Relationship between root structure and root cadmium uptake in maize

A clear description of the mechanism of root cadmium absorption is required in order to understand how this toxic metal is phytoaccumulated. An experiment was carried out in order to test the hypotheses that (1) the Cd uptake is higher for maize roots grown in hydroponics than for those grown in aeroponics, (2) this difference is due to the fact that in aeroponics, root apoplastic barriers are developed more extensively than in hydroponics and (3) the structure of maize roots grown in aeroponics is closer to the structure of roots grown in soil. Apoplastic and symplastic cadmium uptake was measured by exposing the roots to a radio-labelled Cd solution and by the physical fractionation of the metal in the roots. Maturation of the exodermis and endodermis as well as their distance from the root tip were assessed through fluorescent staining and microscopy of successive sections along the main root. The results obtained support the initial hypotheses. Both symplastic absorption and apoplastic adsorption of Cd were higher for maize roots grown in hydroponics than in aeroponics. This can be explained by the fact that the beginning of the barrier formation took place on average at a distance of 12.46% of the root length from its tip, while in aeroponics, root apoplastic barriers developed much earlier, at 2.72% of the root length. Since the characteristics of maize plants roots cultivated in aeroponics were much closer to those cultivated in soil, their kinetic parameters may be considered to be more representative when measuring uptake than those of hydroponically grown plants. (C) 2010 Elsevier B.V. All rights reserved.