EFFECT OF SALICYLIC ACID ON H2O2 METABOLISM DURING GRAPEFRUIT (CITRUS PARADISI MACFAD.) - XANTHOMONAS CITRI SUBSP. CITRI INTERACTION

Citrus canker (Xanthomonas citri subsp. citri: Xcc) is a serious disease that causes significant yield and economic losses worldwide. The endogenous plant growth regulator salicylic acid (SA) was applied exogenously to canker-infected grapefruit (Citrus paradisi Macfad.) at 500 μM to determine its effects on oxidative metabolism. Initial visual symptoms during canker development were water soaking at 2 days after inoculation (dai) followed by hypertrophy and ruptured epidermis between 8-10 dai with the formation of tiny yellow-colored bacterial colonies. Xcc depressed hydrogen peroxide (H2O2) levels in leaves accompanied by reduced superoxide dismutase activity (SOD: EC 1.15.1.1) and higher catalase (CAT: EC 1.11.1.6), ascorbate peroxidase (APOD: EC 1.11.1.11), and guaiacol peroxidase (POD: EC 1.11.1.1) activities. The exogenous application of SA did not affect Xcc population or canker development, however, levels of SOD and POD were elevated and persisted during pathogenesis. SA alone and in association with Xcc suppressed H2O2. SA restored the levels of SOD and POD, but CAT and APOD activities contributed to a substantial decline in H2O2 levels and maintained a favorable environment for Xcc growth.