CHARACTERIZATION OF CADMIUM-BINDING, UPTAKE, AND TRANSLOCATION IN INTACT SEEDLINGS OF BREAD AND DURUM-WHEAT CULTIVARS

High Cd content in durum wheat (Triticum turgidum L. var durum) grain grown in the United States and Canada presents potential health and ec onomic problems for consumers and growers. in an effort to understand the biological processes that result in excess Cd accumulation, root C d uptake and xylem translocation to shoots in seedlings of bread wheat (Triticum aestivum L.) and durum wheat cultivars were studied. Whole- plant Cd accumulation was somewhat greater in the bread wheat cultivar , but this was probably because of increased apoplastic Cd binding. Co ncentration-dependent Cd-109(2+)-influx kinetics in both cultivars wer e characterized by smooth, nonsaturating curves that could be dissecte d into linear and saturable components, The saturable component likely represented carrier-mediated Cd influx across root-cell plasma membra nes (Michaelis constant, 20-40 nM; maximum initial velocity, 26-29 nmo l g(-1) fresh weight h(-1)), whereas linear Cd uptake represented cell wall binding of Cd-109. Cd translocation to shoots was greater in the bread wheat cultivar than in the durum cultivar because a larger prop ortion of root-absorbed Cd moved to shoots. Our results indicate that excess Cd accumulation in durum wheat grain is not correlated with see dling-root influx rates or root-to-shoot translocation, but may be rel ated to phloem-mediated Cd transport to the grain.