Fluorescence microscopy and related enzyme activity of sunflower plants infected by Plasmopara halstedii

Fresh sections of healthy and infected sunflower tissues were examined with the aid of a Jena Universal fluorescence microscope, equipped with exciters BG 12 2 and UG 1 1·5, and a barrier filter GG9. Two weeks after bud-inoculation a blue-yellowish fluorescence was observed in parenchyma cell walls of stems, petioles and leaf main veins taken from infected plants. Fluorescence was more intense in cell wall corners adjacent to intercellular spaces. No such fluorescence was detected in healthy corresponding tissues. As pathogenesis progressed, fluorescence of cell walls intensified and some cells became full of fluorescing material. Peroxidase activity increased 4-fold in leaves and 13-fold in stems as a result of infection. β-Glucosidase activity was remarkably high in sporangial suspensions and in invaded leaves, but not in fungal-free tissues of infected plants nor in healthy ones. It seems likely that scopoletin was the major fluorescing compound accumulated. A causal link is suggested between the increase of fluorescence and the enhancement of β-glucosidase and peroxidase in infected sunflowers.