production en poids sec due à l'exposition à l'ozone; cette perte se situait à -10 % pour les plantes traitées avec de l'eau par rapport aux plantes traitées avec l'EDU et à -19,8 % pour la récolte finale des plantes exposées à l'air non-filtré, par rapport à celles exposées à l'air filtré.
The possible involvement of salicylic acid in systemic acquired resistance of Cucumis sativus against Sphaerotheca fuliginea was studied. Cucumber plants were inoculated with tobacco necrosis virus on the cotyledons and the level of endogenous salicylic acid in the first true leaf was determined by gas chromatography. Salicylic acid increased continously from the second day after virus inoculation to the fifth day, when the same leaf was inoculated with Sphaerotheca fuliginea. In healthy plants, the efficiency of exogenous salicylic acid in inducing resistance was assayed by applying aqueous solutions at different times before Sphaerothecafuliginea inoculation. To evaluate the level of induced resistance, the following parameters were examined by light microscopy: percentage of conidial germination, length of the hyphae derived from single conidia, number of haustoria, percentage of epidermal cells with lignified walls and of necrotic cells underlying fungal hyphae. In treated plants conidial germination was reduced, the total length of the hyphae was shorter, the number of haustoria was lower and the haustoriumcontaining epidermal cells had more frequently lignified walls. Moreover, an evident increase in callose deposition was observed leading to the formation of oversized papillae around the penetration pegs. These results indicate that the application of salicylic acid before inoculation with Sphaerotheca fuliginea reduces the intensity of the infectious process and that salicylic acid is involved in the expression of systemic resistance in cucumber challenged by the biotrophic pathogen Sphaerotheca fuliginea.
Plants of Cucumis sativus were eithe preinoculated with TNV or treated with drops of a 5% solution of CuSO4 on the cotyledons, and 5 days later challenge‐inoculated with Sphaerotheca fuliginea on the first true leaf. The induced systemic resistance was assessed by evaluating the percentage of conidial germination, the length of the hyphae derived from single conidia, the number of cells with lignified walls and that of cells containing haustoria. The number of necrotic cells was also recorded. Compared with controls, in TNV‐preinoculated plants conidial germination was lower, hyphal length shorter and the number of haustoria much reduced. The majority of haustoria was found in cells with lignified walls. Pretreatment with CuSO4, although inducing a slight increase of cell wall lignification, did not appreciably reduce the infection process. The number of necrotic cells was very low in all cases. These results suggest that, in this host/pathogen interaction, hypersensitive cell death has no part in the induced defence reaction, although a major role is played by the lignification process.
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