Cosmos sulphureus Cav. is more tolerant to lead than copper and chromium in hydroponics system

Heavy metals are environmental contaminants that have gained much attention globally. Plant growth and development are greatly affected by toxic heavy metals like lead, copper, and chromium. Cosmos sulphureus Cav. an ornamental plant was grown in hydroponics system against these metals. Plants were exposed to 2, 4, and 8 mg L−1 lead; 5, 10, and 15 mg L−1 copper, and 2.5, 5, and 10 mg L−1 chromium for 3 weeks using control as reference. Lead stress slightly affected the growth of C. sulphureus, however, had extreme signs of phytotoxicity due to copper and chromium. At 8 mg L−1 lead, reductions by 21, 25, 45, and 38% in fresh and dry weight, shoot, and root length were observed, respectively, as compared to control while reductions due to copper/chromium (15/10 mg L−1 in hydroponics) were 79/77, 90/73, 56/60, and 87/90%, respectively. Lead accumulated mostly in roots of C. sulphureus and was observed to be 1000 mg kg−1 at 8 mg L−1. While copper and chromium were also accumulated mainly in roots and were 3 mg kg−1 and 900 mg kg−1, respectively, at 10 mg L−1 in external solution. C. sulphureus without lead-induced phytotoxic effects as well as higher accumulation in roots, strongly advocates this plant as a potential candidate of lead-hyperaccumulator; however, further investigations at the physiological and molecular level are needed to explore the mechanisms for lead homeostasis in roots.