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' (susceptible, S), V. rupestris'du Lot'(intermediate, M), and V. rotundifolia 'Carlos' (resistant, R) were inoculated with sporangia of Plasmopara viticola and examined microscopically after they were stained with a series of reagents. These three cultivars showed necrotic spots without sporulation (R); diffuse necrosis with limited sporulation and short, highly branched aerial hyphae (M); and no visible necrosis with heavy sporulation and long aerial parasite hyphae (S). During histochemical 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 present at 2 days on R but at 8 days on M and at 12 days on S. These data suggest that the presence of trans-resveratrol and flavonoids in the resistant cultivar at a very early stage of infection may play an important role in inhibiting growth of the fungus. Moreover, on the intermediate cultivar, the peroxidase activity, catechins, and lignins appeared 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 limit development of the fungus.