The surface charge density of wheat root membranes

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 m M 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.