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It is widely reported at several places that certain trees such as Peepal (Botanical name:Ficus religiosa) and Tulsi (Botanical name: Ocimum tenuiflorum) release oxygen even at night. My understanding has always been that photosynthesis can not take place in the absence of light. Is this phenomenon real?

The search results appear biased due to the religious connotations attached to the plants above, esp. in India. Here are some links where this is discussed:

ETA: There is another link that purports to explain the same, but uses so much jargon that it becomes impossible to decipher.

Most plants largely uptake Carbon dioxide (CO{-2})and release oxygen during the day (photosynthesis) and uptake oxygen and release CO{-2} during the night (respiration). Some plants such as Peepal tree can uptake CO{-2} during the night as well because of their ability to perform a type of photosynthesis called Crassulacean Acid Metabolism (CAM).

However, it is not true that they release large amounts of oxygen during the night. CAM is one of the three types of photosynthesis pathways occurring commonly in plants; the other two being C3 and C4 pathways.

Of these, C3 is the most common among land plants. CAM occurs primarily in desert plants and epiphytes (plants that live on other plants, usually large trees). CAM plants do not open their stomata during the day in order to minimise water loss. During the night, they open their stomata and fix CO{-2} in the form of malate .

During the day, they breakdown the malate and use the released CO2 through Kalvin cycle to produce sugars, similar to C3 plants. However, CAM is an energy inefficient reaction and hence plants use CAM only during certain conditions.

Peepal tree is a hemi-epiphyte in its native habitat i.e. the seeds germinate and grow as an epiphyte on other trees and then when the host-tree dies, they establish on the soil. It has been suggested that when they live as epiphyte, they use CAM pathway to produce carbohydrates and when they live on soil, they switch to C3 type photosynthesis.

So, Peepal tree would either release or not release CO{-2} in the night depending on if they are epiphytic or not. For other CAM plants, it would depend on if they have adequate water or not, or other environmental factors.

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Some plants store carbon dioxide at night, which they then use during the daytime. But I'm not sure if they also release oxygen at night - en.wikipedia.org/wiki/Crassulacean_acid_metabolism –  Tom77 Apr 13 '12 at 13:17
@Tom77: Interesting. "Crassulacean Acid Metabolism" sounds like a term straight out of science-fiction! That Wikipedia page suggests that the oxygen release (during the "light reactions") only proceeds during the day. –  Oddthinking Apr 13 '12 at 17:10
That discussion of CAM does not discuss oxygen release at all. CO2 fixation and O2 production are not tightly linked in photosythesis. –  adam.r Nov 23 '13 at 2:37
The wikianswers link has no serious content. The "secrets of longevity" link is pure idiotic babbling. I don't think that this counts as a notable claim. These sources seem to conflate CO2 with O2... their dynamics are very different. The O2 from photosynthesis comes from H2O, not CO2. Also the O2 concentration in our atmosphere is 20%, whereas the CO2 level is like 200 parts per million -- so a doubling of CO2 has a trivial impact on O2 levels. –  adam.r Nov 23 '13 at 2:45
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