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.