Sunflowers As Natural Herbicides Biology Essay

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Everybody hates seeing clovers or dandelions in their lawn. They ruin the aesthetics of the meticulously kept grass. Some people physically remove the plants, however others use a simpler method of removing the invaders: Herbicides. All around the world, herbicides are used to increase the yield and quality of crops and maintain gardens, by biologically removing unwanted plants between crops. Before synthetic herbicides, natural herbicides or the physical removal of the plants was used to get rid of the unwanted plants but after the green revolution in the 1940s the use of synthetically made herbicides increased massively. Although the synthetically made herbicides have had a positive effect of increasing the amount of food for a population that has recently reached 7 billion, it has had many negative side effects on all parts of the environment.

Herbicides are made to target specific metabolic pathways in the target plant. This is then sprayed on to the target area. However, due to the spraying the liquid, some of the herbicide evaporates and can rain back down on to a random area, killing the plants with the metabolic pathway that the herbicide targets. These dead plants may have been someone's crop and income. Furthermore if the synthetic herbicides get absorbed into the earth it may make a piece of land toxic for months or even years. Therefore herbicides can have a huge impact on the environment.

It doesn't only affect the surrounding environment but the target ground as well. Bacteria in the soil help to store nutrients, water and to filter pollutants, all essential to plants. Most importantly, the grounds bacteria help transform atmospheric nitrogen into nitrates, which plants need to flourish and grow. The bacteria are also essential in fertilizing the soil. According to the University of Florida EDIS, bacteria help to decompose dead animals and plants tissue into organic matter, like compost, which is useful to plants. By killing these bacteria, herbicides render the ground less and less fertile every year to the point where it is completely infertile.

Dead zone in the Gulf of Mexico

Herbicides also have a major affect on the wildlife. As the herbicides kill the plants, it decreases the food supply for certain animals, such as seed feeding birds. The population of certain birds has decreased due to herbicides eliminating large parts of their food supply, according to Lawrence J. Blus of the U.S. Geological Survey. Furthermore herbicides can be caught in surface runoff and transported into rivers and streams. This can be lethal to aquatic life, often killing all the fish in a particular stream. An extreme example of this is the dead zone in the mexico gulf. It is a 21,756 kilometers2 oxygen low area at the mouth of the Mississippi, USA, as can be seen in the picture to the right. It got created and is maintained by the agricultural surface run off, including herbicides, of the states surrounding the Mississippi. According to Louis A. Helfrich it has resulted in massive fish deaths due to the dead plants rotting and use up all the water's oxygen, essentially chocking the fish. Some herbicides, such as copper sulphite, that are added to water to kill plants are toxic to fish and other water animals at similar concentrations to those used to kill plants.

Furthermore there have been many studies that have shown that herbicides are also bad for human health. Herbicides can be ingested through inhalation of aerosols, dust and vapour that contain herbicides. Humans' organs can also be exposed to herbicides by consuming food and water; and direct contact with skin. Herbicides and Pesticides can cause a number of health problems such as heart congestion, lung and kidney damage, low blood pressure, muscle damage, weight loss and adrenal glands damage. Reuters also confirms that the exposure to pesticides and herbicides "significantly increased the risk of Parkinson's disease". One extreme example where herbicides have killed many people was the use of Agent Orange by the U.S army to destroy foliage that the Vietcong hid in. It is estimates that 400,000 people were killed or mutilate, and 500,000 children born with birth defects.

If we continued use of these synthetic herbicides will cause even more damage, which has the possibility of scaring our already scared planet permanently. Until we find a synthetic herbicide that doesn't have the negative side effects of today, we will have to find a substitute. Concerned environmentalist and humanitarians advise to use organic herbicides. These herbicides are made of natural toxins found in plants and fungi around the world. The benefits of natural organic herbicides are that they will only destroy plant life for a short period of time and won't have long lasting effects on the ground. Furthermore they do not enter in a food chain or stream, and therefore do not destroy any unnecessary wildlife or flora.

It is due to the benefits of organic herbicides, that scientists are looking into the toxic nature of certain plants. Much research is being put into "allelopathic toxins" found in plants such as Fragrant Sumac , Tobacco, Rice, Pea and Sunflowers.


Allelopathy is the "Suppression of growth of a plant by a toxin released from a nearby plant of the same or another species." ( Allelopathic chemicals can be present in any part of the plant. They can also be found in the surrounding soil. The toxic chemicals have a whole variety of actions to inhibit its victim plant, this includes inhibiting shoot/root growth, nutrient uptake, or they may attack a naturally occurring symbiotic relationship thereby destroying the plant's usable source of nutrients. Allelopathic plants prevent other plants from using the available resources and thus increase the chance of survival for the sunflower and its offspring.

Even though scientists are looking to find a herbicidal function for these allelopathic toxins, it will take a couple of years before such a product will enter the market of garden keeping / crop growing.


