Step 4 Conclusions and Discussion                                                                                                            

It is understood that, amongst other things, in simple terms, IAA promotes cell elongation in shoots and inhibits cell elongation in roots.

Therefore, the 2 hypotheses that we will apply to this experiment are that as the concentration of IAA increases from 0.0ppm to 100.0ppm shoot length will get progressively longer and root length will become progressively shorter.

Reading from the graphs the results show the following responses by the seedlings:

The graph of the shoot response is difficult to read as it initially shows a small promotion of growth followed by inhibition with this cycle of promotion followed by inhibition occurring three times with a progressively greater difference between the amount of promotion and the inhibition each time.

The general trend of the graph, if you were to take the average values shown, is a shallow downwards slope from left to right as the percentage of inhibition is much greater than the promotion. The orange line in the amended graph below gives a better idea of this.

This interpretation contradicts the shoot length hypothesis and therefore we have to accept that, in this specific experiment, the hypothesis has not been proven and we need to look at the reasons why.

This situation is common in science and is not considered failure until it is proven that a mistake occurred in the method or elsewhere. Often negative results like this drive the science in a new direction which results in greater or more accurate discovery.



Some questions that we may ask are: Were there mistakes in the method? Were the right light, temperature, pH and moisture conditions maintained? Is there an issue with the seeds used? Is this a species specific response which is different from the expected? Is there another unseen factor at work?

Auxins act on cell walls to cause it acidify its surroundings and this acts on the cell wall components to allow them to expand and grow. The pH (acidity measure) of the Petri dish solutions was not checked as part of the experiment, however an issue with the pH could have affected the result.

Another area to investigate would be the balance of water concentration in the plant compared to the external solution in the Petri dish. Auxin-induced growth by way of cell elongation requires water to be drawn into each cell by osmosis to expand it, however this requires a higher water concentration outside the cells than inside. Therefore, if there was not enough water, shoot growth promotion would cease.

Whilst the Petri dishes were to be place flat in the incubator, if they were turned on their side at any point this would act to gravistimulate the shoots since they can sense such as change. Because auxin is also involved in the gravity reponse of plants their senitivity to the IAA would have changed.



The root response also does not allow for a straightforward interpretation. Initially there is a minimal inhibition response before a greater inhibition of growth at a concentration of 0.10ppm IAA. This inhibition then recovers to a slight promotion of growth followed by a greater inhibition at maximum IAA concentration.

The graph from the results section is copied here for ease of reference.

This interpretation again contradicts the initial hypothesis which is therefore considered null and leaves us with trying to interpret what we observed.

 What could be occurring in this instance is a case of thresholds being reached before initiation of a response.

The first inhibition occurs at an IAA concentration of 0.1ppm. The sudden drop may be due to the concentration reaching the right level to initiate an inhibition of growth response. There is a point when the addition of the added IAA along with auxin produced by the seedling inself will possibly exceed the available concentration of receptors for auxin in the plant.

The change over to a slight promotion response is contrary to expectations and could be a survival response by the plant to prevent damage resulting from an over-concentration of auxin. At a concentration of 100ppm, which is 10 times the preceeding dosage, the plant's protective responses may have been overwhelmed and therefore considerable inhibition occurred for the second time. This could still prove to have a longer-term adverse effect on the plant which didn't reveal itself during this experiment.


Although the seeds used in the experiment were visually checked for contamination and appeared to be healthy it is possible that they came from old stock and this could have affected their normal growth or their auxin responses.

The key to experiments is data, data and more data. The more you replicate your experiment the more data you get and the more likely it is that your results and conclusions will be correct. In hindsight, since this test was one of a number being carried out by a cohort of students, perhaps amalgamating all the data could have produced a more definitive conclusion.

Basic stuff on plant hormones and a bit about me                                                                                                

If I get around to it I will put some details in my profile so you have some idea about who is feeding you this information. In the interim suffice it to say that I'm a 3rd year science student covering the fields of Botany, Zoology and some Palaeontology.

This blog is in the field of botany and applied plant sciences in particular, however as you read it, I'm hoping you keep an open mind about the processes that are being described since some of them are similar to those found in animal biology, in your own body for instance. The fact that they operate in almost all cells of any organism always amazes me.

Plants, like animals, use hormones to regulate various processes in their "bodies". For a general idea, hormone 1 may activate a particular process and hormone 2 may stop that process or slow it down, whereas hormone 3 may act in tandem with hormone 2 to activate an entirely different process.

The concentration that a hormone is found in at any one location also can have a bearing on how it acts, such as the speed that the process proceeds at or how strong its effect is on the organism.

Possibly one of the best known examples is that of human growth hormone which is produced in the pituitary gland on the underside of your brain. Hormones called Growth Hormone Secretagogues activate the pituitary gland to produce growth hormone. When it's secreted and circulates around the body, it activates the process of human growth. In high concentrations you grow fast and the opposite applies of course, for low concentrations.

Its production and secretion is stopped when another hormone de-activates the process and you stop growing. There are cases where persons keep growing their entire life albeit at a slower rate when the production process didn't stop.

So, hormones in animals and plants act like dimmer switches; On or Off and Very bright or Very dim.

Ok that's enough of the simple stuff, you hopefully have an idea how hormones work now let’s look at the experiment next.

