Plants expossed to UV light. Why is it not working?

[carreyesluna]

Hello, so my project is about exposing a common plant (Hedera canariensis) to UV light for different amounts of time and try to proof that the radiation affects the photosynthetic processes of the plants by measuring the oxygen it produces. Plants are divided into three groups: Group A exposed and entire day, B exposed 3 days and C exposed 7 days. After it I take it to my school (the ride since I take it from the light and I arrive is around 20 minutes) and I place it in a photosynthetic chamber (just a chamber of plastic that is completely sealed) and measure the oxygen in produces, I expect that group B will produce less oxygen than group A and C less than the other 2 groups. The plants I got are a little too big for the chamber so I have to push it so it stays sealed and measure for 15 minutes the oxygen (all the sensors and chambers used are from the brand vernier). The thing is that the first time the plant exposed one day produced less oxygen than the control plant-a plant not exposed to UV at all- (which is good. I didn't performed repetitions though) but when I took group B to measure the oxygen it produced the same amount as the control plant. I don't know why this happened, maybe in group A I didn't sealed the chamber good enough or maybe the plants have different number of leafs so if I have more leafs they will produce more oxygen or in the ride from my home to school all damage is repared and they produce normal oxygen. I haven't found an experiment online that is the same as mine so I couldn't compare my procedure with others. The thing is that in other experiments of UV exposure to plants they expose the plants for minutes or hours but never for days (maybe that's my mistake). I also tried to measure the CO2 absorved in the first two groups (I measured it 15 minutes) but it didn't changed at all (maybe if I do measure CO2 it has to be for more time...) or maybe just the specie of plant I'm using is too "strong".
Please help me, maybe I should leave the plants in exposure for different amounts of time of measure another thing that is not oxygen (and fo different periods) or you can find a mistake in what I'm doing. My school is closing for summer break in friday for a week so I just wish I can finish before that and I haven't tested group C yet because of this.
Please help.
PS: Please forgive my grammar and all, english is not my first language.

[sunmoonstars]

Hi,

I haven't worked with plants much, but it looks like you have thought through your experiment very well. I agree, it is possible the ride to school is allowing the plants some time to repair the damage. Culd you find a space at school to dose the plants with light there, so you can mesure right away when you take them out of the UV?

You should measure the 7-days plants - always collect a full data set when you can, don't stop your work based on a partial data set. It is possible 3 days was not long enough to show additional decrease in O2 production, but the 7days may have.

If you have time to collect additional data, try to use some smaller plants that fit in the chamber well, and try to use more than 1 per timeframe - ideally 3 plants per test time.

You bring up a good point about the # of leaves. Perhaps after collecting your data, you can measure the # leaves per plant, and even the surface area of those leaves, so you can calculate the O2 produced per leave area. That would be the most accurate assessment, if there are differences in plant size.

Good luck! SOunds like a good study!

Tonya

[goldenmaps]

So from what I read, your hypothesis is that with increased UV exposure, plants will produce less O2.

My suggestion for any experiment is to first go into it without letting your expectations dictate the data you collect. In other words, try not to manipulate your data to fit your hypothesis and just let the data speak for itself. Having said that, I would strongly recommend you perform your measurements in triplicates (for each condition have at least 3 plants, as Tonya suggested). Then you can take the standard deviation to show how reliable each measurement is. For example if you have 2/3 Group B plants say that O2 output is ~5units and 1/3 is 30units (made up some numbers here), then you know that 1 plant may not be representative of the sample pool. Increasing the number of plants you use increases your robustness.

I also agree with Tonya that it would be best to decrease the amount of time the plants spend between UV and having O2 measured.

I performed a smiliar experiment many years ago, measuring the effects of elevated CO2 on plant growth. And the results were so variable... increasing your sample size definitely helps to see if there is a trend. What I found was that the CO2 meter was not reliable in measuring small changes and that longer treatments were needed to see a significant difference.

Make sure your O2 meter is calibrated and that you have a negative control. Make sure that you aren't measuring residual O2 from the previous plant. Does the chamber allow you to vaccuum O2 before putting a new plant in? Or to blank it?

