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Using Chelator-Buffered Nutrient Solutions to Limit Ni Phytoavailability to the Ni-Hyperaccumulator Alyssum murale

Abstract Nickel (Ni) is essential for all plants due to its role in urease activation. Demonstration of Ni essentiality has required exceptional effort to purify nutrient solutions to remove Ni; thus, an improved technique would make study of Ni deficiency more available to diverse researchers. As part of our research on Ni hyperaccumulation by plants, we developed chelator-buffered nutrient solutions with very low buffered activity of free Ni2+, and tested growth of Alyssum murale (Goldentuft Madwort), A. corsicum (Madwort), A. montanum (Mountain Alyssum) and Lycopersicon esculentum (Tomato). We used a modified Hoagland nutrient solution with 2 mM Mg and 1 mM Ca to simulate serpentine soil solutions. We could use hydroxyethyl-ethylene-diaminetriacetate (HEDTA) to achieve Ni2+ activity levels as low as 10-16 M, and cyclohexane-ethylenediamine-tetraacetate (CDTA) to supply higher activities of buffered Ni2+ compared with HEDTA; however, we were unable to obtain proof of induced Ni-deficiency, even with urea-N supply in a 6-week growth period, apparently because seeds supplied enough Ni for growth. Yields were somewhat reduced at lower Ni activity by the end of the test period, but strong deficiency symptoms did not occur, apparently due to the supply of Ni from hyperaccumulator species seeds (contained 7000–9000 mg Ni kg-1). Chelator buffering supplied controlled levels of Ni2+ for all test species; very low plant Ni levels were attained when seed Ni was low. Reaching clear and strong Ni deficiency appears to require longer growing periods, using seed with exceedingly low initial endogenous Ni, or species possessing higher Ni requirements.