4 RESULTS AND DISCUSSIONS
The laboratory experiments for this research were started in
March 2007 and ended at the end of October 2007. Water hyacinth plants were
collected several times but unfortunately some of them died due to rigorous
conditions, more than 1,000 samples (water and plant mixed) were analyzed and
the results for this research are average values.
4.1 Variations on plant relative growth
4.1.1 Relative growth of water hyacinth plants
The relative growth was calculated to estimate the effects of
zinc and chromium concentrations on plant growth according to exposure time.
The relative growth indicates the tolerance of the plants to different
concentrations of metal and exposure time to these metals. It was observed that
the final fresh weight increased compared to the initial fresh weight in the
first week but it decreased when the metal concentration increased, mostly for
6 mg/L. There is a slight stagnation for the second week and fourth week in
plant growth in terms of metal concentration. The growth decreased with
increasing time and metal concentrations due to a decreasing of essential macro
and micro nutrients for the plants in the experimental water in the small
buckets.
The growth of water hyacinth at different concentrations of
chromium and zinc is shown on Figure 4.1. It was noted that the relative growth
of the plants decreased considerably in the first week only and slightly
decreased with an increasing metal concentration and exposure time. There was
constant trend in the plant growth after 1 week of experiment. However, at
higher concentrations of these metals, plant growth was always inhibited. The
growth of water hyacinth plants significantly increased (P = 0.05) with the
passage of time according to the Figure 4.2.
|
Relative growth values
|
5 4 3 2 1 0
|
|
|
1 wk, 1
1 wk, 3
1 wk, 6
2 wks, 1
2 wks, 3
2 wks, 6
4 wks, 1
4 wks, 3
4 wks, 6
Exposure time (week) and initial conc.
|
Figure 4.1: Relative growth of water hyacinth plants vs
exposure time for different Zn and Cr concentrations
4.1.2 Discussions on relative growth of water hyacinths
For water hyacinth plants treated with Zn and Cr, the relative
growth significantly decreased (P = 0.05) from 1, 3 and 6 mg/L in 1 week but
for 2 and 4 weeks, the relative growth slightly decreased linearly with the
increasing (P = 0.05) of metal concentrations. In the case of zinc and
chromium, however, the relative growth exhibited an exponential decrease caused
by relatively increasing toxicity in contrast to chromium and zinc
concentrations.
The ANOVA 2 shows that for 1 week exposure time, there is a
high effect (difference is significant) of initial concentrations (1, 3 and 6
mg/L) to the growth of the plants (P = 0.05), but for 2 and 4 weeks according
to initial concentrations, the difference is not significant (P = 0.05).
4.1.3. Correlation between final fresh weight and
relative growth
Figure 4.2 shows that there is no correlation between the
final fresh weight and the relative growth of water hyacinth plants. The non
existence of correlation is reported by the stagnation in plant growth due to
the decreasing nutrients and increasing metal concentrations. It was considered
that a decrease in the growth was induced by metal toxicity.
Final Fresh weight vs Relative growth
y = 0.009x + 1.4994
R2 = 0.2977
30 50 70 90
Final Fresh Weight (g)
4
3
2
1
0
Relative growth
Figure 4.2: Correlation between Relative Growth of plants and
Final Fresh Weight
| 
