Salicylic acid-mediated growth promotion, pigment enhancement, and secondary metabolite augmentation in <i>Brassica juncea</i> L. plants under thiamethoxam pesticide stress

Thiamethoxam (TMX) is a second-generation insecticide and belongs to the neonicotinoid class of pesticides. These synthetic compounds are applied to food crops to eliminate pests, particularly insects, and to maintain yields. Consequently, due to their higher solubilities in water, these systemic compounds get incorporated into the edible plant tissues and show impaired growth and developmental aspects. The objective of the present study was to evaluate the harmful effects of TMX pesticide on Brassica juncea L. and to uncover the defense responses exerted by the salicylic acid (SA) to curtail the losses and maintain the integrity of B. juncea field plants for 30, 45, and 60 days after growth. Experimentation with field plants was conducted, and some of the morphophysiological, biochemical, and secondary metabolites associated parameters were investigated in the B. juncea crop. The morphological metrics were found to be lowered in TMX stress conditions. Whereas the contents of photosynthetic pigments, including total chlorophyll content, were lowered, on the contrary, carotenoid and xanthophyll contents were enhanced under the TMX stressed conditions. The gaseous exchange parameters also showed a decline in the observed values. Subsequently, the enzymatic antioxidants and phenolic contents were marked by an increase under the TMX-stressed plants. Notably, the activities of antioxidative enzymes, such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APOX), were significantly enhanced in plants supplemented with exogenous SA. The highest concentration of SA, such as SA3 (1mM), showed significant stress ameliorative potentials in all the studied parameters.