Tuesday 17 April 2012

Can some plants really stop other plants from growing around them?

Welcome to my blog! 
This update is on an experiment we did in class to see  the effect living plants may have on other plants that may be present nearby. This is known as allelopathy. Secondary metabolites cause these allelopathic effects.The most commonly known plant to exhibit this is the black walnut tree. Where a black walnut tree is present, most other plants cannot grow.Walnut trees produce a compund called Juglone, which stops other plants growing.  In this experiment we tested the allelopathic effects of walnuts, oranges and lemons on lettuce seeds (Lactuca sativa). The aim of the experiment is to establish if the listed plants inhibit the germination of lettuce seeds and if they effect seedling growth.
I took some photo's whilst carrying out the experiment, so you can see what it looked like, but bear in mind the quality isn't super as I took them from my phone.

Materials you’ll need:

2g walnuts
2g Orange peel
2g Lemon peel
4 Petri dishes
Filter paper
Pestle and mortar
15ml screw capped tubes
Lettuce seeds

What you’ll need to do:

1) Finely grind 2g of walnut using the pestle and mortar. This should take approximately five minutes.

2) Add the finely ground material to one of the 15ml screw capped tubes and label.
3)Repeat steps 1 & 2 for both the orange peel and the lemon peel.

4) Add 10ml of water to each tube and place in a water bath at 37°C for 10 minutes.

5) Centrifuge the tubes,

6) Label one Petri dish the control. Label the other three dishes: walnut, orange and lemon respectively.

7) Place a piece of filter paper in each dish.

8) Wet the filter paper in the dish labelled control with distilled water.
9) Wet the filter paper of the other dishes with centrifuged extract, adding walnut to the dish labelled walnut, lemon extract to the dish labelled lemon and orange extract to the dish labelled orange.

10) Carefully place 40 lettuce seeds on the filter paper of each dish, ensuring that the seeds do not touch and there is adequate space between the seeds.

11) To make sure your dishes do not dry out, wrap each dish in Parafilm before incubating. Observe results next week.


The results of this experiment showed abundant growth on the control dish. Both roots and shoots of the seedling grew very happily. 
However, no seedling growth appeared on the walnut, lemon or orange peel dishes.

 Orange Peel

 Lemon Peel

Microbial growth was seen on the orange and lemon peel dishes, most notably a blue mould on the orange peel dish, presumed to be Penicillum.
95% of seeds grew on the control dish. 0% germination and seedling growth was recorded on the walnut, lemon and orange dishes. 
The seeds on the control dish were growing happily and looked healthy.
Some of the seeds in the dishes were discoloured as a result of allelopathy. Also, discolouration of the filter paper around the seeds could be seen and various forms of microbial growth.


The results of this experiment suggest that walnut, lemon and orange extracts  inhibit the germination and growth of lettuce seedlings. This is shown by the lack of germination on the plates containing these materials. This would suggest that these extracts contained compounds that inhibit germination and seedling growth. I conclude that there are compounds present in the three plants used that stop the lettuce seeds from growing.