Leaf gas exchange physiology in rice genotypes infected with bacterial blight: An attempt to link photosynthesis with disease severity and rice yield

Abstract

Bacterial blight (BB) of rice caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most devastating disease of rice causing significant yield reduction under serious infestations in many rice growing countries. BB interferes with leaf CO2 exchange processes, enhances leaf senescence and reduces CO2 assimilation. In the present study, 14 rice genotypes were initially studied to understand genotypic variability for resistance against infection by Xoo. Further, to assess the effect of BB on photosynthetic functions, three rice genotypes [O. longistaminata; O. sativa cv. IRBB21 and Pusa Basmati 1 (PB1)] possessing varied levels of resistance/susceptibility to BB were selected. IRBB21 and PB1 were also analyzed to study the effect of bacterial blight on grain yield. Exposure of plants to BB led to significant reductions in net photosynthetic rate (PN), stomatal conductance to CO2 (gs), instantaneous water use efficiency (WUEi), and leaf transpiration rate (E) in susceptible genotype PB1 when compared to resistant IRBB21 and highly resistant O. longistaminata. Under BB infection, O. longistaminata showed highest PN (7.18 μmol CO2 m-2s-1) compared to IRBB21 (4.35 μmol CO2 m-2s-1) and PB1 (0.74 μmol CO2 m-2 s-1), on 216 h of post infection. Due to infection, gs was strongly reduced to 0.046 mmol m-2s-1 on 216 h in PB1 followed by IRBB21 (0.16 mmol m-2 s-1 on 216 h), whereas O. longistaminata maintained highest gs of 0.22 mmol m-2 s-1 on 216 h, indicating better CO2 exchange capacity and resistance against bacterial blight. Regression plots showed significantly positive relationships between PN vs gs and PN vs Ci for all the tested genotypes. Down-regulation in leaf CO2 assimilation physiology as well as loss in photosynthetically active leaf tissue was observed with increment in disease severity, which resulted in substantial yield loss (61.75%) in susceptible genotype PB1. Yield loss was mostly attributed to reduced productive tillers, less number of seeds per panicle, decreased panicle weight and less number of filled grains. Relatively less variation in yield traits was recorded in resistant IRBB21.