Changes in peroxidase activity in sunflower during infection by necrotrophic pathogen Alternaria helianthi

Detailed analysis of threshold levels of resistance in sunflower genotypes to Alternaria helianthi and its relation to peroxidase activity was carried out. Fourteen sunflower genotypes were categorized into resistant, moderately resistant, susceptible and highly susceptible groups based on field reaction on disease rating scale of 0-9. Sunflower genotypes MSH-59 and PF-56 were resistant with disease reaction scale of 3, PC-63, PC-64, RHA6D-1 and G-47 were moderately resistant with disease reaction scale of 5, CMS-859, PC-52, AC-48, R-265, CMS-338 and MR-1 were susceptible with a disease reaction scale of 7, Morden and CMS-62 belonged to a highly susceptible category with a disease reaction scale of 8. Time-course study of peroxidase (PO) activity was analysed in resistant (MSH-59) and susceptible (Morden) sunflower genotypes using spectrophotometry. High PO activity was recorded at 2 and 12 h post-inoculation with A. helianthi. At 2 h post-inoculation, resistant genotypes recorded 12.8 units while the susceptible genotype had 2.4 units of PO activity. At 12 h PO activity increased to 39.2 and 4.2 units in resistant and highly susceptible genotypes respectively. Treatment with antioxidant inhibitors sodium azide and sodium metabisulphite at various molar concentrations reduced the pathogen-induced as well as constitutive levels of PO activity. Pre-treatment of resistant sunflower genotypes with inhibitor followed by pathogen inoculation increased the disease reaction when compared to control. Higher PO activities were recorded in sunflower genotypes with high threshold levels of resistance and lesser in susceptible genotype. The study therefore gives strong evidence for the role of PO as an important enzyme of the central defense system against necrotrophic pathogen A. helianthi and could be used as a reliable biomarker for assessing resistance