Invalidity of the multiphasic concept of ion absorption in plants

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Abstract
According to the multiphasic concept uptake isotherms consist of a number of successive Michaelis‐Menten phases separated by discontinuous transitions. In this paper evidence is provided that the concept is most probably due to an erroneous interpretation of continuous uptake isotherms that yield non‐linear, concave downward Lineweaver‐Burk plots. With reference to the isotherm of sulphate uptake in barley roots (Nissen, 1971) uptake isotherms were simulated in which the uptake rates, computed from the sum of two Michaelis‐Menten terms and a linear term, contained a 4% (or 5%) random error. The same arguments that have been adduced in support of the multiphasic concept would also lead to a multiphasic interpretation of the simulated isotherms. The conclusions concerning a multiplicity of transition points and the multiphasic concept follow from the particular form of statistical test that had been applied rather than from the set of data itself. Characteristics that have been noted in multiphasic uptake appear to follow simply from the multiphasic interpretation of uptake isotherms that yield non‐linear, concave downward Lineweaver–Burk plots.