Phosphorus uptake kinetics, size of root system and growth of maize and groundnut in solution culture

Phosphorus acquisition efficiency of maize (Zea mays L.) and groundnut (Arachis hypogaea L.) was investigated in a flowing nutrient solution culture at constant P concentrations of 0.2, 1 and 100 muM. To calculate the P influx and study changes in plant growth and P uptake in relation to plant age, four harvests were taken. Phosphor-us uptake kinetics of the roots, i.e. maximum influx, I-max, the Michaelis constant, K-m, and the minimum concentration, C-Lmin (the concentration at which no net uptake occurs) were estimated in a series of short-term experiments, based on the rate of depletion of P from solution over a range of concentrations. At 1 muM P, maize was more P efficient producing up to 90% of its maximum yield as compared to groundnut with only 20% of maximum yield. A 3 times faster P uptake rate was the reason for the maize P efficiency. In contrast for groundnut at 1 muM P, a net efflux was observed at some development stages of this crop indicating a much higher P requirement at the root surface for maximum growth. Maize had a 6 times higher I-max value and a 2 times higher K-m value as compared to groundnut. The higher influx of maize was mainly because of the higher I-max. Maize previously grown at low P concentrations had a C-Lmin of 0.1 muM, while groundnut had values of 0.2 and 0.6 muM. Furthermore groundnut previously grown at 100 muM, was not able to absorb P even at 40 muM. Acclimation to low P concentrations in solution by increasing I-max or decreasing K-m was not evident in this study. Differences in P acquisition efficiency between maize and groundnut in solution culture were mainly because of differences in P-uptake kinetics, and to a lesser extent to the size of the root system.