Effect of nitrate concentration on the growth of Duckweeds

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Biology A2 level Course work : Effect of nitrate concentration on the growth of Duckweeds  

Introduction & Method

    At the beginning of the experiment, I put on a lab coat to protect myself from any danger that may occur. I made sure that all my equipments were clean and dry before I started, and I also measured the temperature of the room to confirm it was around room temperature. I then made sure that the ice cube tray was clean, I put a  little label on one end of the tray to indicate my starting point row of the ice cube tray. I then made a little note to myself that the label indicated the row which will contain the 0.0% x 10-3 concentration of nitrate in the solution, and the rows onwards will contain the concentration of nitrogen in solution in ascending orders which I will use (0.0, 0.4, 0.8, 1.2, 1.6 and 2.4 % x 10-3). I

    Whilst pouring the solutions into a glass beaker I put on goggles and gloves to protect my eyes and hands from any contacts with the ammonium nitrate solution, as any contact can lead to irritation of the skin and eye.  I poured 80-90cm3 of the 1st concentration which contained 0.0% x 10-3 of nitrate into a 100cm3 glass beaker. Next I used a clean syringe to measure out 25cm3 of the solution into the 1st well (near my label). The tray had 3 wells in a row and there were 6 rows, I repeatedly added 25 cm3 of the same ammonium nitrate solution into the remaining two wells of that row, so that I will obtain a result of three replicates with each concentration. Using the replicates will give me a more reliable average result, and it will enable me to use more advanced statistical tests, as the class results will be pooled to generate sufficient data. I pored 80-90cm3 of each solution into the glass beaker, to ensure that there was enough for the other two replicate. As I repeated the process for all the other 5 concentrations of nitrate in solutions, each time when I used different concentrations of nitrates I made sure that I washed out and dried the beaker and the syringe, to avoid any contamination of the solutions as this would lead to the concentration of nitrogen and other nutrients to change, therefore affect the outcome of the results. Each time I used a syringe that had accuracy of + - 0.1cm3 , as it would give me a accurate reading of the solution I put inside the wells. I also made sure that I always tapped in every last drop of the solution, to prevent the volume of the solution changing.

    I then collected my duckweeds; I used a spatula to scoop them from the pond water onto the white tile, I used a spatula to get the duckweeds out as it prevented any damage to the rootlets, and as it wasn’t sharp no living organisms inside the pond were harmed. Using a mounted needle and magnifying glass I carefully separated the duckweeds with four healthy fronds, of which I used a ruler to measure that the rootlet was in the length between 0.5 – 1.0 cm3. I had placed the duckweeds on a white tile whilst picking the ones I used, so that I was able to pick out the similar coloured fronds more clearly against the white background. By using a magnifying glass I was able to make sure that the duckweeds I picked were all in similar sizes and also were healthy by not having any damaged fronds or rootlets.  

I picked out 18 duckweeds with four fronds each, so that one duckweed could be placed into each of the wells in the tray. I then decided to pick another 18 duckweeds, so that instead of putting only four fronded duckweed into a well, I would put 8 fronds, each attached t a rootlet, into each of the wells. I did this because although I checked with a magnifying glass to pick out healthy fronds there may still be a chance that some of the fronds are damaged. This would affect the outcome of my results, if not much photosynthesis could take place in one of the four fronds, so by putting 8 fronds into each well ensured that the maximum amount of photosynthesis, and therefore plant growth could still take place even if there are unhealthy fronds. I used a mounted needle to separate the duckweed easily, without damaging their fronds.

    Each time I collected the duckweeds I placed them into a glass beaker which contained tap water, because if I left the duckweeds on the tile without water this could lead to the weeds drying out, and become damaged easily, due to it’s small size and as I was finding quite a large number of particular shaped weeds which took me a quite a long time. Also I did not put any of the duckweeds into the wells until I collected enough, as I wanted the experiment to start at the same time to ensure reliability of the all the replicates, as they will be exposed to the same environment.

    After collecting enough duckweed, I carefully placed duckweeds with a total of 8 fronds using a mounted needle and magnifying glass into each of the wells of the tray. I then placed the ice cube tray in another room underneath a light bank, where factors such as temperature and light (intensity and quality) were constant all the way through the investigation, as this would encourage the reliability of the outcome. I also used the ice cube tray to put my duckweeds in as it contained wells that were all the same size and at the same level of height.

    I will observe the duckweeds over a 10 day period and check the water in the solutions, as uptake of water and evaporation will take place. In order to fill the wells with water, I will add tap water with a syringe. Each time I will fill the wells to the top with tap water; this ensures that there is the same amount of water in each well, so therefore this increases the reliability of the outcome results. To do this I will use a syringe, as this prevents adding too much water at once, which leads to the overflow of water, and therefore contamination of solutions in their neighbouring wells; the contamination will change the amount of nitrates in each solution.

    I will check each day and add water to prevent any wells from drying out or become too concentrated with minerals and nutrients as this can cause problems and affect my results.

    I have chosen to add tap water to make up for the loss of water in the wells, this is because tap water contains high levels of minerals in it such as Calcium, Magnesium, sodium and certain trace elements essential for plant growth. Tap water also has a high PH, which affects and encourages the nutrient uptake by plants. Therefore it is important that I use tap water and not distilled water, as it ensures that other minerals do not become a limiting factor on plant growth. Consequently I am not using distilled water as it is clean of minerals.

    However many tap water are not good for plants, often they are treated with chlorine or other chemicals, to make them safer to drink and so these are bad for many plants. To control the experiment, I will water the duckweeds using the same tap water.  




The previous graph shows the individual results I obtained during the experiment. The graph shows two major patterns labelled trend A and trend B, found through out the range of nitrates exposed to the Duckweeds. Trend A on the graph is between 0%-0.4% x 10-3 concentration of nitrates in solution, this trend shows a rapid increase in growth of duckweeds (increase in the number of fronds) as the concentration of nitrates increases. This increase in trend A lasts until the growth of the fronds reaches the optimum which is at 0.4% x 10-3 concentration of nitrates. This pattern suggests that the growth of duckweeds in this investigation is most favoured at 0.4% x 10-3 concentration of nitrates, as this is the concentration where the maximum number of fronds grew.  

    Trend B is between 0.4%- 2% x 10-3 concentration of nitrates, this trend shows that as the concentration of nitrates increases from 0.4% onwards the growth of the duckweeds decreases. This is due to the unfavourable amount of nitrates exposed to the duckweeds.

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Trend A:

At 0% x 10-3 of nitrate concentration, the number of fronds increased from the initial 8 frond to an average of 20.67 fronds, this increase took place without the presence of any nitrates in the solution. This is because there were already some nitrates present inside the cell, in the required form such as proteins, coenzymes and other nitrogen containing components, essential for the plant to survive and grow. Since nitrogen is already available inside the plant cells the duckweeds in this concentration are able to carry on growing until nitrogen becomes a limiting factor. The presence of ...

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This detailed account just gains 5 stars because the author has a clear grasp of concepts and explains ideas well using A level biological language. There are some, mostly minor, errors. 5 Stars