Effect of exogenous H₂O₂ on antioxidant enzymes of <i>Brassica juncea</i> L. seedlings in relation to 24-epibrassinolide under chilling stress

Abstract

Hydrogen peroxide is most stable molecule among reactive oxygen species, which play a vital role in growth and development of plant as signaling molecule at low concentration in response to various abiotic and biotic stresses. Exogenous application of H₂O₂ is known to induce chilling tolerance in plants. Brassinosteroids are plant steroid hormones known for their anti-stress properties. In this study, effect of exogenous H₂O₂ on antioxidant defense system of <i>Brassica juncea</i> L. seedlings was investigated in 24-epibrassinolide (24-EBL) treated and untreated seedlings under chilling stress. The surface sterilized seeds of <i>B. juncea</i> L. were germinated in petriplates containing different concentrations of H₂O₂ alone and in combination with 10^-8M 24-EBL. Chilling treatment (4 ºC) was given to 10-days old seedlings grown in different treatments for 6 h daily up to 3 days. 24 h recovery period was given to chilling treated seedlings by placing at 25ºC ± 2ºC and harvested for antioxidant enzymes on 14^th day after sowing (DAS). Treatment of 24-EBL in combination with H₂O₂ (15 and 20 mM) helped in reducing the toxicity of seed and seedlings due to H₂O₂ exposure on their germination rate, shoot and root length respectively. 24-EBL treatment at seed and seedling stage helped in alleviating the toxic effect of H₂O₂ through antioxidant defense system by increasing the activities of various enzymes involved in antioxidant defense system such as catalase (CAT, E.C. 1.11.1.6), ascorbate peroxidase (APOX, E.C. 1.11.1.11), and superoxide dismutase (SOD, E.C. 1.15.1.1). In conclusion, exogenous pretreatment of H₂O₂ to seeds of <i>B. juncea </i>L. adapted the seedlings to tolerate chilling stress, which was further ameliorated in combination of H₂O₂ with 24-EBL.