Why do plants close their leaves at night?

15 April 2014

Question

Why do some plant close their leaves at the evening?

Answer

Helen - It's a lovely question and the word for it is - Nyctinasty is the word for plants that close their leaves and their petals sometimes at night. And they do it in different ways, it's rather nice. I don't know if you've ever seen Clover? If you went out and looked at some clover at night you would see that they've raised their little leaflets up and pressed them together and other plants fold them downwards. We already talked today a bit about light and how it affects organisms and these plants also respond to light. They have a circadian rhythm, a body rhythm, an internal clock if you like. And this does seem to have an effect on how they hold themselves and the kind of, the nuts and bolts of how it happens is a little thing - a joint-like organ at the base of the leaves. That's called the pulvini. They're basically little blobs of cells and these can change shape based on the pumping of ions. So, potassium and chloride ions get pumped in and out of different parts of these little organs. And then because of osmosis, water then shunts backwards and forwards and either pops these leaves up or squashes them back down again. And then that movement of those ions is affected by blue light in the daytime and by red light which happens at more kind of dusk time and into the night. We think it happens probably to protect themselves from getting cold and in other cases, there are plants - have you ever come across this fantastic thing in the tropics called sensitive shy grass. If you touch it, it collapses instantly. It's the same organ that's doing this, the pulvini is the same sort of reason it's happening but they respond to touch rather than to light and they're really fun. So, if you ever find them, you flick them and they just all collapse down and they think that so they don't get eaten by herbivores that come along.

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