The Absorption and Accumulation of Salts by Living Plant Cells

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Abstract
Chemical methods for the quantitative determination of rubidium in aqueous plant extracts and saps were investigated but abandoned in favour of a spectrographic technique which is fully described. The results of quantitative determinations of the rubidium and bromide absorbed by potato discs from solutions of rubidium bromide, and the effects upon this absorption of oxygen, of time, and of the specific surface of the discs are described. The effects of these variables upon the accumulation of bromide which were previously described have been confirmed. All the variables which affect bromide absorption affect the uptake of rubidium in a similar manner. The absorption of rubidium is of two distinct kinds. The first phase in the absorption process is a relatively rapid uptake of rubidium unaccompanied by bromide. This process is unaffected by oxygen, is not confined to the surface cells, and ceases after a short time. The second phase occupies a protracted period during which both rubidium and bromide are absorbed in equivalent amounts. During this phase the absorption is confined to a few layers of cells at the surface and is determined by oxygen concentration in the manner already described for the bromide ion. The two types of absorption are described as ‘induced absorption’ and as ‘primary absorption’ in the sense used earlier. The former is merely a property of the substances in the tissue, the latter is a process which tends to increase the free energy of the tissue, therefore demands that work should be done, and is a property of the organized living cell. The relationships of the two types of absorption process to time have been described and they can be expressed in terms of equations. The effect of the surface and thickness of discs upon the absorption of bromide and rubidium is interpreted quantitatively. The distance from the surface of the discs at which ‘primary absorption’ ceases is the same for rubidium and bromide, and coincides with the values previously determined for the depth of the layer of tissue which exhibits enhanced respiration. The ‘induced absorption’ of rubidium may be observed in discs killed by alcohol. The effect of concentration upon this process is similar to the adsorption isotherm. The effect of concentration upon the absorption of bromide by living discs is much less conspicuous and the requirements of the absorption isotherm are not rigidly met. The effect of concentration also involves the factor of surface and thickness of disc. Unequal absorption of the bromide and rubidium arises from the superimposed effect of two distinct processes one of which causes the absorption of rubidium only and the other tends to cause equal uptake of rubidium and bromide.