LINKS AND REFERENCES                                                                          

In case you don't follow my methodology, here's a link to another blog on the same subject as a different view may make  things a bit clearer: kevinsplantscience

The following website was originally set up by the Biotechnology and Biological Sciences Research Council (BBSRC) and contains a great overview of auxins along with details on other plant hormones : Info on plant hormones

Most of my reading for this subject has come from two books, namely;

Plant Physiology 4th Ed., 1992, Frank Salisbury and Cleon Ross, Wadsworth Publishing Co. California, USA, Pages 357 - 372

Biochemistry and Molecular Biology of Plants, 2000, Bob Buchanan, Wilhelm Gruissem and Russell Jones Eds., American Society of Plant Physiologists, Maryland, USA, Pages 884 - 895

The paper named below is only about 3 pages and shows one of the early experiments using auxin that had been isolated. It's interesting in that it describes the observed effect on a cellular level in as far as could be determined for the time. It can be accessed from the NUIG library website.

Yin, H.C., Effect of auxin on Chlorella vulgaris,Wm. G. Kerckhoff Laboratories of the Biological Sciences, California Institute of Technology, 1937

There are few papers accessible which deal with auxins in general terms and most are not accessible from the NUIG library website due to their age. Most accessible are the modern papers which deal with auxin-induced plant responses on very specific issues and are only worth viewing if you intend to investigate the hormone in some detail.

Step 3

Data collection and Results                 

During the week the seedlings, having been given water, warmth and light,have germinated and developed SHOOTS and ROOTS.

The next task was to see how much effect, if any, the IAA had on the development of theses shoots and roots.

The video shows the Petri dishes starting on the left with the 0.0001 ppm IAA to the 0.0 ppm IAA control (open dish) on the right. The form at the end shows the way the information on lengths was collated.

This was done by measuring the lengths of the shoot and root of each of the five seedlings in each of the Petri dishes containing the different IAA concentrations.

Once they were all measured and the information written down separately for shoots and roots an average growth length for shoots and an average growth length for roots was calculated for each Petri dish/IAA concentration.

These average growth lengths for each set of shoots or roots was then compared to the average length of growth for the corresponding shoot or root control which contained no IAA. This was done using the formula below:

Percentage of
stimulation/inhibition = { Average sample shoot length - Average control shoot length } x 100
                                        ---------------------------------------------------------------------------
                                                               Average control shoot length

Of course, for roots, the roots values were used  :-)

By doing this it was possible to determine to what extent, expressed as a percentage of the control growth lengths, did the IAA treatment effect the shoots or roots in comparison to the control.

The resulting data were placed in graph form for easier interpretation;

If you consider the zero % line to be the Control with no IAA in it then you can read the effect of the IAA concentrations relative to this

No data is of use unless it is given some interpretation to determine whether it upheld or refuted you original hypothesis; What occurred? Why did it happen that way? Was there something that could have been wrong with the experiment? The interpretation of the results along with other questions will be the subject of the next blog. 

Steps 1 and 2

Auxins are a group of hormones in plants that are involved in various processes and one of these processes is cell elongation which is the way that plants grow and bend.

Indole-3-acetic acid (IAA) is one such auxin and this experiment is an investigation into how it affects the growth of shoots and roots at various concentrations.

28th Feb - Week 1

 Step 1

Production of 7 different concentrations of IAA and one Control of distilled water.

This was done by serial dilution of an initial IAA solution which was at 0.01% concentration. That's 0.01 parts per hundred which, if you multiply both figures by 10,000, is the same as 100 (or 10²) parts per 1,000,000

Written as 10² ppm, if you dilute this by a factor of 10 each time, for six times, you get further IAA concentrations of 10ppm, 1ppm, 10^-1ppm, 10^-2ppm, 10^-3ppm and 10^-4ppm.

This was done by pouring 10ml of the initial solution into a test tube and pouring 9ml of distilled water into each of 6 more test tubes.

Then 1ml of the initial solution was taken from the first tube and added to the distilled water in the second tube thereby diluting it by a factor of 10 to form 10ppm. The tube was so labelled. 

Then 1ml of this tube's solution was added to the distilled water in the next tube thereby diluting this by a further factor of 10 to 1ppm. This tube was labelled 1ppm.

This process continued until the last of the six tubes had a total of 10ml of solution in it. 1ml of this solution was removed and discarded.

An 8th and final tube had 9ml of distilled water only poured into it to act as a control, to see what would occur if there was no IAA present in order to better gauge what was happening in the other 7.

Step 2

Preparation of 8 Petri dish cultures

8 Petri dishes were each lined with round cross-hatched acetate and 5 radish seeds placed in a row at the intersection of two lines. Lining the seeds up like this is to allow an easier comparison of their respective shoot and root lengths. A round piece of filter paper was placed over each set of seeds.

Each of the Petri dishes had been labelled in the same parts per million (ppm) as the test tubes.

From each corresponding tube 2ml of the solution was dropped on the filter paper and it was gently pressed in place to ensure that there were no air bubbles which could result in a seed drying out.

Finally, each dish was spread with a piece of absorbant cotton wool and the remaining 7ml of solution in each case was poured onto the cotton wool and the 2nd half of the dish put in place..

The dishes were sealed with tape to prevent leakage. Then were then all placed in a stack, taped together and placed with the dishes standing on their edges in an incubator for 2-3 days at 25 degrees C.

Hi. Over the next few weeks I will be blogging about an experiment to investigate the action of a particular plant hormone on growth of roots and shoots.

To give you a taste, check out the youtube link Plant hormone experiment