The activities of enzymes including peroxidase, polyphenoloxidase, phenylalanine ammonia lyase, tyrosine ammonia-lyase or catechol-O-methyltransferase in rice leaves were surveyed to elucidate the mechanism of action of probenazole in connection with the resistant reac tion of host plant against rice blast fungus. The activities of enzymes were increased evidently in the treated-inoculated rice leaves with probenazole and rice blast fungus conidia than in the leaves of treated-noninoculated, nontreated-inoculated or nontreated-noninoculated one. The above results seem to be closely correlated with the facilitated formation of a physical barrier in and around the invaded cell, which was observed microscopically.

After treating rice plants by submerged application with probenazole no changes were noted in the siliciferous cells in the leaves, no morphological changes in the leaf tissue and no anti-fungal activity in leaf exudates were detected. When rice plants were treated by submerged application of probenazole and subsequently inoculated with the blast fungus (Pyricularia oryzae CAVARA) peroxidase activity (oxidation of phenolic substances) increased and an anti-conidial factor in the rice leaf increased. Also the increase of the factor was observed in rice plants treated by the chemical but not inoculated. A part of the appearance of induced resistant characteristics were thought to be similar to a reaction of resistant rice plant. The induction of resistant characteristics were largely inhibited by a heat shock upon rice plant indicating that the event was host mediated reaction. Probenazole (submerged application) appears to control the blast disease in rice by a combination of the events, such as inhibiting sensitive stages in the life cycle of the blast fungus and also activating the host defence.

The structures of substances A and C have been elucidated by chemical and spectroscopic analyses. Both compoundsgave stearic acid on catalytic hydrogenation. Based on the analysis of 400 MHz^-NMRspectral data, substance A was determined as 1 3-hydroxy-d.?-9, trans-l 1 , cis-1 5octadecatrienoic acid and substance C as 9-hydroxy-trans-10, as-12, ds-1 5-octadecatrienoic acid. Substance A was identical with a reduction product of 1 3-hydroperoxylinolenic acid prepared by treatment of a-linolenic acid with soybean lipoxygenase.

Plant hormones were tested for their effect on the occurrence of rice blast disease caused by Pyricularia oryzae Cav., using a compatible race and cultivar combination. The degree of disease occurrence, as measured by the number of lesions or by the types of lesions, was suppressed by the treatment with auxins (2, 4-D, NAA, IAA), tryptophan or Ethrel (an ethylene releasing agent), whereas stimulated by the treatment with kinetin or abscisic acid (ABA). Gibberellin also tended to stimulate the occurrence of the disease with considerable elongation of the leaves and sheaths.The suppressive effect of IAA or Ethrel turned into stimulative by the combination with kinetin or ABA and the effect of IAA was also interfered by the combination with the antagonists of auxin. The number of lesions on the leaves treated by kinetin with Ethrel was more numerous than that was treated with kinetin alone.While the number of lesions on the leaves treated by ABA with IAA or Ethrel was fewer than that was treated with ABA alone. The lesions on the leaves were conspicuously of acute type after the treatment with kinetin and were of chronic type after the treatment with ABA. The relationship between plant hormones and resistance of rice plant to the rice blast disease was discussed.

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