The Role of Potassium Phosphite in Enhancing the Activity of Defense Enzymes in Rice Infected by Sheath Blight

Background: Rice (Oryza sativa L.) serves as the primary food source for over half of the world's population.The disease caused by Rhizoctonia solani has significantly impeded rice production, resulting in substantial economic losses and posing a threat to food security.Currently, the most suitable method to control this disease is the use of commercial fungicides.However, the use of fungicides results in increased costs and harm to the environment and human health.Therefore, the utilization of biocompatible chemical compounds is regarded as an innovative and effective strategy for integrated product management and combating various stressors.One such compound is potassium phosphite, which works to reduce disease by directly affecting the pathogen and indirectly stimulating the host's defense responses.Methods: The seeds of local Tarom and Khazar cultivars were prepared and disinfected to produce identical, pathogen-free seedlings.Subsequently, the seeds were germinated and transferred to plastic pots filled with sterile soil.Finally, the pots were moved to the growth chamber.Some of the seedlings were treated with potassium phosphite while others served as the positive control.All the plants were then infected with R. solani.Leaf tissues from the treated and control seedlings were sampled at 0, 24, 48, 72, and 96 hours after infection.The leaf extract was then obtained to measure enzyme activity, including catalase (CAT), polyphenol oxidase (PPO), and superoxide dismutase (SOD).Disease severity and enzyme activity were studied in a factorial design using a completely randomized design with two cultivars, Tarom (resistant) and Khazar (susceptible), and two treatments of potassium phosphite and R. solani, each with three replications. Results:The results of the analysis of variance showed that the effects of the treatments were significant in all sources of changes in the severity of the disease and the activity of PPO and SOD enzymes.However, the effects of the treatments were not significant in the treatment number for the CAT enzyme activity but were significant in other sources.The severity of the disease was more pronounced in the control plants than in the plants treated with potassium phosphite, and in the sensitive variety (Khazar) than in the resistant variety (Tarom).Additionally, in both the control and treated plants, the disease progressed more by the 10 th day after infection than on the 21st day.Enzyme activity was significant in both the resistant (Tarom) and sensitive (Khazar) varieties, as well as between the two potassium phosphite treatments in the presence of the pathogen.The highest level of enzyme activity was associated with the Tarom variety and potassium phosphite treatment in the presence of the pathogen while the lowest level was linked to the Khazar variety and R. solani treatment.The CAT enzyme activity increased in both treatments and cultivars at 24 h after infection, and then gradually decreased at 48 and 72 h.At