Effects of exogenously applied melatonin on growth, photosynthesis, ion accumulation and antioxidant capacity of canola (BRASSICA NAPUS L.) under chromium stress

The present investigation was performed to examine the positive role of exogenous application of melatonin on biomass production, photosynthetic pigments, total soluble proteins, nutrients uptake and oxidative defense on canola under chromium (Cr) stress. Two week old plants of a canola cultivar Ac-Excel were subjected to three stress levels of chromium (Cr) stress (0, 50 and 100 µM) and four levels of melatonin (MT) (0, 1, 5, and 10 µM) were applied foliarly. Cr stress significantly reduced the plant growth attributes, chlorophyll contents, total soluble proteins and total free amino acids, catalase (CAT), peroxidase (POD), superoxide dismutase (SOD) activity and yield attributes of canola. Foliar application of melatonin significantly increased plant growth status in terms of shoot height, number of leaves, fresh and dry biomass, enhance accumulation of total soluble proteins under Cr stress. Overall, foliar application of melatonin considerably improved stress tolerance of canola plants by up regulating the ROS scavenging enzymes (catalase, peroxidase). Chlorophyll a fluorescence measurements (F/sub o/, Fm, F/sub v/, F/sub o/Fm, Fv/F/sub o/, Fv/F/sub m/ and other JIP-test parameters) revealed that chromium stress caused PSII photoinhibition by damaging donor end of PSII (oxygen evolving complex) and acceptor end of PSII (over reduction of QA). However exogenous application of melatonin protected the oxygen evolving complex of PSII and helped out in maintaining PSII activity. Overall melatonin induced growth improvement in canola cultivar under chromium stress was seemed to be associated with activation of antioxidant defense system which protected PSII from oxidative damage thereby resulting in enhanced photosynthetic capacity. (author)