HISTOCHEMICAL RESPONSES OF LEAVES OF IN-VITRO PLANTLETS OF VITIS SPP TO INFECTION WITH PLASMOPARA-VITICOLA

Leaves excised from in vitro plantlets of Vitis vinifera 'Grenache' (s usceptible, S), V. rupestris'du Lot'(intermediate, M), and V. rotundif olia 'Carlos' (resistant, R) were inoculated with sporangia of Plasmop ara viticola and examined microscopically after they were stained with a series of reagents. These three cultivars showed necrotic spots wit hout sporulation (R); diffuse necrosis with limited sporulation and sh ort, highly branched aerial hyphae (M); and no visible necrosis with h eavy sporulation and long aerial parasite hyphae (S). During histochem ical studies, a blue autofluorescence (trans-resveratrol) was detected at 1 day on R, at 2 days on M, but not at all on S. Flavonoids were p resent at 2 days on R but at 8 days on M and at 12 days on S. These da ta suggest that the presence of trans-resveratrol and flavonoids in th e resistant cultivar at a very early stage of infection may play an im portant role in inhibiting growth of the fungus. Moreover, on the inte rmediate cultivar, the peroxidase activity, catechins, and lignins app eared at a late stage of infection. The production of these molecules seems to restrict the development of this pathogen. In the susceptible cultivar, these reactions presumably were too late and too weak to li mit development of the fungus.