Biochemical and Physiological Mechanisms Underlying Sclerotinia Sclerotiorum Resistance in Brassica Juncea : Differential Cultivar Responses

Sclerotinia sclerotiorum is a devastating necrotrophic fungal pathogen that causes stem rot in Brassica crops, leading to substantial yield losses worldwide. This study examines physiological, biochemical, and anatomical differences between susceptible (Varuna) and tolerant (RH1222‐28) Brassica juncea cultivars under S. sclerotiorum infection, emphasizing antioxidant and enzymatic defenses. Phenotypic evaluation revealed that while Varuna developed extensive lesions, RH1222‐28 exhibited significantly restricted disease symptoms. Varuna had stronger stems, but RH1222‐28 coped better with the disease because its biological defenses were strong. RH1222‐28 exhibited reinforced cell walls and compact vascular bundles, strengthening its structural resistance. At the same time, it maintained reactive oxygen species (ROS) balance through sustained radical scavenging activity, unlike Varuna, which showed a prolonged oxidative burst and greater tissue damage. These two pathways—structural fortification and ROS homeostasis—emerged as central to RH1222‐28's superior tolerance. Principal component and correlation analyses further confirmed that RH1222‐28's tolerance is associated with coordinated antioxidant defenses, phenylpropanoid pathway activation, and anatomical fortification, offering valuable insights into mechanisms of resistance against S. sclerotiorum in Brassica juncea .