(the growing shoot).
My investigation involves determining the relative toxicity of one heavy metal chloride (lead) on
cress. I expect growth to be inhibited at low concentrations, with death of the plant occurring at
slightly higher ones. I will measure the height of the plants after 5 days growth.
AIMS
The aim is to investigate the effect of varying concentrations of a heavy metal ion on the growth of
cress seedlings. Variables are the concentrations of the heavy metal, all other variables will be
controlled where possible.
HYPOTHESES
PbCl2 will inhibit growth of cress at low concentrations, higher concentrations of the salt will have
greater effect on cress growth.
METHOD-PILOT
Short experiments were conducted for several reasons:
To determine whether the heavy metal compound actually had a negative effect on cress growth.
To determine appropriate conditions.
To establish how long it takes for cress seedlings to have grown sufficiently to observe
differences in growth.
Pilot involved planting cress on cotton wool (soaked in concentrations ranging from 0.2M to
0.002M of the heavy metal salt). This pilot proved that the heavy metal salt did have an adverse
effect on growth. At 0.2M almost all seeds failed to germinate; lower concentrations would be
needed. It was also noticed at this stage that at low concentrations shoot growth was relatively
unaffected, but root growth was greatly decreased. However, roots became very tangled with
cotton wool and were hard to remove. Because of this I decided not to measure dry mass and to
grow the seeds on filter paper.
EXPERIMENTAL PROCEDURES
In order to control the variables, 40 seeds were planted in each Petri dish. All dishes received the
same amount of light at constant temperature in a Dewpoint propagator. The propagator will also
keep a constant humidity and prevent evaporation of the various solutions. If the solutions of lead
chloride had suffered evaporation, the concentration of the solutions would alter. This would
make it difficult to ‘top up’ the solutions. The seedlings were all measured after 5 days. Distilled
water alone acted as a control.
The only variable was therefore the concentration of metal salt.
5 x distilled water (control)
5 x 0.00025M PbCl2 , 5 x 0.001M PbCl2 , 5 x 0.01M PbCl2 , 5 x 0.02M PbCl2 , 5 x 0.1M PbCl2 and
5 x 0.2M PbCl2
APPARATUS
Dewpoint propagator
35 Petri dishes
Lead Chloride
Filter paper circles
Acetate circles (marked with grid)
1500 cress seeds
Distilled water
Measuring cylinders, 1x500 cm 3 , 1x100 cm 3
Chinagraph pencil
Syringes – 50 ml, 20 ml , 10 ml and 5 ml
Spatula
Plastic gloves
Eyeglasses
SAFETY PRECAUTIONS
Plastic gloves and eyeglasses were worn when handling toxic lead chloride to avoid
unnecessary contact. If any solid lead chloride is spilt, this must be removed into a bucket and
the area well rinsed with water.
Any solutions should be clearly labelled as TOXIC.
All excess chemicals will be disposed of following the school safety policy.
METHOD
1. A 0.2M solution was prepared; the appropriate amount (see appendix for calculations) of
metal chloride was added to l000 cm 3 of water. This was diluted to give further
concentrations.
2. An acetate circle was prepared for each Petri dish. This was ruled with a grid of 4 x 5
squares,(draw)
3. Filter paper was then placed over the top of this and 4 cms 3 of the 0.2M solution was added.
This will make the grid visible.
4. Then 2 cress seeds were placed in each square.
5. The Petri dish was labelled using a chinagraph pencil- indicating concentration and date
planted.
6. Stages 2-5 were repeated for another 4 Petri dishes, each containing 0.2M PbCl2.
7. Stages 2-6 were then repeated for each of the other PbCl2 solutions.
8. The dishes are then placed in a Dewpoint propagator for 5 days. This propagator controls
temperature, light and humidity.
9. After 5 days growth, a random number table was used to remove a maximum of 10
germinated seedlings from 5 squares in each Petri dish. This gave a random sample,
while avoiding having to measure all 40 seedlings per dish. To ensure uniformity, growth
was measured as the total length of the plant.
