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When a plant starts to flower, the flower stalk grows towards the light. If a potted plant has a flower stalk, a pedicel, already growing in towards the sunlight, then the same potted plant is turned 180° the pedicel will bend so the flower is pointed towards the light again. What is the drive behind this action? I understand a flower would start to grow towards the light because it has a better chance at being noticed by pollinators or a greater chance of catching a breeze to disperse it’s pollen or better chance at catching pollen in the wind. But, what physiological action causes it to make that bend? Is it as simple as the photosynthesis that is taking place in the stalk?

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There are several theories proposing different mechanisms, and different species may do this in different ways.

One mechanism is that light affects the production of auxins, which are hormones controlling the rate of growth. There is more auxin on the darker side of the plant, producing curved growth.

Auxins also affect the overall biochemistry of the plant, including changing the internal pH value of the plant cells, which can cause the cell walls to become softer or stiffer and make the plant "droop" in one particular direction. That can explain temporary shape changes, for example plants which "follow the sun" on a daily cycle, compared with different growth rates on the light and dark side of the plant which are more permanent.

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Phototropism is the force responsible for this 'leaning towards the light', in this case, positive phototropism. It's not just the flowering stems either, most plant shoots exhibit positive phototropism; the production of auxin on the darker side of the plant is increased and elongates the cells on one side only, causing the plant to lean or bend towards a light source https://en.m.wikipedia.org/wiki/Phototropism. There are examples of negative phototropism - some stems on climbing plants exhibit this, climbing up away from the light; lateral shoots produced off those growths then exhibit positive phototropism.

The effect of positive phototropism is often more noticeable in houseplants; rotating them regularly will offset this effect and stop the plant developing a pronounced lean.

  • Sorry, your answer is also correct. alephzero answered the question first, so I marked her's correct. I appreciate your answer. I believe it is just as educational. – GardenGems Jan 3 at 19:20
  • No worries - I'm more intrigued by your use of the feminine pronoun in regard to alephzero, who I did not realise was a fellow female... – Bamboo Jan 3 at 19:28
  • Sorry, I just put pronouns where they do not belong. That is my fault. I also give my plants pronouns. – GardenGems Jan 3 at 19:51
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Plant stalks lean toward the light, because the light, in 'striking' the near-side of the stalk inhibits the activity of the plant growth hormone: auxin. Then the far-side of the stalk, whose growth is not being inhibited, continues growing; and given that the back of the stalk continues growing while the front halts: the stalk will bend.

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