Disease resistance chemical response

Immunization of cucumbers by (L lagenarium, C. cucumerinum, P. 1achrymans or TNV generates a systemic increase in peroxidase activities (. TJ, ] 9, 8U) > Like 1 i gni f ic a t ion and phytoalexin induction, peroxidase activities also rise more quickly in response to infection in leaves of immunized plants, even though total activity eventually may be highest in infected susceptible leaves (77). Several other stimuli can induce local (mechanical and chemical injury) or systemic (senescence, ethylene) peroxidase increases that are not accompanied by increased disease resistance. Thus, enhanced peroxidase activity per se may not be a defense mechanism, but may be a necessary adjunct with appropriate chemical substrates for processes important in disease resistance, e.g., lignification, suberization, and me 1anization. 

Chemical factors are also involved in the resistance of plants to disease and in the competitive ability of a plant to survive within a community of plants. Plant stress may also generate a chemical response giving rise to compounds known as the phytoalexins, the nature of which will depend on the chemistry of the host plant (18, 19). Such response to injury or infection is of great Interest because it has stimulated investigations of the nature of the bloregulatory processes involved. 

Research in recent years has shown that protective chemicals in plants can operate in disease resistance. Some of these chemicals occur in disease-free plants, but fungicidal compounds can build up in tissues in response to attempted infection. When this occurs, these defense chemicals are known as phytoalexins. Papers to be presented at this conference by Cutler et al., Harborne, Bailey,... 

Earlier research on phenolic substances in relation to plant disease resistance was concerned not only with the possible protective effects of preformed phenolic compounds, but also whether these compounds are mobilized or their synthesis is promoted at the site of infection. Another aspect which has received attention is that the phenolic compound might be liberated from its glycoside or sugar ester at the Infection site in response to Infection (5J. Chemical changes in Infected tissue due to the activity of polyphenol-oxidases and peroxidases, leading to the production of qulnones and other fungicidal compounds, have also been Investigated (6). 

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