Effects of ouabain and phytotoxic metabolites from Verticillium dahliae on the cell membranes of cotton plants

Twenty-one-day-old cultures of Verticillium dahliae yielded an extract which was toxic to the susceptible cv. of cotton (KH-2) but not to three resistant cvs (H-4, Varahlaxmi and C-Indore). Cuttings from 8-week-old cotton plants of the susceptible cv KH-2 developed typical wilt symptoms 8 h after being treated with the toxin preparation. Loss of electrolytes from leaf and root tissues was significantly greater than that from water treated controls and began immediately after exposure to the toxin preparation. Root tissues of the susceptible cv. also showed a significant loss of β-cyanin within 15 min of exposure to the toxin preparation. Significant losses of K⁺ and Na⁺ occurred from leaf tissues, beginning immediately after treatment, but permeability to Ca²⁺ was not affected.

Ouabain, at 0.2 mm concentration, induced wilting in cuttings of the susceptible cultivar similar to that caused by the toxic preparation, but the resistant cvs were not affected. Ouabain also induced loss of electrolytes from leaf and root tissues of the susceptible cultivar but it had no effect on permeability to β-cyanin.

The results suggest that Vd-toxin sensitive K⁺ and Na⁺ ion transport systems are present in the plasma membranes of the susceptible cv. but not in the resistant cv. These sites are also sensitive to ouabain treatments. Other sites in the plasma membranes of the susceptible cv. determine permeability to β-cyanin, but these are not affected by ouabain treatments. Thus, the Vd-toxin preparation does not induce non-specific changes in the cell membranes leading to uncontrolled losses of all constituents, because permeability to Ca²⁺ ions is not affected.