Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/1997
Publication Date: N/A
Interpretive Summary: Concerns about human consumption of foods that contain cadmium (Cd) have led international agencies to propose strict limits on the permissible levels of this heavy metal in unprocessed food products intended for export. Durum wheat grown in the northern Great Plains region of the U.S., where Cd occurs naturally in the soil, has been shown to accumulate Cd to levels that exceed the proposed limits. In order to devise agronomic methods that will reduce the levels of Cd in durum wheat, it is important to understand the biological processes that lead to excessive Cd accumulation. This study was designed to examine the first physiological steps in Cd movement from soil solution to mature grain: movement of Cd into roots and subsequent translocation to shoots. Cd fluxes were measured and compared in seedlings of durum and common bread wheat, which does not accumulate excessive levels of Cd. The results demonstrated that the rate of Cd influx into seedling roots was not different in the two wheat types. The data also showed that Cd moved more slowly from roots to shoots in the durum variety. These findings suggest that root uptake of Cd and movement of Cd to the shoot are not critical factors in accumulation of Cd in grain. Additional studies focusing on the movement of Cd into developing grains of durum wheat may shed light on the causes of Cd accumulation in durum wheat grown in the northern U.S., and lead to field practices that ultimately reduce Cd levels in exported grain.
Technical Abstract: High cadmium (Cd) content in durum wheat grain poses potential health and economic problems. In an effort to understand the biological processes that result in excess Cd accumulation, Cd uptake by roots and xylem translocation to shoots in seedlings of bread and durum wheat cultivars were studied. Accumulation of Cd in roots was somewhat greater in the bread dwheat cultivars. However, the difference was partly due to increased nondesorbable apoplastic binding of Cd in the bread wheat line. Desorption of Cd from the apoplasm was rapid, with most adsorbed Cd removed in less than 5 min. Concentration-dependent influx kinetics in both cultivars were characterized by smooth, non-saturating curves which were readily dissected into linear and saturable components. Experiments designed to inhibit membrane transport of Cd suggested that the linear component represented apoplastic binding of Cd. The saturable concentration-dependent component likely represented carrier-mediated influx of Cd across the plasma membran of wheat root cells. Km values were somewhat higher in the durum wheat line and Vmax values were similar in the two cultivars. Time-dependent trans- location of Cd to shoots was much greater in the bread wheat cultivar than in the durum cultivar. This was a result of a larger proportion of root- absorbed Cd moving to shoots in the bread wheat line. Results indicated that excess Cd accumulation in durm wheat is not correlated with root influx rates or root-to-shoot translocation, but may be related to symplasmic transport of Cd.