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(Original post by maggyxxp)
June 2016 Paper 2

Lead ions can be toxic to plants. Different plants are killed by different concentrations of lead ions. The toxic concentrations for different plant categories are shown in the table.

Toxic lead concentration /ppm Tolerance category of plant
<300 low
300-999 medium
1000-2000 high
>2000 very high

Devise a laboratory investigation to determine the tolerance category of tomato plants to lead ions.

Please please please help me with this one, especially with the preparing plants (should i use plantlets here?) and results bit!
Here are some ideas:

. Include a 5th control plant (that'll you'll add no lead to but keep everything else the same). This is so that you can see the difference the independent variable is making (i.e The concentration of lead)

Control variables (The variables that you keep the same)

. Try to choose similar looking plants grown in similar conditions up to the period when you would start the experiment

. Include the same amount of soil, same type of soil and same size of pot (so that the distribution of the lead solutions will be roughly equal).

. Keep the plants in the same place (so that they get equal exposure to sunlight).

. Water them with the same quantity of water.

. Give them the same volume of lead solution.

The independent variable is the concentration of the lead solutions.

. You could run it over e.g a 2/3/4 week period, giving say 100cm3 of the 4 different lead solutions to each of the 4 respective plants at the start of the investigation.

Recording data

. The dependent variable could be something like growth rate or just growth - use a ruler to measure stem height of each plant e.g every 2 days during the experiment at a set time.

.Doing growth rate could be something like this: Record the starting heights of each plant before adding the lead solution and then take a recording e.g Every 2 days over a 2 week period. You could calculate growth rate every 2 days via (m2-m2/T) where T is time (e.g 2 days) and M2 is the recent height measurement, M1 is the previous height measurement. You could then take an average.

. You could also record dead or alive - At the end of the period, see which tomato plants are alive and which ones are dead or visibly damaged. Record your results in a table alongside the growth data.

. Repeat the experiment x amount of times.


You would probably have to describe how the data from your results shows how you can deduce the tolerance category of the species of plant. In theory the poisoned plants will at least show a decline in growth rate or could stop growing all together.

Those are my thoughts anyway.
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Report 4 years ago
Assuming this for A level, and a 6 marker? I would start off by saying getting 6 seeds from the same tomato plant to ensure genetic similarity, plant each one in equal amounts of soil with varying amounts of lead ions, prob best naming the ranges in the table, have a control that is just grown in soil. Control all other abiotic factors, type of soil, amount of water and sunlight etc. Do repeats to ensure reliability, after a set period of time measure how tall the plant has grown, or you could weigh at the start and weigh at the end to see how much mass it has gained, you can then see at what tolerance it stops gaining mass and ends up losing mass, this will show it has too many Lead ions to tolerate. Oh and repeat the experiment 3 times atleast.

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