The surface charge density of wheat root membranes

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Abstract
Seedlings of winter wheat (Triticum aestivum L. cv. Hildur) were grown at 18°C for 7 days in darkness in a complete growth medium in the presence or absence of 1 mM KCl to produce roots with different ion contents (high and low K+ respectively). The roots were homogenized, the 3 000 g, 10 000 g, 30 000 g (further fractionated by two phase partitioning) and 100 000 g pellets isolated, and their surface charge densities (σ) determined by the use of 9‐aminoacridine fluorescence. The average σ for all membrane fractions weighted for protein content was the same (−18 mC m−2) for low and high K+ roots. The K+, Na+, Mg2+ and Ca2+ content of roots was determined and used to calculate an average σ following the procedure of Bérczi et al. [Physiol. Plant. 61: 529–534 (1984)]. The predicted value (−11 mC m−2) does not deviate much from the experimentally determined value. It is concluded as a useful working hypothesis that the average surface charge density is constant and that the ionic content of plant cells is regulated such that the average surface potential is constant.