Induction of defense response of Oryza sativa L. against Pyricularia grisea (Cooke) Sacc. by treating seeds with chitosan and hydrolyzed chitosan

Seeds of rice (Oryza sativa L.) were treated with chitosan and hydrolyzed chitosan at 100, 500 and 1000 mg L⁻¹. After 18 days of germination, spore suspension of Pyricularia grisea was applied. The enzyme activity of phenylalanine ammonia-lyase, β-1-3-glucanase, chitinase and chitosanase in leaves of rice seedlings was evaluated after 24, 72, 120 and 168 h of inoculation. Blast affected area (%) was evaluated 7 and 14 days after spraying spore suspension. Chitosan performance to elicit defense response induction was associated with the concentration and type of chitosan. The activity of most of the enzymes tested was induced in leaves of treated seeds with chitosan and hydrolyzed chitosan at 1000 and 500 mg L⁻¹, respectively. The highest enzyme activities were observed with hydrolyzed chitosan after 72 h however, compared to chitosan, the activity was not maintained during the entire post-inoculation period. The highest control (0 = no lesions) of P. grisea in rice seedlings was observed at 1000 mg L⁻¹ in both chitosan and hydrolyzed chitosan treated leaves. Symptoms of infection by P. grisea were evident after 14 days evaluation date, but according to the standard scale proposed by the International Rice Research Institute, these symptoms fell into the resistance category of blast diseases.