Submitted to: International Journal of Phytoremediation
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/9/2010
Publication Date: 9/1/2011
Citation: Cassina, L., Tassi, E., Morelli, E., Giorgetti, L., Remorini, D., Massai, R., Chaney, R.L., Barbafieri, M. 2011. Exogenous cytokinin treatments of a Ni hyper-accumulator, Alyssum murale, grown in a serpentine soil: Implications for phytoextraction. International Journal of Phytoremediation. 13(S1):90-101. Interpretive Summary: Cytokinin treatments are used in commercial horticulture to increase yields or reduce plant stress for many crops. The present study was undertaken to learn if cytokinin treatment would increase the value of the phytomining crop Alyssum murale. Alyssum was grown in an Italian serpentine soil rich in Ni, and given four treatments for six weeks: 1) control; 2) foliar spray with cytokinins; 3) soil application of cytokinins; and 4) both foliar and soil treatments with cytokinins. A commercial cytokinin product was used in the tests, with 3 applications at two week intervals. Shoot biomass yield was significantly increased by the cytokinin treatments by 50-75%. Interestingly, shoot Ni concentration was not significantly changed by the treatments, but the higher yields means that harvestable Ni in biomass was significantly increased by 50-75% in the different treatments. Observations of leaf stomata confirmed that the cytokinins had the expected effect of increased stomatal openings, which increased transpiration as expected. In contrast with some other suggestions, tissue phytochelatins (metal binding peptides derived from glutathione) were not increased by the cytokinin treatments, and the molar levels of phytochelatins were far lower than that of Ni indicating that phytochelatins are not important in Ni hyperaccumulation or tolerance. Thus spray application of cytokinin on field crops of Alyssum murale for Ni phytomining may be a commercially useful practice.
Technical Abstract: Application of exogenous plant growth regulators was examined as a viable technique to increase the efficiency of plant metal phytoextraction from contaminated soils. The aim of this study was to investigate the alteration of Ni phytoextraction by Alyssum murale, a Ni hyperaccumulator, following the application of cytokinins. The following parameters were investigated: Ni accumulation, plant growth, gas exchange, stomata behavior and the concentration of non-protein thiols (glutathione, y-Glu-Cys and phytochelatins). In a pot experiment, A. murale plants grown in a serpentine soil were treated with a mix of naturally occurring cytokinins. Results showed that Ni accumulation in plants ranged from 4000 to 7000 mg kg-1 shoot biomass confirming the hyper-accumulation ability in the soil used. Cytokinin treatments produced a significant increase in plant biomass and transpiration rate whereas no significant variation in Ni accumulation or the concentration of non-protein thiols was observed. The results suggest that A. murale is a plant species sensitive to cytokinin treatment and that cytokinin treatment is a potentially useful method for increasing the phytoextraction capability by increasing biomass. Moreover, for first time, evidence was obtained that the Ni hyperaccumulation mechanism is independent of water flux and transpiration rate.