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Mechanics of phototropism?

Posted by allotrope 5a ON (My Page) on
Wed, Jul 28, 04 at 20:42

I'm trying to remember about how plants that are phototropic (+ or -) move. Are they using cellular turgor (sp?) pressure? For example when my Phalenopsis grow leaves, the new leaf is very phototropic, I must rotate the plant every 2-3 days to try and keep the leaves facing up (purely for personal esthetic reasons). Once fully grown the leaf is no longer phototropic, why is that? Some plants actually follow the sun on a daily basis, are they using the same mechanics?

Follow-Up Postings:

RE: Mechanics of phototropism - partial answer

Hmm, after a little research I can probably answer my own question. The phototropic mechanism used by the growing leaf of the orchid is a hormonal response to the light. The plant pumps more hormones (auxins) to the side where there is less light thus that side of the leaf is growing faster which bends it towards the light. When the leaf has matured the production of hormone is "turned off" ie no longer produced, therefore the leaf is no longer phototropic.

While plants who follow the sun (circadian movement) use a different mechanism. They use turgor pressure for the mechanical movement of the cells. There is a good article that describes this but be warned is looks like it's from a scientific publication so the reading can be a little arduous.

It still would be worthwhile for a laymen's explanation for both mechanism and included as a FAQ (as someone already requested).

Here is a link that might be useful: Phototropic mechanism using turgor pressure

Ugh! - corrections

Man, I should learn how to read :-( Movement of a plant following the sun is NOT "circadian rhythm". Circadian rhythm refers to an internal clock not the movement of the sun. Phototropism seems to be strickly homone driven and associated with growth. Is this correct?

This however brings up the question of what do we call the movement of plants that move with the passing of the sun? and what mechanism is it using hormonal or turgor? I can't remember the plant that moves with the passing of the sun, does anyone know? Or am I dreaming. Or is it moving because it has a very active growth?

Also the link I gave takes you to the middle of the article, here is the link to the first page.

Here is a link that might be useful: Article

RE: Mechanics of phototropism?

Are you looking for the word heliotropism?
The hormone responsible for this movement is still auxin, which makes cell walls facing the sun get longer. All this results in a slight bending.

RE: Mechanics of phototropism?

Yes it is heliotropism, what is confusing me is that the article that I quoted seem to imply that heliotropism is really phototropism which again they imply is only related to growth. However it appears that heliotropism is not associated to growth. I did a quick google search on heliotropism and found very little. There are a couple of papers on the subject but it seems to be an odd duck (few plants have this ability, even within the same species).

RE: Mechanics of phototropism?

One that always fascinated me though I've never done any reading on it is Sunflowers. Many always face the flower toward the sun and will follow it accross the sky. Only the flower not the plant.Another thing is they resemble the sun.Can you imagine how much energy they're
devoting to this seemingly worthless habit?? lol I hadn't thought much on the mechanics of it I just want to know why lol

RE: Mechanics of phototropism?

garyfla, the reason sunflowers turn to the sun is to maximize the exposure to insects. the competition between plants mean that the plants visited by most insects are better able to reproduce. Insects primarily visit flowers which has greatest sun exposure.

I guess this becomes increasingly critical for plants with dark colored centers which tend to absorb light rather than reflect. I don't think you see white colored flowers exhibit this behaviour ?

RE: Mechanics of phototropism?

That most certainly be true.But sunflowers can't grow in the shade anyway lol.Also the insect has no choice.No flowers means no food which would mean no insects . A flower is a glorified highly specialized sex organ no doubt but I suspect there's a lot more to it.Otherwise all flowers would be the same color and shape and let's don't forget aroma. Seems rather a waste of energy to produce a flower in the first place.After all, ferns and fungi get along great without them If turning toward the sun is so effective why don't most flowers do it?? Phototropism is easy to explain .Obviously without the proper light the plant couldn't exist in the first place. But heliotropism
of a flower would seem to be detrimental.
I'm sure it has some important function to the plant but
it's certainly not obvious. Ah, the mysteries of nature lol.

RE: Mechanics of phototropism?

If you buy into Dawkin's selfish gene theory...passing your genes onto the next generation is the MOST IMPORTANT function of a plant. Really the green stuff just exists to pass those genes on.

I have no idea of how ferns or fungi accomplish this. I believe ferns use water to spread their spores (???).

But the beauty of angiosperms is their move away from water as a means of mixing up the gene pool and the population structure. The rest of the stuff below applies to those angiosperms that put their energy into out-crossing.

Really, if you think about plants are just like that Dixie Chick's song, Earl. They are stuck in one place and all they can choose is Earl...unless they find out some way to get out. So to avoid the abusive husband (and the associated "inbreeding depression") they have to move out.

So if your #1 goal is to PASS ON YOUR GENES...not to just get big or be beautiful...

then a flower that helps you do that more successfully isn't a's the whole point.

Now there are only so many bumblebees (Lear jets) in the world and if you are stuck in an area without enough bumblebees, you'd better learn to like riding Greyhound.

So plants have evolved relationships with so many different types of dispersion forces (insects, bats, moths, wind and the aformentioned water) to shake up their gene pools by acting as pollen distributors.

So I think heliotropism is not helping the plant accomplish it's major goal.

Besides why put all that energy into having that big old sunflower head and not do everything you can to ensure it's successful?

RE: Mechanics of phototropism?

For a complete explanation, including the motor, the sun sensing site and the movement mechanism, see this article, titles Sun Stalkers.

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