Exogenous melatonin regulates chromium stress induced feedback inhibition of photosynthesis and antioxidative protection in Brassica napus cultivars

Abstract Developing heavy metal chromium (Cr) tolerant cultivars is mainly constrained due to poor knowledge of the mechanism behind Cr stress tolerance. In the present study, available germplasm of Brassica napus (canola) was screened out for Cr stress tolerance using Dual-PAM-100 and biochemical attributes and used to selected one Cr stress tolerant (Ac-Excel) and one sensitive (DGL) under melatonin (MT) treatments for further studies. Plant oxidative enzymes and PSII activity were found to be key physiological discriminating factors. Changes in PSII and PSI efficiency, oxidative damage, higher proline content and antioxidants were further evaluated in selected Cr tolerant and sensitive cultivars. Cr stress reduced PSII efficacy and ETC in both canola cultivars. MT application increased NPQ and Y(NPQ), higher increase in NPQ and Y(NPQ) were observed in Cr stress tolerant cv AC-Excel, indicating that the MT treated tolerant cultivar have higher excitation of PSII by more heat dissipation via photo-protective component of NPQ under Cr stress. Reduced PSI efficiency and increased donor end limitation of PSI in both canola cultivars further confirmed the downregulation of electron transport from PSII. However, Cr stress sensitive cv DGL had poor efficacy to manage over-excitation of PSII via buildup of Y(NPQ) through increased oxidative stress. Tolerant cultivar exhibited higher proline content and antioxidant enzymes activity than Cr sensitive cultivar. Overall, MT induced tolerance in canola cultivars can be related to PSII activity, Y(NPQ), and antioxidants potential which can be effectively used for canola cultivars selection under Cr stress.