Quantitative correlation between short-term accumulation of protoporphyrin IX and peroxidative activity of cyclic imides

Using Scenedesmus , a series of peroxidative cyclic imides was assayed with respect to degradation of chlorophyll (bleaching), light-induced ethane formation, growth inhibition, and protoporphyrin IX (proto IX) levels. The p I 50 values of the first two parameters, comprising a range of about four decimal powers of inhibitory activity, were quantitatively correlated with protoporphyrin IX formation by regression equations. Applying constant concentrations (0.3 and 1 μ M ) of each compound a correlation could be established with the proto IX level attained after a short-term (1-hr) incubation under moderate light intensity. Long-term treatment of the cells with cyclic imides (4 hr and more) produced excess protoporphyrin, apparently not needed for the phytotoxic effects. Including our previous data, proto IX formation could also be quantitatively linked to the inhibition of root tip growth of Echinochloa (sawa millet) by cyclic imides. All these cell parameters, obtained with green algae or higher plants, are correlated and can be taken as reliable means to quantitate the phytotoxic activity of peroxidizing herbicides.