Picolinic acid stress imposed on rice leaves is not alleviated by silicon

Abstract

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 CO2 assimilation rate and transpiration rate and high internal CO2 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 PA-mediated 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.