"For the kinetics of ion uptake by plants, the Michaelis- Menten equation has been modified to include the param- eter Cmin, and the term I, designating unidirectional influx, has replaced the term V (Fig. 2.11). Very often, though, only the net uptake of ions is determined experimen- tally, which is the net result of influx and efflux across the plasma membrane (Fig. 2.12). Efflux can become similar in magnitude to influx, particularly at extreme low or high external concentrations, and therefore can be an important component in determining net uptake (Fig. 2.13; Elliott et al., 1984; Britto and Kronzucker, 2006). It is also note- worthy that, at a given external concentration, the efflux of a particular mineral nutrient can be many times higher from roots of plants sufficiently supplied than from roots of defi- cient plants (e.g., McPharlin and Bieleski, 1989; Lee et al.,
The efflux of ions and other solutes is affected by sev- eral factors: (i) the integrity of the plasma membrane, (ii) the presence of transport proteins allowing efflux, (iii) the electrochemical driving force for transport, and (iv) the concentration of the solute in the cytoplasm. In pea, for example, the initial high rate of net uptake of sulphate by S-deficient roots placed in a solution containing sulphate decreases to about 30% within one hour due to a marked increase in sulphate efflux, despite a slight increase in influx (Deane-Drummond, 1987; Bell et al., 1995). Similarly, for nitrate and ammonium, the efflux component can account for a high proportion – almost 40–50% of the influx – most probably due to the high concentrations of nitrate and ammonium in the cytoplasm (Britto and Kronzucker, 2003, 2006)."
White, P. J. (2012). Ion Uptake Mechanisms of Individual Cells and Roots. Marschner’s Mineral Nutrition of Higher Plants, 7–47. doi:10.1016/B978-0-12-384905-2.00002-9
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