How does a Sunflower know which way the Sun is?

Lbeard

How does a Sunflower know which way the Sun is?

Carawaystick

Lbeard wrote: »

How does a Sunflower know which way the Sun is?

There's a thing phototropism, in plants, so they tend to follow the light (photo being Greek for light)
There's geotropism, which is for plants to know up from down, so roots go down and shoots go up from seeds, irrespective of their direction when planted (geo is Greek for earth)

How it works is that say a sunflower faces the sun straight on
On an hour, the sun will have moved 15 degrees, so the cells on the right of the plant will be getting more sun, so will signal to the plant, which will turn right.

So either the cells near the sun will produce more of a chemical messenger, (or less, I don't actually know which) and that messenger will cause the plant to turn right(to the light)
(If its less messenger chemical, that will reduce the tendancy to turn away from the light)


You can see something similar with crocus' which close up when the sun goes away, or daisies, which close up when the light, rather than sun fades

Lbeard

Carawaystick wrote: »

There's a thing phototropism, in plants, so they tend to follow the light (photo being Greek for light)
There's geotropism, which is for plants to know up from down, so roots go down and shoots go up from seeds, irrespective of their direction when planted (geo is Greek for earth)

I've just looked it up on Wikipedia. Auxins are responsible for both the geotropism and phototropism.

So either the cells near the sun will produce more of a chemical messenger, (or less, I don't actually know which) and that messenger will cause the plant to turn right(to the light)
(If its less messenger chemical, that will reduce the tendancy to turn away from the light)

Light is very powerful at initiating chemical reactions. According to wikipedia, the Ph is different on the darkside of the plant. The plant cell walls expand and break free from each other, and that breaking gives the mechanical twist. So, the plant doesn't know where the sun is.

I know nothing about biology. When I thought about the sun flower, I imagined it required a complex messaging system. But with photo activated reactions, the messenger doesn't have to send messages anywhere - it just reacts locally to what it gets.

You can see something similar with crocus' which close up when the sun goes away, or daisies, which close up when the light, rather than sun fades

It's fascinating. It's one of those questions small children drive their parents crazy with "How do the daisies know it's time to go asleep?"