It also sounds like you're measuring every day (from the statement: The thing is that the first time the plant exposed one day produced less oxygen than the control plant-a plant not exposed to UV at all"). I think it's better to measure only after the time you indicated. For example, after 3 days, measure Group A and B. After 7 days measure Group A and C. Are group B plants the same as group C plants? I mean, are you just measuring the group B plants again on day 7 and calling it group C? Or are they a a completely different group of plants?

In summary, my advice is to
- Have 5 plants for each test group: (treated and untreated)
- Measure each plant 3 times
- "Blank" or get rid of O2 in the chamber before measuring each plant to avoid cross contamination
- make sure the chamber is sealed
- Measure the treated groups 2x (at 3 days and 7 days, along with the untreated).

Good luck with this experiments! Sounds cool!

[carreyesluna]

Culd you find a space at school to dose the plants with light there, so you can mesure right away when you take them out of the UV?

You should measure the 7-days plants - always collect a full data set when you can, don't stop your work based on a partial data set. It is possible 3 days was not long enough to show additional decrease in O2 production, but the 7days may have.

You bring up a good point about the # of leaves. Perhaps after collecting your data, you can measure the # leaves per plant, and even the surface area of those leaves, so you can calculate the O2 produced per leave area. That would be the most accurate assessment, if there are differences in plant size.

Good luck! SOunds like a good study!

Tonya

It also sounds like you're measuring every day (from the statement: The thing is that the first time the plant exposed one day produced less oxygen than the control plant-a plant not exposed to UV at all"). I think it's better to measure only after the time you indicated. For example, after 3 days, measure Group A and B. After 7 days measure Group A and C. Are group B plants the same as group C plants? I mean, are you just measuring the group B plants again on day 7 and calling it group C? Or are they a a completely different group of plants?
In summary, my advice is to
- Have 5 plants for each test group: (treated and untreated)
- Measure each plant 3 times
- "Blank" or get rid of O2 in the chamber before measuring each plant to avoid cross contamination
- make sure the chamber is sealed
- Measure the treated groups 2x (at 3 days and 7 days, along with the untreated).

Hello!
Thanks a lot for your advices, actually I followed them and asked to my school permission to leave mi uv light with my plants on, they told me they couldn't allow me to leave it on by night but I could do it on the day. I thought this would be a problem because I couldn't do test groups experiments that lasted more than 1 day,but I came across a similar paper in which they left their plants for 10 minutes, 30 minutes and one hour. Just to try I used the same plants I had that didn't fit as well in the chamber but I decided to seal the chamber with an special type of paper and measure the changes for more time while I held down the chamber. With this I eliminated the time between exposition and testing and also the exchange of air from outside and inside the chamber and It worked! groups that were exposed for more time produced less O2, it was really small difference (around 1.20%) but it was constant. Continuing with the answers I will also measure the surface of leaves so my results can be presented in a better way. At the end my test groups for now are 10 minutes, 30 minutes, 1 hour and 6 hours. In answer to "goldenmaps" I will test 5 plants to have a greater pool and I haven't thought in the residual O2 so my school has 2 photosynthetic chambers and I will be measuring one plant's o2 in one, then letting it clean with air while I use the other and so on. I don't know yet about measuring each plant 3 times, the thing is that from my experience of waiting 20 minutes to arrive to school and measure the O2 it seems like the repairing of the plants is quick and I'm afraid that if I measure the same plants 3 times when I arrive to the second repetition the plant will have a normal O2 production rate.
I really thank you both a lot, you have helped me a lot.
I will perform repetitions tomorrow and I hope they work as well, what I have to ask now is, do you think that if the test time is so limited (from 10 minutes to 6 hours) and the change (even when it's constant through the repetitions) is so little the results are significative? I think there was an statistical way of knowing it but since I'm on vacation my teachers cannot help me. I think that if they are not my paper cannot be presented, it's not a paper for phd or anything but it is for the IB diploma.
Greetings