maize and soybean seedlings were grown for several weeks in flowing nutrient solution culture with P concentrations ranging from 0.03-100 μmol P L-1 kept constant within treatments. P uptake kinetics of the roots were then determined with intact plants in short-term experiments by monitoring P depletion of a 3.5 L volume of nutrient solution in contact with the roots. Results show maximum influx, Imax, 5-fold higher in plants which had been raised in solution of low compared with high P concentration. Because P concentrations in the plants were increased with increase in external P concentration, Imax was negatively related to % P in shoots. Michaelis constants, Km, were also increased with increased pretreatment P concentration, only slightly with soybean, but by a factor of 3 with maize. The minimum P concentration, Cmin, where net influx equals zero, was found between 0.06 and 0.3 μmol L-1 with a tendency to increase with pretreatment P concentration. Filtration of solutions at the end of the depletion experiment showed that part of the external P was associated with solid particles. It was concluded that plants markedly adapt P uptake kinetics to their P status, essentially by the increase of Imax, when internal P concentration decreases. Changes of Km and Cmin were of minor importance. © 1990 Kluwer Academic Publishers.