the roots. For all treatments, Au uptake in the roots and shoots was
greater at 360 h than at lesser exposure times.
Fig. 3a shows that BJ andMSexhibited an increase in uptake with
an increase in concentration. BJ had significantly higher uptake
compared to MS at low concentrations (5–20 ppm) with approximately
3-fold greater uptake than MS. However, the rate of increase
in uptake with an increase in concentration (5–20 ppm) slowed
beyond these concentrations. This might be because of an increase
in the fixed charge at the root cell surface which is believed to
increase the resistance to metal uptake by slowing metal ion diffusion
through the cell wall. At higher concentrations (40–80 ppm),
MS exhibited higher comparative uptake than BJ.
At low solution concentrations (5–20 ppm) the roots transport
greater quantities of Au to the shoots in BJ than MS (Fig. 3b). Au
mobility to shoots is greater at lower concentrations indicating
lower retention in the roots. At higher concentrations Au transport
occurs at slower rates as indicated in Fig. 3b. A maximum TF
of 0.14 was achieved at the lowest solution concentration studied
(5 ppm) after 48 h exposure time. The TF for BJ decreased in general
across the lower concentrations (5–20 ppm) and increased with an
increase in concentration (40–80 ppm). Transport of Au to shoots in
MS was different from BJ; the results show less transport compared
to BJ. Amaximum of 0.087 TF was achieved at 5ppm concentration
and 48 h exposure time. MS showed similar trends to BJ with a
decrease in TF from 5 to 20ppm and an increase in TF from 20 to
80 ppm.
These results suggest that a large quantity of Au is immobilised
within the root tissues, e.g. in the apparent free space (AFS) associated
with pectin and protein fractions. AFS is freely accessible to
ions where cation exchange and complexation at the cell wall components
take place. It can be inferred that the uptake of Au increases
over a period of 360 h, but the rate of uptake decreases considerably
after 72 h.
Experiments at low Au concentration over a period of 360 h
show that although the plants were totally necrotic after 72 h, they
continued to absorb Au in appreciable amounts. In general, the
uptake of Au by MS and BJ shoots increased with time exposure.
The data clearly suggest that at shorter exposure times, the absolute
quantity of Au accumulated by BJ shoots was 3–4 orders of
magnitude greater than for MS shoots (Fig. 3c). At longer exposure
times the accumulation of Au by BJ was greater still, however
it was only twice the uptake by MS shoots. Thus BJ is a better
accumulator of Au at low concentrations and longer exposure
times.