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Name: Sheena
Status: student
Grade: 9-12
Location: TX
Country: N/A
Date: 1/4/2005


Question:
Hi, i'm doing a science project about the affects of relaxants and stimulants on a soybean plant's height and its cell walls. I am using sugar, tobacco, caffeine, and aspirin all with water on 25 different plants. The height was affected greatly but I have looked through a microscope and still cannot tell a difference in the cell walls of the plants. Another problem is that the aspirin and caffeine plants did not grow at all so i couldn't even take a sample of them to look at under the microscope. Do you know if the cell walls are affected at all in my project?


Replies:
Would recommend recording the before and after sizes or incremental growth measurements of the plants, and comparing the measurements with a chart and simple statistics (averages, etc.) Perhaps not possible to view any changes in the cell walls with a light microscope since either no change in the wall's thickness or would need an electron microscope. Possibly could look at the size of the plant cells instead.

Anthony Brach PhD.


To begin with, I would urge you to repeat the experiment and control all variables you can think of. Be certain that your treatments are the only conceivable differences between the groups of plants. The fact that your caffeine- and aspirin-treated plants did not grow at all makes me suspect that you need to perform more repetitions of the experiment.

Based on my knowledge of plant growth hormones, I would expect sugar to have no direct effect on cell walls. However, it will be a good food source for bacteria and fungi and thus it will tend to make the plants susceptible to bacterial and fungal diseases. Tobacco is very toxic stuff in general. Tobacco leaves contain dozens of different toxins, many of which could affect your plants in ways that are very difficult to predict.

Aspirin (acetylsalicylic acid) is chemically similar to the well-known plant hormone salicylic acid. To my knowledge, salicylic acid is not involved in growth; it is generally involved in responses to stress or injury and it triggers thermogenesis (heat production) in some plants. Aspirin is somewhat acidic, though, and acidic solutions can cause plant cell elongation by "loosening" the cell walls. However, this effect is only seen when the acidic solution is applied directly to the cell walls.

Caffeine is chemically similar to the well-known plant growth hormone indole acetic acid (IAA, also known as "auxin"). IAA is very clearly involved in plant growth - in particular in the growth of stem and root cells. I don't know if caffeine might have a similar effect, but it seems conceivable to me. IAA seems to work by stimulating the cells to pump acid into their cell walls, thereby making them "looser" or more "stretchy". This mechanism is debatable, though.

I very much doubt that you will be able to see any cell wall structural differences with a microscope. If there are growth differences, you would be more likely to see them in terms of internode length. I would also caution you against making any connection between these effects and the effects seen in animals due to these substances. In animals, these stimulants affect the nervous system; plants don't have nervous systems and the mechanisms by which plant hormones work are very different from the mechanisms seen in animals. Plant physiology is quite different from animal physiology.

Christopher Perkins



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