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On an exam of mine, I had the following question:

The interaction between the guard cells and stomata in a plant leaf can be seen in the diagram below.

enter image description here

In which scenario could the guard cells and stomata be in situation 2?

(A) High humidity

(B) High light intensity

(C) Low temperatures

(D) Nighttime

The answer choices aside, I was thinking the answer should be along the lines of "a scenario in which the temperature is really hot", since an open stomata on a very hot day would cause a plant to lose a lot of water. The only answer choice that seems to fit this is "high light intensity" (although I'm not sure if high light intensity would necessarily cause high temperatures, but this seems to be the closest thing to what I predicted). However, the exam key is saying that "D", nighttime, is the correct answer. My teacher explained that since plants don't need to do the light reactions during the night, they apparently close their stomata because they don't need to intake CO2. But this doesn't make sense to me: why would a plant close its stomata during the night instead of keeping it open to stockpile CO2 for use in the day? If I'm not mistaken, this is precisely what CAM plants do (although there's no mention of whether the plant in question is a CAM plant).

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I found a paper and this is the link https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3258058/ it says that Stomatal opening is induced by low CO2 concentrations, high light intensity and high humidity. By contrast, closing is promoted by high CO2 concentrations, darkness, drought and the plant hormone ABA. Another paper, Open or Close the Gate – Stomata Action Under the Control of Phytohormones in Drought Stress Conditions https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3652521/ During the night, elevated levels of CO2 in the leaves were observed due to respiration. It has been proved that CO2 has a positive effect on the stomatal closure process

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