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Segments from dark-grown cucumber (Cucumis safivus 1.) hypocotyls were used to study defense reactions occurring upon fungal infection and induced by elicitors in the same tissue. The segments were rendered resistant to infection by Colletotrichum lagenarium either by growing the seedlings in the presence of dichloroisonicotinic acid (DCIA) or by preincubation of the cut segments with DCIA, salicylic acid (SA), or 5-chlorosalicylic acid (5CSA). This resistance appears to be due mainly to inhibition of fungal penetration into epidermal cells. In the resistant hypocotyl segments, the fungus induced, at the time of attempted penetration, an increased deposition of phenolics, which were visualized by autofluorescence. These phenolics were located mainly in the epidermal cell wall around and in the emerging papillae below appressoria and were quantified either as lignin-like polymers by the thioglycolic acid method or as 4-OH-benzaldehyde, 4-OHbenzoic, or 4-coumaric acid liberated upon treatment with alkali at room temperature. Pretreatment with DCIA, SA, and 5CSA induced little chitinase activity, but this activity greatly increased in resistant tissues upon subsequent infection. These observations indicate that resistance is associated with an improved perception of the pathogen stimulus resulting in the enhanced induction of diverse defense reactions. When the cut segments were pretreated with DCIA, SA, or 5CSA and then split and incubated with chitosan fragments, the deposition of cell wall phenolics was also enhanced. These pretreated and split segments also exhibited an increase in the rapid production of activated oxygen species induced by an elicitor preparation from Phytophthora megasperma f. sp. Glya. Pretreatment of the segments with methyl iasmonate neither induced resistance nor enhanced indudion of cell wall phenolics upon fungal infection, although we observed in the corresponding split segments some increase in chitosan-induced cell wall phenolics and in elicitor-induced rapid produdion of activated oxygen species.Localized infection of certain plants by pathogens can result in the development of resistance either near the area of first inoculation or spreading systemically to other plant organs (for citations, see Rasmussen et al., 1991). This SAR is nonspecific with regard to the first, inducing pathogen as well as to the second, rejected pathogen. These observations make the participation of severa1 defense mechanisms likely. Accordingly, various "pathogenesis-related proteins" are systemically induced in the resistant tissue. These proteins are Supported by Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie.

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