Citric acid and 5-aminlevulinic acid assisted phytoremediation of chromium through sunflower

Abstract

Environmental pollution caused by heavy metals are of great concern now a day as it pose a great threat to human, animals and plants by entering into food chain and living tissue. A large number of heavy metals are being under consideration worldwide due to their potent environmental effects. Chromium (Cr) is one of them causing serious environmental damages. The irrigation of important food crops and vegetables with industrial and municipal wastewater might have human and animals to a significant health risk. For the remediation of heavy metal contaminated soils and water, the most economically effective, ecologically safe and environmental friendly technique is phytoremediation. Few hyper-accumulator edible crops such as, brassica and sunflower are subject to accumulate higher concentration of heavy metal from moderate to highly contaminated soils. Keeping in view the above scenario, this research was conducted to study the morpho-physiological and biochemical mechanisms of sunflower involved in Cr tolerance. Six Faisalabad Hybrids, FH-614, FH-600, FH-619, FH-620, FH-425, FH-612 varieties were selected and one best Cr tolerant variety was chosen for further experiments. The present study was divided into four sub studies. Prior to pot experiment, a short seed germination test was run under different Cr levels (0, 10, 50 and 100 µM) in petri dishes. The selected varieties were grown in Cr spiked soil (0, 5, 10 and 20 mg kg-1) in a pot experiment under natural conditions. Plant height, root length, fresh and dry biomass of root, stem and leaves, chlorophylls and SPAD value, carotenoids, water use efficiency (A/E), transpiration rate (E), stomatal conductance (gs) and photosynthetic rate (A) were measured. The enzymatic activities of antioxidants, such as ascorbate peroxidase (APX), superoxide dismutase (SOD), guaiacol peroxidase (POD), catalase (CAT), reactive oxygen species such as hydrogen peroxide (H2O2), malondialdehyde (MDA), electrolyte leakage and soluble proteins both in roots and leaves were also measured. Cr contents in roots, stem and leaves were determined. The best tolerant variety of sunflower FH-614 was screened and selected for further experiments. Then, same variety of sunflower (FH-614) was grown in Cr spiked soil (0, 5, 10 and 20 mg kg-1) and citric acid (CA) (0, 2.5 and 5mM) and 5-aminoluvulinic acid (ALA) (0, 10 and 20 mg L-1) with factorial arrangement to investigate the promoting role of ALA and chelating effects of CA in Cr stressed plants alone and in combinations. All morpho-physiological and biochemical attributes of sunflower were measured under Cr, CA and 5-ALA. The statistical data revealed that Cr significantly decreased the agronomic traits of all six selected hybrids along with biochemical and physiological changes. The second study (Cr +CA) showed same trend of Cr stress but the application of CA alleviated the Cr induced toxicity and also enhanced the Cr concentration and accumulation in plant tissues. The third study (Cr+5-ALA) depicts similar results to second study but the combined application of CA and 5-ALA under Cr stressed plants in fourth study showed additive effects in alleviating Cr induced toxicity and accumulation of Cr in plant tissues. In combined application, CA enhanced the mobility and solubility of Cr in soil while 5-ALA regulated the normal functioning of plant metabolism which capable plant to accumulate higher concentration of Cr and translocated it to higher parts (stem and leaves). The present study suggested that the combined application of CA and 5-ALA to sunflower plants grown in Cr contaminated soils can enhance the phytoextraction of Cr and a suitable management technique for the Cr contaminated soils. The possibility of combined CA and 5-ALA for other heavy metals and plants required more detailed studies in future to combat heavy metal pollution.