may explain the higher retention of Au in the roots of BJ. A maximum
of 188 mg of Au g−1 dry weight was observed in BJ roots when
exposed 10,000 ppm Au solution for 24 h. MS roots accumulated
193 mg of Au g−1 dry weight at the same conditions. The Translocation
Factor (TF), defined as the ratio of metal accumulated in the
shoots to metal accumulated in the roots, was between 0.15–0.50 in
BJ and 0.25–0.97 in MS. These results indicate an increase in TF with
an increase in concentration and thus increase in metal transport
from roots to shoots was observed with increased concentration of
Au in the solution.
The roots of both BJ and MS in general showed a greater UR
defined as the ratio of Au concentration in plant tissues to the
concentration in the solution to a maximum of 994 in MS and
906 in BJ. On average, MS exhibited a higher root and shoot UR
than BJ. Fig. 2a shows a decrease in the relative uptake of Au
with an increase in solution Au concentration. The greater UR of
the roots suggests restricted translocation of Au to shoots once
absorbed. Although the reason for this restricted translocation was
not able to be established from these experiments, it is possible
that significant quantities of Au were retained in the root vacuoles,
and/or apoplasts. This result suggests that plant roots rapidly
deplete Au ions from the solution and hence have a high affinity for
Au+