Microbiology Laboratory, Annamalai University^ Annamalainagar, Madras, India The Role of the Parasite and its Metabolites in Triggering Host Physiology By R. SRIDHAR, D . CHANDRAMOHAN, and A. MAHADEVAN With J figures One of the problems in infection is the role of the parasite and its meta- bolites in altering host responses. Activation of respiration, and phenolic com- pounds and oxidative enzymes in host plants in response to parasitic infection has been frequently observed (FARKAS and KIRALY 1962, TOMIYAMA 1963, MAHADEVAN 1966, RUBIN and ARCICHOVSKAJA 1967). Many physiological altera- tions in host plants are caused by nonspecific factors including medianical damage and metals. Recently ALLEN (1966) emphasized that the visible host reaction to an invading microorganism is not usually caused directly by pathogen produced toxins but the characteristic symptoms are the result of a sequence of reactions in whidi host metabolites play a role. The role of a parasite and its metabolites in triggering the host responses is reported here. Materials and Methods Wilt susceptible cotton plants [Gossypium herbaceum variety Kalyan) grown in loosely packed soil for 12 days were removed and root systems washed and suspended in beakers containing various fungal extracts and enzymes. Fusarium oxysporum f. vasinfectum (Atk.) Snyder and Hansen was grown in Czapek's liquid mediuin for 13 days; the mycelium was removed, washed repeatedly in distilled water and placed in a dialyzing membrane containing sterile distilled water. The membrane was suspended in a beaker containing the plants which at the end of 24 h were removed and analyzed. In another lot, the fungus mycelium was ground (1 g in 2 ml sterile distilled water) in a Waring Blendor for 3 minutes, a few drops of toluene added, allowed to autolyze for 24 h at room temperature (26 ± 3°C), and filtered. Plants were suspended in the filtrate for 2 h and analyzed. In a third experiment, the plants were suspended for 2 h in the homogenized mycelium, removed, washed and analyzed.