The Relation of External Rubidium Concentration to Amounts and Rates of Uptake by Excised Potato Tuber Tissue.

A number of studies have indicated that the amount of salt taken up by plant tissue corresponds to the external concentration in the manner of a Freundlich adsorption isotherm (for references see 2, 10, 28). The idea that adsorption is involved in uptake seems to be gaining general acceptance at least for the initial phase of uptake (2, 16) despite the fact that the validity of the relation and of the conceptas a part of the accumulation mechanism-has been questioned (12, 28). The present report is concerned with the relation of ion concentration in the external solution to the amount of uptake by excised tissue, as possibly an adsorptive process, and the probable significance of this relation to accumulation. The fact seems well established that the time course of cation uptake consists of at least two, easilv distinguished, time phases: 1) an initial non-metabolic process which is presumed by many to be an adsorption, adsorption-exchange, or exchange process (1, 24, 26, 28, 30, 32), and 2) a metabolic process, which is dependent on aerobic respiration and in which accumulation may proceed against a concentration gradient (2, 3, 4, 17, 21, 28, 36). Phase one of the uptake, or loss, is essentially independent of temperature and aeration as shown by isotopic exchange of potassium (5); it corresponds in exchange to relative concentration of two ions as expected by the law of mass action (36); and it shows ion selectivity (13, 22). Phase two of the uptake has a high temperature quotient (Ql0= 1.5 to 3.0) (4, 33) and is dependent on oxidative metabolism; it has been reported to be directly dependent on external concentration (14, 20, 23, 34) though evidence to the contrary has been reported for intact plants (18). In former years there was a strong tendency to view initial ion penetration as a diffusion and permeability process (3, 27); however, more recently a trend has developed to consider the first steps in uptake to be a binding reaction, eg., an ion carrier complex, either in a non-metabolic process or in energetically coupled reactions (27). This view has received subst.ntial support recently with the demonstration that in some cases of related ions the effect of one ion on another in uptake by roots fits the AMichaelis-AMenten equation for competitive inhibition as in an enzymatic process, whereas with other ions uptake is uncompetitive (8, 9). This concept seems not to be inconsistent,