Effect of Temperature on the Radial Exchange of Labelled Water in Maize Roots

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Abstract
The temperature dependence of the efflux kinetics of labelled water in isolated maize roots has been studied. The purpose of these experiments was to obtain the energy of activation, ΔE (kcal/mole), of the rate-limiting step in this radial exchange process under various experimental conditions. Estimates of ΔE were obtained from linear relations between ln{D'w} and the reciprocal of the absolute temperature; values of the apparent diffusion coefficient, D'w, of labelled water in the root were found from an analytical treatment of the efflux data in terms of a cylindrical diffusion model. The energy of activation for labelled-water exchange in normal roots was 14.9 kcal/mole. The corresponding value for ‘dead’ (boiled) roots was 3.9 kcal/mole. These values of ΔE substantiate the view that in normal roots the penetration of water across the membranes of the root cells constitutes the rate-determining step in the efflux whereas in ‘dead’ roots extracellular diffusion of water is the source of rate-control. Similar temperature dependence studies were performed on the efflux kinetics from normal and ‘dead’ roots treated with 10−5 M phenylmercuric acetate (PMA). The energies of activation for labelled-water exchange in normal and ‘dead’ roots under these conditions were respectively 15.5 and 5.3 kcal/mole. Moreover, the results of the efflux experiments on PMA-treated roots were considered to indicate that this inhibitor produces an alteration in some structural aspect of the rate-controlling ‘membranes’.