Allelopathy

Do not mess with this plant. It could be deadly.....

Allelopathy:

Introduction:

The word allelopathy comes from the Greek words allelon (‘of each other’) and pathos (‘to suffer’).  A literal translation would be ‘to cause suffering to each other’. In scientific terms, allelopathy is the inhibition of growth of a plant species and microorganisms caused by another plant species using chemicals called allelochemicals. These allelochemicals are secondary metabolites and may affect the root/shoot growth or nutrient uptake of a plant, meaning it will not thrive in this environment. They are released by the plant into the surrounding area and can be found within the plants leaves, roots, flower, surrounding soil, fruits and stems.  The 3 main types of secondary metabolites are terpenoids, alkaloids and phenylpropanoids. Terpenoids are extensively used by humans but some can be dangerous.  Allelopathy plays an important role in competition between plants and is important for species distribution and abundance.

The earliest studies of allelopathy were carried out by Theophrastis. He found that chickpeas used up all the soil nutrients and destroyed weeds. Sorghum bicolor (Sorghum) is a cereal crop which is grown in Africa. It produces a phenolic acid in its roots called sorgoleone. Sorgoleone disrupts mitochondrial functions in plants, inhibiting photosynthesis. Sorghum is currently being researched as a weed killer. 

Pteridium aquilinum (Braken) releases phenolic glycosides as its secondary metabolite when damaged. This phenolic glycoside produces cyanide which in turn will kill whatever has eaten it and inhibit grazing. Eucalyptus oil, leaves and roots also have allelopathic properties. A study was carried out on the effect of Eucalyptus tereticornis oil on seedling growth of Amaranthus viridis. It was found that the Eucalyptus oil greatly hindered the growth of Amaranthus viridis. http://www.tandfonline.com/doi/full/10.1080/17429145.2010.539709 (link to the paper).

Oranges have essential oils in their skins which have been proven to affect the germination of certain plants. This is due to the acidic pH in the orange peel.

Daffodils (Narcissus spp.) contain lycorine and narcissen which causes antimitic activity in plants thus leading to inhibition of growth. 

Garlic (Allium sativum) has been proven to destroy symbiotic relationships between tree roots and mychorrhizal fungi.

Walnut trees produce a secondary metabolite known as juglone. Juglone inhibits the growth of any other plant species around the base of the tree. This allows the walnut tree to utilise all of the nutrients and water in the soil without being affected by other species of plants. This also prevents creepers and lianas from growing on the walnut tree, as they can damage and shade the tree. 

Secondary metabolites of plants are quite often utilized by humans. Spotted knapweed (Centaurea maculosa), releases cathechin into the soil which kills plants. Plants in Europe have developed a slight resistance to this chemical. However, in North America this is not the case and Centaurea maculosa is a problem in the vegetation. Spotted knapweed was brought to North America by the English so that the settlers would not feel so homesick. Catechin is used in green tea and a Japanese soft drink. 

Humans use plant secondary metabolite toxins in low doses for medicinal purposes. St. John’s Wort (Hypericum spp.) contains hyperforin and hypericum which have been used to treat depression. Digitalis purpurea (foxglove) which contains digitoxin and digoxin has been used to treat patients with heart problems. Using too much of the secondary metabolite can be harmful to human.

Some more information on allelopathy can be found here: 

http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=2816&Type=2

Experiment:

Apparatus used in experiment

• ·         Materials:

• ·         Garlic

• ·         Daffodil leaves

• ·         Walnuts

• ·         Orange Peel

• ·         5 Petri dishes

• ·         Parafilm

• ·         Mortar and pestle

• ·         Lettuce seeds (Lactuca sativa)

• 15ml screw-capped tubes

Procedure:

1. Weigh out 2g of each plant tissue and grind in a mortar and pestle. 
2.  Place the ground tissue into the 15 ml screw-capped tube and label. 
3. Add 10ml of water at 37°C and mix thoroughly. Leave the tube for 10 minutes. 
4. Centrifuge the tube.
5. Repeat steps 1-4 for all other plant tissues. 
6. Label the Petri dishes: one as control and the rest as one of the plant tissues.
7. Place a piece of filter paper into each Petri dish and wet each one with the sample it has been labeled with. In the control, just put distilled water.
8. Place 40 lettuce seeds onto each Petri dish ensuring they are not touching and they have enough space to grow.
9. Cover each Petri dish in parafilm and incubate.
10. Record the results after approximately one week to allow sufficient germination time.

Results:

Lettuce seeds in Petri dishes with different plant juices

 

 The Control (plate with distilled water) had almost 100% germination. All seeds had roots and tall green shoots as there was no secondary metabolites to affect it. The dish with the daffodil shows that most of the seeds have produced shoots but no roots, some root hairs were produced. The plate with the garlic had strong root growth. The orange plate had some seeds which did not germinate. The seeds that did germinate had short shoots and no roots. The dish with the walnut germinated showing short roots and microbial contamination. 

Discussion:

The control plates gave the expected result. There was 100% lettuce seed germination (root and shoot growth). This was because there was no secondary metabolites acting upon the seeds. The plate on which the seeds soaked in daffodil grew shoots but only some root hairs and no proper roots. Daffodils secondary metabolites affect antimitic activity thus leading to inhibition of growth. Growth was not totally inhibited in this experiment however, shoots formed and small root hairs also. This could be due to the fact that the flower of the daffodil was used and not the stem (which should have been used). Garlic is a very good microbial inhibitor which also inhibits the growth of roots. In this experiment however, there was root growth and strong root hair growth. This could be due to not enough garlic being crushed or possibly not enough garlic being spread onto the cotton wool. The orange peel plate showed no growth of roots and short shoot growth. The acidic pH makes it difficult for the seeds to germinate (which accounts for the seeds which did not germinate) and inhibits root growth, which is the expected result. On the walnut plate, there was strong root and root hair growth. This was not the desired result as there should have been no germination. There was also penicillin growth on the plate.

It is clear to see that certain plant secondary metabolites can inhibit another plant from flourishing. In a lab situation, trying to mimic allelopathy can often be difficult and many different aspects can affect the results. However, if this experiment was carried out in the natural environment, different results could be observed.