Multiphasic kinetics were indicated for uptake of ammonium and potassium by intact citrus seedlings as measured by a continuous flow technique. Uptake at low concentrations, below 10"' M, was biphasic. The patterns for ammonium uptake by 60-day-old and 180-day-old seedlings were similar, but the rates of uptake per g dry weight of root were greater for the younger plants.

Seedlings of rice (Oryza sativa), soybean (Glycine max) and sour orange (Citrus aurantium) were grown for 20 to 125 days under controlled conditions in nutrient solutions wiht up to 16 different concentrations of NH4+, H2PO4‐, K+, Ca2+, Mg2+ or Zn2+. Nutrient concentrations differed by up to 4 orders of magnitude (H2PO4‐ and Zn2+) and were kept constant or within certain limits by changing solutions daily. Dry weights and concentrations of N, P, K, Ca, Mg or Zn were determined for roots and tops (or roots, stems and leaves). The relationship between tissue concentration of an element and external concentration of the corresponding nutient ion was invariably multiphasic, with phases separated by sharp breaks or jumps. The kinetics of accumulation were similar to those of short‐term uptake of the same ions. Reanalysis of previously published data (including data for Mn2+) for other plants yielded, similarly, bi‐ or multiphasic isotherms for accumulation. Accumulation patterns and growth were in several instances correlated, with separate phases coinciding with regions of poverty adjustment, luxury consumption and toxicity. Implications of multiphasic kinetics of long‐term nutrient accumulation for membrane properties, fluxes and regulation include: (i) Membranes and uptake mechanisms must remain relatively constant throughout the life of the plant with respect to affinities for ions and concentrations at which transitions occur. (ii) Rate‐limitation occurs at the plasmalemma of the root cortical cells. (iii) Uptake is at all times under multiphasic control by the external solution.

Uptake of manganese by intact citrus seedlings can be represented by three phases of a single, multiphasic isotherm in the range 10−8M–2× 10−4M. The phases are separated by marked jumps and the kinetic constants increase upon transition to higher phases.