Dual effect of picolinic acid in the photosynthetic performance of rice plants infected by Bipolaris oryzae

Abstract

Brown spot, caused by Bipolaris oryzae, is one of the most important diseases of rice. The non‐host toxin α‐picolinic acid (PA) has great potential to be used to enhance plant resistance against pathogen infection. The present study investigated the effect of spraying PA [0 (control), 1, 3, and 5 mg/mL] on the photosynthetic performance of rice plants (cultivar Metica‐1) infected or not with B. oryzae. Moreover, whether the PA treatment, especially at the highest concentration, could affect brown spot development was also evaluated. The chlorophyll a fluorescence parameters such as variable‐to‐maximum chlorophyll a fluorescence ratio (Fv/Fm), photochemical yield [Y(II)], yield for dissipation by down‐regulation [Y(NPQ)], the yield for non‐regulated dissipation [Y(NO)], and electron transport rate (ETR) as well as the concentration of photosynthetic pigments were determined. Based on the in vitro assay, PA inhibited mycelial growth of B. oryzae in a dose‐dependent manner and conidial germ tube length only decreased at 5 mg PA/mL. Conidia germination was not affected by the PA treatment. Necrotic lesions caused by PA were observed on leaves of non‐inoculated plants at 3 and 5 mg PA/mL. Symptoms of the brown spot were reduced on plants sprayed with 1 and 3 mg PA/mL compared to the control treatment. Brown spot lesions and those originating from PA toxicity overlapped for inoculated plants sprayed with 3 and 5 mg PA/mL. The photochemical performance of non‐inoculated plants was hampered by treatments with 3 and 5 mg PA/mL. Greater concentration of photosynthetic pigments and less impairment on the photosynthetic performance of inoculated plants sprayed with 1 mg PA/mL were noticed based on the values of Fv/Fm, Y(II), Y(NPQ), Y(NO), and ETR compared to inoculated plants non‐sprayed with PA. In conclusion, spraying rice plants with a low concentration of PA could decrease brown spot severity while preserving the photosynthetic capacity of the infected plants. The cellular damage generated by spraying the plants with the highest PA concentration did not favour the infection process of B. oryzae.