Effect of Flavin Inhibitors on Photoactivation of Oospores of Phytophthora cactorum

When dark-grown mature oospores of Phytophthora cactorum were activated to germinate by exposure to 5 uW cm^-2nm^-1 of fluorescent light at 20–22°C in the presence of certain flavin inhibitors such as KI, salicylhydroxamic acid and phenylaceric acid at 40. 1. and 0.1 mM respectively, photoactivation and hence subsequent germination of oospores were inhibited without appreciable irreversible effect on oospore viability. Likewise, when applied during the light period, NaN₃ and KCN at 1 mM reduced photoactivation but had a minimal effect on dark reactions. Diphenylamine, an inhibitor of certain carotenoids, had no effect on photoactivation of oospores. The data suggest that the photoreceptor pigment for activation of oospore germination is a flavin.