RESULTS SUMMARY.
Whilst all the seedling lengths were only measured to the nearest whole millimetre, all of the
means are given to one decimal place.
Control of water
CONC. (M) Mean Length (mm) Size of sample
0.00000 29.7 39
PbCl2
CONC. (M) Mean Length (mm) Size of sample
0.00025 29.0 39
0.00100 27.9 40
0.01000 10.2 39
0.02000 7.2 40
0.10000 5.6 39
0.20000 4.1 38
OBSERVATIONS
The seedlings displayed a leaf necrosis, they were all dark blue/green.
This was probably a result of inhibition of root proteins (chelates) which transport metal ions (e.g.
iron). Discolourisation was therefore caused by iron deficiency in leaves.
It was also observed that at low concentrations, shoot height was relatively unaffected, but root
growth was greatly inhibited. This implies that inhibition of root proteins occurs before mitosis in
the meristem is inhibited.
CONCLUSION
Results from the investigation show that the lead chloride has a negative effect on cress growth
(as measured by height). The general trend is of decreasing growth with increasing
concentration of PbCl2. The greatest effect of lead chloride is up to 0.02M. Any further increases
have less effect.
Therefore even small concentrations of lead could inhibit enzyme action or mitosis, reducing cell
division and therefore growth. This is supported by the t tests carried out where all but the
0.00025M concentration show a significant effect (see appendix). The t values also support the
greatest significance up to 0.02M as the t value increases less for higher concentrations.
The effect of the lead in such small concentrations has implications for mining waste tips which
may leave the soil with low lead concentrations, but high enough to inhibit enzyme action and
mitosis, therefore slowing or preventing recolonisation.
Further experiments using other plants e.g. mosses and lichens would be needed to see if these
groups were also affected.
SOURCES OF ERROR
It is possible that the lead chlorides were not properly mixed. This would lead to pockets of high
and low heavy metal concentration, respectively encouraging and inhibiting growth. This would
lead to poor results.
Also, not all dishes may have received the same volume of solutions.
MODIFICATIONS
Lower concentrations should have been used to determine the point at which lead affects
germination (down to 0.0001M).
If the cress had been allowed to grow for longer, more difference in growth might have been
recorded. It might have been preferable to have used nitrates, which are more water-soluble
than chlorides.
Dry mass measurements could have been taken.
(graphs to show effects of lead chloride concentration on growth(height ) of cress seedlings)
(Height(mm) on left, concentration on bottom(M))
APPENDIX
Make up of solutions:
PbCl2: RMM = 278
\0.2M ⇒ 55.6g PbCl2/1000 cm 3 distilled water.
From the 0.2M solution prepared serial dilutions were carried out to give 0.1M, 0.02M, 0.01M,
0.001M and 0.00025M solutions.
Lead Chloride concentration (M) 0.2M lead chloride (cm 3 ) Distilled water (cm 3 )
0.20000 1000.00 0.00
0.10000 500.00 500.00
0.02000 100.00 900.00
0.01000 50.00 950.00
0.00100 5.00 995.00
0.00025 1.25 998.75
0.00000 0.00 1000.00
STATISTICAL ANALYSIS
t test
The t test is a test of significance. It is used to tell whether two results are statistically significant.
In general when using more than 30 measurements per set of data, the value of should be
compared with the critical value at ∞ degrees of freedom. This critical value is 1.96 for p = 0.05,
and if the calculated value of t is greater than (or equal to) this critical value then the null
hypothesis can be rejected.
In all the following cases, the null hypothesis is that there is no difference between the data from
the two samples being compared.
t values calculated for control v various heavy metal concentrations
Lead chloride concentration (M)
0.00025 0.00100 0.01000 0.02000 0.10000 0.20000
t value 1.61* 3.72 38.26 45.45 51.78 59.79
The results marked with a *, are not significant, all other results are significant.
