INSIGHTS INTO THE NUTRITIONAL PHYSIOLOGY OF NICKEL

Nickel (Ni) is essential for plants, yet its physiological role is poorly understood. Ni-deficient and Ni-sufficient pecan [Carya illinoinensis (Wangenh.) K. Koch] trees were compared regarding the impact of Ni nutritional status on reduced nitrogen (N) forms present in xylem sap at spring bud break. High performance liquid chromatography (HPLC) of xylem sap of Ni-sufficient trees found organic reduced N-forms to be primarily ureides (73%; citrulline > xanthine > ureidoglycolate > allantoic acid ≈ allantoin ≈ uric acid ≈ urea), followed by amide-N (26%; asparagine), and amino-N (1%; tryptamine and β-phenylethylamine). Nickel deficiency reduced xylem sap concentration of xanthine, asparagine, and β-phenylethylamine, yet greatly increased citrulline and allantoic acid. These data indicate that pecan is likely a ureide-N transporter and that Ni deficiency potentially disrupts ureide catabolism and urea cycle functionality; thus, potentially disrupting normal N-cycling during early spring when N reserves are being remobilized to sinks.