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. Nyctinasty is the word for the process by which plants close their leaves and their petals at night. And they do it in different ways.

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 leaflets up and pressed them together; 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 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 organs. Then, because of osmosis, water shunts backwards and forwards and either pops these leaves up or squashes them back down again.

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 leaves from getting cold; in other cases there are plants 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. But they respond to touch rather than to light.

So, if you ever find them, you flick them and they just all collapse down. It's believed that this happens so they don't get eaten by herbivores that come along.

Comments

It's really good advice and info

Put it in soil and this grew. The leaves come together at night and open during the day. It's green and the leaves are paired on each side of the stem.

Hi - the Answer posted was interesting but didnt explain why leaves are lowered at night. Is it to save on energy ? I dont know !

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