Science Buddies: "Ask an Expert"

Hello,
I am working on doing a science fair project on photosynthesis. I want to measure the concentration of oxygen that the plants produce over 2-4 weeks. To do this, I want to put 2-3 plants in an air tight container with a NeuLog oxygen sensor to measure the oxygen. My worry is that the rate of photosynthesis will start to slow because there will be a lack of carbon dioxide in an air tight box. I want to try and measure the slowing of photosynthesis due to abiotic stress conditions, not because of lack of carbon dioxide. Do you have any suggestions or comments on how I might be able to maintain a balance of gases?

Thanks, Taytum

Hi Taytum,

Sounds great that you're thinking very far ahead in your project design. I personally would not be concerned with the plants being choked off from CO2, as long as your design is proper.

Remember that any good science experiment will have a control group. I don't know what your abiotic stress conditions are, but basically your best plan here is to have a control with none of those stressors and monitor its oxygen levels side-by-side with your experimental conditions. Hopefully, there will still be a significant and noticeable difference in photosynthesis rate that you can monitor. Larger containers will also help, as there's going to be more air for the plant to work with.

However, the key point that you may have forgotten is that plants do both photosynthesis and cellular respiration! Meaning, plants are constantly producing CO2 as well as using it up, so in an airtight space, they'll do much better than an animal would. In the 2–4 week timespan, I don't think you'll be able to choke off your plants. In general, I believe photosynthesis rates are faster than respiration rates in terms of CO2 expenditure (which is why you'll get a result in the first place), but for further details on what conditions are optimal, what plants might be exceptions, and what other things affect it, I'll leave you to do more background research.

If you are still concerned, you could hook up a CO2 input pump so that the containers have a steady CO2 source without the input gas changing your oxygen readings. However that may be more resource-intensive and difficult to setup.

–Eugene