If a plant has enough water, minerals and energy it will grow right? Well sort of… but there is more to it – like why do plants bend towards the light and not just grow straight? And how come the stem grows up but the roots grow down? It isn’t as if a plant has eyes to tell it where the sun is.
Plants are packed full of hormones, sending messages around to its different parts. Where humans have the creatively named ‘growth hormone’, plants have hormones called auxins.
Auxin is produced in the stem tips and roots, and controls the direction of growth in response to different stimuli including light and gravity. Having been made in the tips of the stems and roots, auxin is moved in solution by diffusion to older parts of the plant. In the stem, the auxin causes the cells to change in elasticity. More elastic cells absorb more water, and can grow longer.
Strangely though, stems and roots respond differently to high concentrations of auxins. Whilst the stem cells grow more, the root cells actually grow less. So auxins make plants grow, but why do they bend towards the light? How do they know to do this when they don’t have eyes?
The bending happens because the light hits the one side more and breaks down the auxins in that side of the stem. So then growth slows down on the ‘light’ side. The faster growth on the ‘dark’ side causes the shoots and leaves to turn towards the light – which is ideal for the plant for photosynthesis.
Auxin is produced in the tips of growing shoots. If the tips are cut off, then no auxin can be produced and so no plant growth. If the tips are covered, whilst auxin is still produced, light cannot break it down and so phototropism cannot occur: the plant just grows straight up and does not bend towards the light.
Auxins have the opposite effect on root cells. In roots, auxins cause less growth. The shaded side of roots contain more auxins, and so they grow less. This enables the ‘light’ side of the roots to grow more and bend away from the light.
And if that wasn’t weird enough, we have opposites happening with auxins and gravity too. In a horizontal root, the bottom side contains more auxins and grows less, so the root bends downwards in the direction of gravity. So positive geotropism. But of course, the stem responds differently.
In a horizontal stem, again the bottom side contains more auxins because it is less directly hit by sunlight. But because auxins cause growth in stems, the bottom side grows more causing the stem to bend upwards, against the direction of gravity. So negative geotropism.
But I am giving auxins too much credit; they don’t work alone. They have a partner in crime; cytokinins. You don’t need to know anything about these hormones other than the fact that they work alongside auxins. There is another plant hormone that you do need to be aware of… Gibberellins.
Once a seed germinates, the roots and shoots start to grow. But for this, the seed needs energy. Luckily, the seed releases a hormone called gibberellin which causes the starch in the seed to turn into sugars and provide the seed with energy to grow. As well as causing shoot growth, gibberellins can also stimulate flowering and fruits in some plants. And they also work with auxins to cause stem elongation.
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1:58 What do you mean by 'shaded' side of roots. Also the root was kinda moving towards the light just like the horizontal shoot in the video at the 1:58 mark. If you could explain, that'd be awesome. Cheers!
vry cool i show my class from africa dey ave neva seen plant before
I got a 9 in Biology because of FuseSchool
Might be here forever…mightened we
This is a pretty helpful animation, just a small part on the tropism section though:
'The original theory was that auxins were destroyed by light but recent evidence suggests that levels of auxins in the shoot stays the same regardless of whether they are kept in the dark or in the light'
(Fuller, Ann 2015 Edexel Biology B A level. Page 184 paragraph 3)
just wondering your thoughts about that, can't see any other method of causing a phototropism
That's not the structure of a gibberellin, shown at 3:12…
Please do I apply directly to the root or by spraying, thanks
hi guys! i need help, i have searched whole internet but couldnt find the ans, i gave my Tomato plant (NAA) which has been recovering from transplant shock, kept under low LED/HPS lights, after NAA the plants started wilting & dropping badly, now seems like they are dying, i didnt overdose NAA, i follow the description mentioned in the bottle, what it might be? lack of light or wrong time to give auxin?
Really helpful fr studies thnk u guys..
fuse school hit 100k XD
One thing I don't get: You say that the light breaks down the auxin on the side closest to the light, but my science book says simply that the 'hormone moves away from the light as it diffuses downwards'. Which one is right? Really helpful video otherwise – thank you so much!
This is cool
Thank you!!!!!