Comparison of L-Histidine Effects on Nickel Translocation into the Shoots of Different Species of the Genus Alyssum

The work dealt with the influence of free L-histidine on nickel (Ni) translocation into the shoots of the hyperaccumulator plants Alyssum murale, A. fallacinum, A. corsicum, A. tenium, A. lesbiacum, A. bertolonii, A. pintodasilvae, and A. obovatum and of the closely related non-hyperaccumulator Aurinia saxatilis (formerly Alyssum saxatile). The Ni concentration in the xylem sap was determined by graphite furnace or flame atomic absorption spectrophotometry. If plants were not treated with L-histidine or L-alanine, the highest Ni concentration was found in the xylem sap of A. murale and A. corsicum. When the plants were pretreated with L-histidine, the Ni loading into the xylem vessels increased in only two hyperaccumulator species, A. pintodasilvae and A. obovatum, and in the non-hyperaccumulator A. saxatilis. The plant pretreatment with L-alanine did not increase the Ni level in the xylem sap. This indicates that the stimulation of Ni xylem loading is histidine-specific and not characteristic of any amino acid. Therefore, the role of histidine in the selective nickel accumulation in the shoots may considerably differ even in closely related plant species of one genus. This may presumably be accounted for by both different contents of endogenous histidine in the roots and specific patterns of the metal transport and distribution in different species.