However it is going to take a couple of years before allelopathic toxins become a commercial product available in garden stores. This should not discourage people from having natural herbicides in their near future. It is even possible for them to make the toxins themselves. Around 10% of the herbicides used in the world are used for garden use, and if we change our methods in this field we will already be making an active effort to save our environment and preserving it for our future generation.

epidermal surface of a sunflower leaf including trichomes

One of the most available allelopathic plants around the world is sunflowers. The sunflower family are full of sesquiterpene lactons, a group of sesquiterpenes with a wide range of bioactivity. Sesquiterpenes are a class of terpins, a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers. The sesquiterpene lactons argophylline A, argophylline B niveusin B and niveusin C were isolated and according to M. Oofsdotter of the International Rice Research Institute affect the hormonal systems of plants and showed antiauxin activity. Antiauxin activity is the inhibiting of auxins, plant hormones which play an essential role in the coordination of many growth and behavioural processes in the plant life cycle, by competing for the same growth receptors. This supported by Professor Azim U. Mallik of the biology department of Lakehead University, Ontario. According to M. Oofsdotter most toxic terpins compounds of sunflowers, are located in the trichomes on the back of the leaf, fine outgrowths or appendages on plants. By being located on the back of leafs, the toxins are probably easily leached by rain, of, and dew in the environment.


The evidence above clearly states that the largest amounts of the sesquiterpene lactons, which have toxic effects, are found in the trichomes of the sunflower leaves. It is therefore natural for us to deduce that the leaves will have the largest allelopathic affect on plants.

However other parts of the sunflower will also have trichomes on their surface cells. Also it seems that the trichomes, containing the sesquiterpene lactons, are deliberately placed on the leaves in order to affect the growth of other plants. This then suggest that the sunflower is actively fighting off other plants that will be using the surrounding environments nutrients and water.

With support of the information above, I believe that the roots of the sunflower will have the least affect on the plant. Due to water seeping into the earth, the sesquiterpene lactons will be released into the close proximity of the roots they originated from. If there are plants very close to the roots they will naturally be affected by the toxins, but it may also back fire and affect the sunflower.

Furthermore, as the roots of the sunflower go quite deep into the ground, the secretion of toxins would be a waste of nutrients. Because the roots are so deep, the toxin would move further down into the soil, somewhere where the roots of other plants don't reach are, or where there of roots of trees that do not get affected by the minute amount of toxins given off by the roots

In addition, because the sunflower seems to be actively fighting other plant, the stem of the plant will also have trichomes, as water will also drip on the stem, transporting the sesquiterpene lactons to the soil below. However they will have less of an effect then the leaves, due to the fact that there are more trichomes on the leaf then the stalk, as that is not one of its main functions.

Moreover I believe that there will be no difference between the lower parts of the sunflower and the higher parts, including leaves and stem as gravity will have no effect on the creation of the sesquiterpene lactons in the trichomes.



To investigate which section of a sunflower (dried, crushed and turned into an aqueous solution) has the greatest allelopathic affect on the dry mass of mustard plant (sinapis alba)



The section of the sunflower sun dried, crushed and turned into an aqueous solution


Biomass of Mustard plants.

Sunflower section


Type of Mustard plant

Age of mustard seed

Amount of seeds added to each cup

Type of sunflower

Amount of soil used

Weather/growing conditions

Structure used to contain soil

Amount of water and aqueous solution given to plants

Time lapse

Water sprayer

Drying method

Crushing method



10 large sunflower of the same species

100 plastic cups

8000 grams of earth

12000 ml of water

240 mustard seeds

5 water sprayer



2 months


take the 100 cups and poke 5 holes in the bottom using a pen

put 80 grams of earth in each of the 100 cups

make a 1.5 cm hole in the earth of each of the cups using the pen

add two mustard seeds in to all the holes

fill the holes with earth so that the mustard seed is completely covered

water the plants for 30 days giving equal amounts of water using one of the water sprayer

group the cups into groups of 20

leave the sunflowers in the sun to dry

once they are dry, cut the sunflowers up into the sections shown above

crush the sections very finely

weight out 300 grams of all the dried and crushed sections and add them to separate beakers of 1000 ml of water

After 30 days designate each group of 20 plastic cups to a section of the sunflower including the controlled

Give each aqueous solution made in step 11 a separate water sprayer that sprays out equal amounts of water

Water the groups equally with their designated aqueous solution for 5 days

At the end of the 5th day, remove all the plants from the earth and leave them to dry for 2 days

Measure the weight of each plant using the scale and record data

Changes made in method

One hole drilled into bottom of the cups using drill

The sunflowers were immediately cut, due to suspicion that allelopathic toxins were of an aqueous state and would evaporate if put in the sun.

Control group was split into two. One was the active control which were watered with a commercial herbicide while the other was watered with normal tape water.

50, instead of 300, grams of all section of the sunflower were added to the water

The groups were water for 22 days with their individual aqueous solutions

Seedpods counted




Due to the large error bars on both graphs and the T-tests conveying that there is no significant difference between the control plants and all the plants given an aqueous solution, there seems to be no correlation between the different parts of sunflower and their allelopathic properties. However, even though there is no significant statistical difference, the average size of the halo created by the unemotional tear is slightly

greater in size than that produced by the emotional tear, seen from the average values,

1.7 and 1.6 respectively, and illustrated on the bar graph. This suggests that that there

may be slightly more substances acting against bacteria in the unemotional