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There's a claim that Sansevieria trifasciata (Mother-in-law’s Tongue) generates oxygen from carbon dioxide during the night.

This seems surprising to me: that process requires energy; plants generate O₂ from CO₂ through photosynthesis, requiring light. So not photosynthesis, and no CO₂ absorption, at night.

Am I missing something, or is this claim false?

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up vote 7 down vote accepted

This is an interesting topic!

Crassulacean acid metabolism is a second CO₂ fixation pathway where CO₂ is absorbed at night. The CO₂ is fixed into maleic acid HOOC-CH₂CH(OH)-COOH which stores some of the CO₂ in the form of carboxyl groups. During the day carboxylases release the CO₂ for fixation during the day.

This is an adaptation where the stomata open at night to take in the CO₂ and are closed during the day - the plants sort of 'hold their breath'. The energy for this process is derived for glycolysis which were stored during the day (nice detailed review here).

The paper says that about 6% of plants (including Mother-in-Laws Tongue) do this and you can imagine with night breathing how closing the stomata during the day would really help with dehydration. Its interesting to note that this is found in a wide variety of plants "CAM is found in five taxonomic classes, comprising monocots and dicots, encompassing 33 families and 328 genera". That says that CAM is a pretty old development in plant evolution and not so expensive that it is easily lost.

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Shigeta submitted his answer as I was writing this!

Sanseveria is one of a wide group of plants (mainly succulents) that adopt a photosynthetic strategy referred to as crassulacean acid metabolism (CAM).

Recall the basics of photosynthesis. The light-dependent reactions use energy from captured photons to generate ATP and NADPH, with the generation of O2. In the light-independent reactions (the Calvin cycle) this ATP and NADPH is used to fix CO2 to form three carbon sugars, which can then enter standard metabolic pathways to produce other metabolites.

In order to obtain CO2 for the fixation reactions plants have pores in their leaves called stomata to promote gas exchange (via diffusion). This presents a challenge for plants that grow in arid conditions: if they open their stomata during the day they lose water as water vapour. Many such plants use CAM to solve this problem. Basically what these plants do is to only open their stomata at night, capturing CO2 in the form of organic acids (oxaloacetate, malate, maleate) which are stored in vacuoles. Then, during the day, these acids can be metabolised to release the CO2 again for use in the photosynthetic reactions.

Notice that in this scheme the O2 will still be generated during the day. So when does it leave? Does it have to wait until the stomata open at night, or is it able to diffuse out through cells (recall that O2 readily diffuses through biological membranes)? I have been unable to find a definitive answer to this question so far (this is old literature), but my guess is that it is lost throughout the day.

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bread and butter! – shigeta Feb 2 '13 at 23:39

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