Physiological and biochemical responses in sunflower leaves infected by Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a necrotrophic fungal pathogen that causes severe yield loss of sunflower, an economically important oilseed crop. Despite years of effort to understand resistance mechanisms of sunflower toward S. sclerotiorum, such knowledge, however, is still far from complete. We assessed physiological and biochemical responses following S. sclerotiorum invasion, over a 72-h time-course, in two sunflower varieties-a resistant variety (var.) Fengkuiza No. 1 and a susceptible var. 7101. Activities of several antioxidant enzymes were found to be altered by pathogen infection in both varieties. Moreover, the concentration of two stress-inducible phytohormones-salicylic acid and abscisic acid-were dramatically elevated by S. sclerotiorum infection. Similarly, upon infection, levels of total protein, carbohydrates, and chlorogenic acid were significantly higher in leaves of the resistant var. than in those of the susceptible var., indicating a positive correlation between resistance and these indicative indexes. In contrast, the malondialdehyde (MDA) level was found to be negatively correlated with sunflower resistance to S. sclerotiorum. Findings from this study improve knowledge of sunflower resistance toward S. sclerotiorum and provide an inventory of potential indicators for future sunflower screening and breeding with an aim toward enhanced resistance.