Multiphasic Uptake of Potassium by Corn Roots

Abstract

Concentration-dependence data (LV Kochian, WJ Lucas Plant Physiol 1982 70: 1723-1731 [1983] 73: 208-215; [1985] [429][430][431][432][433][434][435][436]; LV Kochian, J Xin-zhi, WJ Lucas [1985], 79: 771-776) for potassium uptake by com roots have been claimed to be resolvable into a saturable and a linear component representing two separate uptake mechanisms. A comprehensive analysis shows, however, that the data are not compatible with this concept. They can, in contrast, be precisely represented by multiphasic isotherms, in agreement with many findings for uptake of potassium and other solutes. Solute uptake in plants is proposed to be mediated by multistate entities having camer-like properties at low extemal solute concentrations and channel-like properties at high concentrations.in ice-cold CaSO4 + 1 mm RbCl) periods, thus yielding values which represent plasmalemma influx (6). AnalysisThe function for the sum of a saturable and a linear component (v = VmaxS/(Km + S) + kS) was fitted to the uptake rates (weighted by l/v2) by a computer program for nonlinear least squares (1). The individual phases in the multiphasic function (v = VmaxjS/(Kmj + S), where j denotes the phase number) was fitted by a program minimizing deviations in log v (cf. Ref. 2). Significance of fit to adjacent phases was determined by an F-test (20).As a measure of goodness of fit, MSD' was used:Despite extensive studies over a quarter of a century, there is still no consensus about the number and nature of uptake mechanisms in plants (13,17,21). Recently, the existence of a 'linear component' has been widely accepted, in part on the basis of work (6-9) purporting to show that uptake of potassium by corn roots can be resolved into a saturable and a linear component. This claim runs counter to numerous findings (12-17, 19, 20 and references therein) for potassium and other solutes that uptake is mediated by a single multiphasic mechanism for each solute or group of related solutes. In the present paper the claim for two separate uptake mechanisms for potassium is examined by analysis of the underlying concentration-dependence data. An abstract of this analysis has appeared (18). MATERIALS AND METHODSwhere N is the number of points, wi the weight factor 1 /vbs2, v; obs the observed uptake rate (original or estimated data), vi pred the uptake rate predicted from the model, and df the degrees of freedom. Because of the division by df differences in the number of adjustable parameters in kinetic models do not affect their comparison by this measure.MSD was supplemented by the Runs test (3) which gives the cumulative probability with which a number (csn) of'runs' of positive or negative residuals would occur for a given number of such residuals. This test can be used directly to evaluate the fit of a model to a set of data, e.g. the very low probabilities for the 'saturable + linear' interpretation in some experiments are independent of the multiphasic interpretation.

Sulphate uptake and xylem loading of mycorrhizal beech roots

“…Kinetic data were analysed as described by Nissen (1989). Computer programs were used to test whether multiphasic isotherms or separate uptake systems were present.…”

“…Nissen (1989Nissen ( , 1991 proposed that single structures, presumably membrane transporters mediating nutrient uptake, undergo all-or-none transitions with increasing nutrient concentrations. At low concentrations the transporters are energy dependent and have a carrier-like character, whereas at higher concentrations they allow facilitated diffusion showing channel-like characteristics.…”

“…To calculate apparent K m and V max of sulphate uptake and to clarify if the data better fit the multiphasic or dual models, data shown in Figure 3 a and b were tested by a computer program (Nissen, 1989). A comparison could not be made owing to the …”

“…Linearization of these kinetics according to Eadie-Hofstee revealed two ' phases ' (not shown). The data of the kinetics were statistically analysed using the computer programs described by Nissen (1989) designed to test whether the data are better fit by multiphasic isotherms or by dual transport mechanisms. However, due to the high scattering of data points, no final conclusion about the mechanism active in mycorrhizal beech roots could be drawn in the present study.…”

Potassium Fluxes in Chlamydomonas reinhardtii (I.Kinetics and Electrical Potentials)

“…Despite extensive kinetics studies of the absorption of K+ in higher plants, beginning with the pioneering investigations of Epstein and co-workers in the 1950s, mechanisms of K+ transport based on kinetics data remain controversial (Bange, 1973;Epstein, 1976;Borstlap, 1981;Nissen, 1989). Details of energy coupling are uncertain (Poole, 1974;Cheeseman and Hanson, 1980;Behl and Raschke, 1987;Glass and Fernando, 1992) and the biochemistry of highaffinity K+ transport constitutes virtually unexplored territory.…”