Abstract

Four mustard cultivars possessing different degrees of resistance to Sclerotinia stem rot (Sclerotinia sclerotiorum) were used to study the biochemical and enzymatic basis of resistance. These include two moderately resistant cultivars (RGN-13 and RRN-505) and two susceptible cultivars (Bio-902 and T-59). Phenol content in all the mustard cultivars tested was found to increase at all three stages of inoculation, that is, 60, 75 and 90 days after sowing in diseased plants as compared to their respective healthy plants. Reduction in the contents of total sugars and reducing sugars were observed in diseased plants of all four cultivars irrespective of their susceptibility towards S. sclerotiorum. Peroxidase (PO) activity was observed to be high in moderately resistant cultivars as compared to susceptible ones. Maximum increase in PO activity was measured in diseased tissue of moderately resistant cultivar RRN-505 followed by RGN-13 and minimum PO activity in susceptible Bio-902. Significant increase in polyphenol oxidase (PPO) activity was recorded in moderately resistant cultivars as compared to susceptible cultivars. The phenylalanine ammonia lyase (PAL) activity was maximum in moderately resistant cultivar RGN-13. The PAL activity increased with days of inoculation in healthy as well as in diseased plants. Significant decrease in catalase activity was observed in diseased plants as compared to healthy ones and this decrease was more pronounced in susceptible cultivar T-59. The pathogen related enzymes polygalacturonase trans-eliminase (PGTE), pectin trans-eliminase (PME), polygalacturonase (PG) and cellulolytic (Cx) activities were found to be lower at initial stages of enzymes activity.

Key words: Enzymes, phenol, peroxidase, phenylalanine ammonia lyase and polyphenol oxidase.