Multiphasic uptake of phosphate by corn roots

Abstract

Abstract The concentration dependence of phosphate uptake was studied using root sections of corn (Zea mays L. cv. Ganga 5). Detailed and wide‐range (57 concentrations in the range 1 μmol m−3‐75 mol m−3), precise (average SEM < 2.5%, n= 6) and reproducible (similar patterns in three independent experiments and for 5, 10, 15, 20, 25 and 30°C) data revealed six (or seven) concentration‐dependent phases separated by ‘jumps’ or sharp breaks. These transitions were independent of temperature and occurred over relatively narrow concentration ranges (0.0001–0.0004, 0.08–0.31, 1.0–3.5, (7.5–10), 18–20 and 57–59 mol m−3). The intermediate phases obeyed Michaelis‐Menten kinetics, whereas sigmoidal kinetics were observed at lower concentrations. Uptake within each of the two highest phases increased more rapidly with increasing external phosphate concentration than predicted from Michaelis‐Menten kinetics but also saturated more rapidly. The latter finding is not consistent with free diffusion across the plasmalemma at high external phosphate concentrations. Kinetic models yielding continuous isotherms, e.g. the sum of one or two Michaelis‐Menten terms and a diffusion term, cannot account for the data.