Organic acids exuded from plant roots significantly modify uptake and long-distance translocation of metals. Little is known about the effect of amino acids on metal ion uptake by plant roots. The present study investigated the effects of exogenous amino acids (histidine and glycine) in a nutrient solution on root uptake and xylem sap transport of cadmium (Cd) in triticale (× Triticosecale cv. Elinor) and bread wheat (Triticum aestivum L. cv. Back Cross Rushan). Plant seedlings were grown in a Cd-free modified Hoagland nutrient solution to which 1 µm Cd was added with either 50 µm histidine or 50 µm glycine or without amino acids at 4 weeks after germination. A control treatment consisted of a nutrient solution free of Cd and amino acids. In bread wheat, addition of histidine to the Cd-containing nutrient solution resulted in a higher operationally defined symplastic Cd fraction but a lower apoplastic one in the roots. In triticale, addition of either amino acid decreased the symplastic Cd fraction but increased the apoplastic one. Addition of histidine to the nutrient solution increased Cd concentration in wheat xylem sap but had no significant effect on Cd concentration in triticale xylem sap. Compared with the Cd-only treatment, the glycine-containing treatment led to significantly reduced Cd concentrations in xylem sap of both plant species. Wheat plants supplied with histidine and Cd accumulated greater amounts of Cd in their shoots than those supplied with Cd alone. Glycine had no significant effects on the Cd content of wheat shoots but decreased it in triticale shoots. Results indicate that the effects of amino acids on plant root uptake and xylem sap translocation of Cd depend on the type of amino acid supplemented. This finding is of great importance for selecting and/or breeding cultivars with Cd-toxicity tolerance.
Understanding how complexes with amino acids in soil solution affect plant zinc (Zn) uptake may aid in optimising plant Zn nutrition. We investigated the influence of histidine and glycine in nutrient solution on apoplastic and symplastic uptake and root-to-shoot translocation of Zn in a triticale (×Triticosecale cv. Elinor) and a bread wheat cultivar (Triticum aestivum cv. Back Cross Rushan). Six-week-old seedlings of the two cultivars were transferred to a nutrient solution containing 20 µm Zn, to which 50 µm histidine, 50 µm glycine or no amino acids were added. Control plants were transplanted to nutrient solution with no Zn or amino acids. Higher concentrations of Zn were found in root and shoots of wheat and triticale plants supplied with Zn than in the control plants without Zn supply. Among the treatments with Zn supply, histidine enhanced, whereas glycine reduced, symplastic root Zn concentration in bread wheat. Both amino acids decreased the symplastic root Zn concentration but had no effect on apoplastic Zn in triticale. Both amino acid treatments also reduced Zn concentrations in the xylem sap of the two plant species compared with the treatment with Zn addition only. In bread wheat, the effect was stronger for glycine than for histidine; in triticale, it was the same for both amino acids. The concentration of Zn in xylem sap was always greater in wheat than in triticale. Addition of histidine to the nutrient solution increased the total amount of shoot Zn in triticale but not in bread wheat, compared with the treatment with Zn addition only, whereas glycine had no significant effect on total shoot Zn in either of the plant species. The results show that histidine, but not glycine, can enhance Zn uptake and translocation into the aboveground parts of triticale. We suggest that this ability of histidine was due to the formation of strong complexes with Zn.
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