Exogenous Application of Jasmonic Acid Alleviates Chromium Stress in Mustard (Brassica juncea (L.) Czern) by Enhancing Growth and Biochemical Parameters, Regulating Antioxidant Enzymes Activity, Lipid Peroxidation, and Cellular Viability

Jasmonic acid (JA) is a prominent class of stress hormones involved in the survival of plants in stressful conditions. JA is directly linked to plants' ability to withstand heavy metal stress. The present study was conducted to evaluate the role of JA in mitigating chromium-induced stress by assessing various morpho-physiological, biochemical parameters and antioxidant machinery in Brassica juncea (L.) Czern (variety PM-25). A completely randomized experiment was conducted in earthen pots containing mustard grown under different levels of chromium stress (T0-control; 50, 100, and 200 mu M) supplemented with graded levels of JA (25 and 50 mu M, alone and in different combinations). Plants were evaluated to examine phenological features and biochemical parameters like total chlorophyll content, total soluble protein (TSP), total soluble sugar (TSS), proline content, and nitrate reductase (NR) activity at three different growth stages. The activity of antioxidant enzymes like SOD, CAT, and APX, along with histochemical staining and confocal microscopy, to locate reactive oxygen species, were also performed. The results suggest that Cr stress (100 and 200 mu M) hampered the growth and yield of mustard seedlings, showing a decrease of - 42 and - 77% in root and shoot fresh weight, - 26% in chlorophyll levels, - 30% in TSP, - 19% in TSS, and - 35% in NR activity over control. However supplementation with JA (50 mu M) improved growth features like root and shoot fresh weight by 128 and 131%, chlorophyll content (43%), TSP (60%), TSS (93%), NR activity (83%) and proline content (140%), in comparison to control. JA (50 mu M) augmented the activity of antioxidant enzymes and decreased ROS levels as depicted by improved cell viability visualized using confocal microspcopy. Hence, supplementation of JA (50 mu M) could be an effective strategy to alleviate the deleterious effects of chromium stress (200 mu M) in mustard plants so as to realize its improved and sustainable production under abiotic stress.