Biochemical and physiological responses of rice plants to the combined occurrence of drought stress and infection by Bipolaris oryzae

Abstract Due to global climate change, an increase in the occurrence of water stress in rice fields is expected, considerably influencing the development of diseases, such as brown spot of rice, caused by Bipolaris oryzae . The present study investigated the combined and individual effects of drought stress and B. oryzae infection on rice at physiological and biochemical scales. All treatments presented high severity levels, with severity over 50%, however less disease development was observed in the drought treatments at 72 h after inoculation (hai). The reduction in severity in this group of plants was related to the preservation of the quantum efficiency of photosystem II and the concentration of photosynthetic pigments. However, the greatest negative physiological impact on plants exposed to water restriction, coupled to B. oryzae infection, was determined by the water potential in the rice xylem. B. oryzae infection intensified oxidative stress, especially in plants previously subjected to drought stress, as indicated by increased activities of superoxide dismutase (23% at 72 hai), peroxidase (37% and 25% at 48 and 72 hai, respectively), phenylalanine ammonia lyase (53% and 41% at 48 and 72 hai), and proline accumulation (31%). In conclusion, plants previously exposed to water stress and B. oryzae infection, despite moderate reduction in brown spot symptoms, displayed a greater oxidative stress in rice leaves associated with reduced levels of water potential in the xylem. Despite a lower level of disease, the combined effect of both stresses, even sequentially, caused severe physiological damage to the rice.