# Do high-light plants also need extra CO2?



## solarz (Aug 31, 2010)

Suppose we have a tank with low light plants and high light plants. Suppose that the tank is not CO2 injected and has low light at first. If the low light plants are doing well while the high light plants are struggling, would simply increasing the amount of light allow the high light plants to do well too, or would we need to inject CO2 as well?


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## Zebrapl3co (Mar 29, 2006)

Tough question to answer. It depends on how high you are planning to go. If you increase the light enough for the high light plant to survive and you regularly trim back the low light plants. It'll be a working model. The high light plant will survive, but it won't thrive/glow.
Usually, when we say high light, we meant at a point where high light plant can thrive at a maximun growth rate. If this is the case, then you must inject CO2 to keep up.

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## solarz (Aug 31, 2010)

Zebrapl3co said:


> Tough question to answer. It depends on how high you are planning to go. *If you increase the light enough for the high light plant to survive *and you regularly trim back the low light plants. It'll be a working model. The high light plant will survive, but it won't thrive/glow.
> Usually, when we say high light, we meant at a point where high light plant can thrive at a maximun growth rate. If this is the case, then you must inject CO2 to keep up.


That's what I'm wondering about. If a plant is dying due to low light, would increasing the light and not increasing CO2 allow it to survive and grow (albeit slowly)?

I guess this boils down to: "what does it mean to be a high-light plant?". Does it mean the plant requires more energy to photosynthesize the same amount of CO2 (as compared to a low-light plant), or that the plant needs to photosynthesize at a higher rate in order to survive?

I found something that deal with photosynthesis of terrestrial plants, which I think should apply to aquatic plants as well:

http://prakashamarasooriya.wikispaces.com/file/view/Adaptations+for+Photosynthesis.pdf
http://www.marietta.edu/~spilatrs/biol103/photolab/sun_shad.html

According to the above, high light plants are capable of absorbing more light and producing more sugar, but low light plants are more "efficient" at it. The "compensation point" is the point where the plant would be capable of growing from the photosynthesis. I think that's the critical point in amount of light needed: light intensity needs to be higher than the plant's compensation point in order for the plant to survive.

The second link does a comparison of the curves. Under low light conditions, low light plants actually photosynthesis at a greater rate than high light plants. This tells us that for the same amount of CO2 availability, simply increasing the amount of light does indeed benefit high light plants.


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