Alterations on rice leaf physiology during infection by Bipolaris oryzae

Brown spot, caused by Bipolaris oryzae, is one of the most destructive diseases of rice. This study investigated some physiological and biochemical changes on rice leaves infected by B. oryzae. Rice plants (cv. Oochikara) were grown for 35 days in hydroponic culture and inoculated with B. oryzae. Leaf samples were assessed for disease severity and also collected to determine lipid peroxidation (equivalents of malondialdehyde acid-MDA), electrolyte leakage (EL), and pigment concentrations. Gas exchange parameters were also evaluated. Brown spot severity increased over time, which was associated with increased MDA concentration and high EL. Chlorophyll (Chl) concentrations were significantly lower at 144 h after inoculation (hai) as compared to leaves from non-inoculated plants. At 144 hai, the concentration of total carotenoids (Car) was less than on 72 hai, and the Chl a + b/Car ratio was significantly lower on inoculated plants as compared to their non-inoculated counterparts. Overall, chlorophylls were more affected by fungal infection than were carotenoids. The net carbon assimilation rate, stomatal conductance, and transpiration rate were reduced by about 65% in plants at 144 hai as compared to non-inoculated plants, but the internal CO2 concentration was unresponsive to infection. Changes in photosynthesis were largely related to both impaired light capture ability and decreased mesophyll capacity to fix CO2. It can be concluded that the infectious process of B. oryzae on rice affects the leaf physiology mainly through the damage to the cells, basically at membrane level.