The relationship between the external concentration of a nutrient ion or other solute and the rate of its uptake by roots or other plant tissues remains controversial.
Does uptake of a particular nutrient occur by one or two mechanisms and is there also free diffusion across the plasma membrane? In the case of sufficiently detailed and precise data it will be shown that models consisting of the sum of one or two Michaelis-Menten terms and a diffusion term cannot account for the uptake.
The multiphasic model, which states that uptake of an ion or molecule is mediated by a single mechanism which changes characteristics at certain discrete external nutrient concentrations, is in contrast consistent with a large body of data for nutrient uptake in a variety of plants and tissues.
Long-term uptake (accumulation) of nutrients by plants is generally taken to remain constant above a certain low external nutrient concentration.
Detailed data for nutrient accumulation (expressed as tissue concentration) by seedlings of rice, soybean and sour orange were, however, invariably multiphasic and similar to those for short-term uptake of the same ions.
The properties of multiphasic uptake mechanisms differ at low and high nutrient concentrations, not only with respect to the values of Km and Vmax but also with respect to metabolic dependence (active uptake at low concentrations, facilitated diffusion at high concentrations) and regulation by internal ion concentration (negative feedback at low external ion concentrations, no regulation at high concentrations). An understanding of multiphasic kinetics in molecular terms is still lacking, but it has been shown that phase transitions are caused by interaction of ions or solute molecules with a separate transition site which, in contrast to the uptake site, is only accessible from outside the plasma membrane.
It seems that the shape of the uptake isotherms (graphs relating uptake rate and external nutrient concentration) should be considered in the formulation of nutrient solutions.
Especially interesting is the finding of an essentially concentration-independent phase for uptake of nitrate and other ions.