Rice blast, caused by Pyricularia oryzae, is the most important disease in rice worldwide. This study investigated the effects of silicon (Si) on the photosynthetic gas exchange parameters (net CO2 assimilation rate [A], stomatal conductance to water vapor [gs], internal-to-ambient CO2 concentration ratio [Ci/Ca], and transpiration rate [E]); chlorophyll fluorescence a (Chla) parameters (maximum photochemical efficiency of photosystem II [Fv/Fm], photochemical [qP] and nonphotochemical [NPQ] quenching coefficients, and electron transport rate [ETR]); concentrations of pigments, malondialdehyde (MDA), and hydrogen peroxide (H2O2); and activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), and lypoxigenase (LOX) in rice leaves. Rice plants were grown in a nutrient solution containing 0 or 2 mM Si (-Si or +Si, respectively) with and without P. oryzae inoculation. Blast severity decreased with higher foliar Si concentration. The values of A, gs and E were generally higher for the +Si plants in comparison with the -Si plants upon P. oryzae infection. The Fv/Fm, qp, NPQ, and ETR were greater for the +Si plants relative to the -Si plants at 108 and 132 h after inoculation (hai). The values for qp and ETR were significantly higher for the -Si plants in comparison with the +Si plants at 36 hai, and the NPQ was significantly higher for the -Si plants in comparison with the +Si plants at 0 and 36 hai. The concentrations of Chla, Chlb, Chla+b, and carotenoids were significantly greater in the +Si plants relative to the -Si plants. For the -Si plants, the MDA and H2O2 concentrations were significantly higher than those in the +Si plants. The LOX activity was significantly higher in the +Si plants than in the -Si plants. The SOD and GR activities were significantly higher for the -Si plants than in the +Si plants. The CAT and APX activities were significantly higher in the +Si plants than in the -Si plants. The supply of Si contributed to a decrease in blast severity, improved the gas exchange performance, and caused less dysfunction at the photochemical level.

Blast, caused by Pyricularia oryzae, is one of the most important diseases affecting rice yield worldwide. This study investigated the potential of silicon (Si) to counteract the effect of picolinic acid (PA), a non-host selective toxin produced by P. oryzae, on the photosynthetic performance, on the activities of defense enzymes, and the antioxidative metabolism of rice plants. Plants were grown in nutrient solution containing 0 or 2 mM of Si and non-sprayed or sprayed with PA (0.5 mg PA/ml; 20 ml per plant) at 35 days after emergence. The PA-sprayed plants exhibited necrotic lesions on leaves that resembled those caused by P. oryzae infection. In the presence of Si, the necrotic lesions on the leaves of PA-sprayed plants were less in number and of reduced size. The necrosis of leaf tissues caused by PA spray was accompanied by lower values of net CO 2 assimilation rate and transpiration rate and high internal CO 2 concentration values indicating the occurrence of biochemical limitations in photosynthesis regardless of Si supply. For plants non-sprayed with PA, there was no change in the activities of the enzymes involved in host defense (chitinases (CHI), β-1,3-glucanases (GLU), polyphenoloxidases (PPO), and phenylalanine ammonia-lyases (PAL)) and on the antioxidative system (superoxide dismutases (SOD), catalases (CAT), peroxidases (POX), ascorbate peroxidases (APX), and glutathione reductases (GR)). The activities of CHI, GLU, PAL, GR, POX, and SOD were greater on the leaves of PA-sprayed plants in the absence of Si than on its presence. Silicion did not alleviate the physiological and biochemical aspects of PAmediated stress on rice leaf blades. We hypothesize that Si forms a physical barrier. Such physical impediment makes the diffusion of PA into the leaf blades more difficult rather than actively affecting the physiology and biochemistry of the plant.

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