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.
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).
