Investigating the effects of varying pH levels on the germination of cress seeds

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I will be investigating the effects of varying pH levels on the germination of cress seeds.

A seed is a fertilised and ripened ovule, consisting of the plant embryo, varying amounts of stored food material (endosperm), and a protective outer seed coat (aleurone layer). The embryo secretes hormones such as cytokinin, and indole acetic acid, which promote cell division. The endosperm contains insoluble food stores including starch, proteins, and lipids, all of which are used in the initial germination stages. The aleurone layer contains proteins, and under the influence of gibberellic acid (manufactured and secreted by the embryo) synthesises amino acids into hydrolytic enzymes such as   -amylase and a protease. These enzymes are then used to hydrolyse starch  glucose, and proteins  amino acids for further growth and development.

The scale used to measure the acidity or alkalinity of a solution is the pH scale, which generally ranges in values from 0-14. Acidic solutions have pH values below pH 7, which is the value of a solution considered to be neither acid nor alkaline i.e. neutral. In pure water, the concentration of hydrogen ions is equal to 10ˉ  ; when an acid is added to pure water, the hydrogen ion concentration increases above this level.

The pH level of the solution in which the seeds are germinating affects the availability of elements. Extreme acid/alkaline conditions may adversely affect plant growth by altering selected nutrient availability.  Moreover, extreme acid/alkaline conditions can be very corrosive, and would more than likely denature vital enzymes due to their delicate nature as proteins.

Slightly acidic conditions can be advantageous to germinating seeds, as the higher concentration of Hˉ ions encourages the uptake of nutrients.

I therefore predict that when grown in a variety of acidic pH values, the success of germination will decrease as the acidity increases.

Null hypothesis: success of seed germination is not dependent on pH level.

To prove/disprove this null hypothesis, I will use the Chi-squared goodness of fit test with my results to show if there is statistical evidence to reinforce that the null hypothesis is/is not correct. This statistical test will compare the observed numbers of seeds that germinate with the expected number of seeds that will germinate (i.e. that the same number will germinate in each sample), and show if any difference of germination success amongst the different acidic conditions is significant (thus disproving the hypothesis) or occurred by chance.

To carry out the initial experiment, I will use the following equipment:

The following solutions will be produced, each allocated to one sample:


Acids can cause burns and irritate the respiratory system. Eye protection should be worn.


Sulphuric Acid: Very corrosive; causes severe burns. Solutions equal to or stronger than 1.5 M should be labelled ‘corrosive’.

Can be dangerous when mixed with water; always add acid to water (never the reverse) when diluting.

Can be dangerous when mixed with hydrochloric acid; hydrogen chloride given off.

Hydrochloric Acid: May cause burns. The vapour is very irritating to the respiratory system. Solutions of 1 M should be labelled ‘irritant’.


From an ethical perspective, some may disagree with placing the cress seeds in acidic, and thus potentially harmful conditions; moreover, some may also disagree with the disposal of the cress seeds/germinating cress seeds as it would be potentially/essentially killing living organisms.

The loss of these (potentially) living organisms could contribute to long-term aid for other plants on a much larger scale. I therefore believe that this is justified as the experiment could be developed and the results could lead towards a solution to the problems of acid rain on a global scale.


  • I will measure out each solution (using volumes as previously described) into separate bottles, label each one, and measure and record the pH value of each one with the Universal Indicator.
  • I will then cut out circles of cotton wool to fit the petri dishes. I will ensure that the total weight of the petri dish with the cotton wool is kept equal amongst all the samples i.e. they will all weigh the same.
  • I will add 20cm³ of each solution to the corresponding sample petri dish.
  • I will count and add 40 cress seeds to each sample petri dish.
  • I will add a further 10cm³ of each solution to the corresponding sample petri dish.
  • I will weigh each sample, and record the results.
  • I will place all of the samples in an area where natural light is available. I have chosen a shelf directly parallel to a window, where there are no obstructions blocking the light.
  • After 24 hours, I will weigh each sample and record the results. I will also take note of any physical changes that have occurred.
  • I will add a further 10cm³ of each solution to the corresponding sample Petri dish.
  • If germination has begun, I will measure and record the length of any sprouts or stems.
  • I will repeat the previous 3 steps every 24 hours for a further 6 days.
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As well as pH levels, there are other variables which can change unintentionally, and thus affect the outcome of the experiment. These include:

  • Volumes and concentrations of solutions
  • Temperature
  • Light availability
  • Wind/Movement

The concentrations will be varied intentionally, and therefore do not need to be controlled. The volumes of solution that each sample receives will be kept constant amongst all the samples. I will attempt to control the remaining variables by keeping all of the samples in the same room, in the same area, with closed windows and doors. In the event ...

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