what bad thing is going to happen at night, that they feel the need to expend the energy
A food-for-thought question from a biochemist who doesn’t know much about water lilies specifically: your question assumes that it’s the opening/closing that requires energy, but another possible mechanism might be if the default, relaxed position was closed, and energy needed to be expended during the day to keep the flower open (or the inverse, where the open position is the default, relaxed position, but then at night, energy is continually expended to keep it closed). I suggest this based on how I know many of the molecular mechanisms of plants rely on turgidity (swollenness from water) to evoke structural changes.
It doesn’t change the question materially, it just involves looking at the question from different angles. I don’t know much about water lilies though, I didn’t even know that they close at night. I might report back here if I find anything interesting.
Okay, well this is actually super cool, thanks for driving my attention there. It seems that the word for this particular behaviour is "Nyctinasty
I think the short answer to your question appears to be “we don’t know”, because nyctinasty has been observed for millennia but there isn’t a clear, conclusive answer. In some plants that are open at night, it may be that they are pollinated by bats or nocturnal insects, but one of the open questions here is “does nocturnal nyctinasty happen via the same mechanism as diurnal nyctinasty?”. By that, I mean “does a nocturnal plant (like moonflower) use the same ‘muscles’ as a plant that’s open during day, like water lily?”.
I don’t recall it mentioning water lilies, but a book that you’d probably enjoy if you’re curious about this stuff is What a Plant Knows by Daniel Chamovitz. I remember there is some really interesting stuff in there about plant circadian rhythms, and the information is presented in an accessible and entertaining manner
I did try googling this first.
Surface area is evaporative exposure and frost exposure, might not matter much to temperate acclimated plants but it may be an evolutionary holdover. Also plants don’t really expend energy to modulate their bodies. It’s more about moving water around their internals.
Moving may also prevent some fungus or algae colonization.
I want to be clear that I’m not arguing with you, but when you talk about moisture evaporation it’s kind of funny because the water lilies are sitting in a lake lol
Yes but they still have interior and exterior moisture, and evolved from plants that weren’t always in a lake. And I suppose occasionally grow in temporary bodies of water.
Also plants don’t really expend energy to modulate their bodies. It’s more about moving water around their internals.
Yes, but no. The energy expenditure is there, just not in the same way as animals and motile single-celled organisms. It’s, to borrow from software engineering, “left-shifted”. Plants invest the energy into building structures in their bodies that can be actuated via environmental changes. So, for example change in temperature between day and night may change hydrostatic pressure, which in turn causes motion (opening and closing the blossom).
Probably has something to do with their (preferred) pollinators not being active at night. If they are still opened the pollen will more easily blow away maybe?
David Attenborough is your friend:
nope. staying at a lake cottage
