Auxins are one of the families of plant hormones, still rather elusive substances, synthesised in small quantitied by plants somewhere in their tissues (i.e. root tips) to send growth messages across to other tissues either locally or elsewhere .
Of these substances, auxins were the first to be discovered, the most common form they take in the family being IAA (indole-3-acetic-acid). Once they were discovered, we tried to replicate their effects, synthesising substances in the lab: those, together with the natural plant hormones, are collectively known as plant growth regulators.
The current hypothesis is that plant hormones are difficult to trace because they are active in such small quantities, and possibly different mixes of the same substances have different effects. If you read a range of books, they will all say slightly different things about plant hormones, but they generally agree that auxins:
- affect cell elongation in stem and roots, in particular, they are behind tropic responses. For example, take phototropism, the plant's growth response to light. Positive phototropic plants grow towards the light (i.e. Kalanchoe stems bend towards the light: mind where you put your plant! Tropic responses are irreversible). Negative phototropic plants grow away from the light (i.e ivy-leafed toadflax - Linaria cymbalaria - which grows on walls, has stems that, after flowering, bend away from the light to direct seed pods towards wall crevices, where the seeds will get a chance to survive).
The way auxins work is by accumulating in the shady side, stimulating elongation of the cells, so that they get longer than the ones in the sun, and as a result the stem bends away from it.
- Auxins are mainly produced in the stem and root tips and move directionally (top to bottom and bottom to top respectively). When produced in the stem tip, they help maintain apical dominance, which means that the uppermost bud grows tall and the other ones beneath it, in the leaf axils, do not grow. Once the uppermost bud is removed, the other buds start growing. Because of that, pruning generally makes plants bushier. I noticed that quite strikingly in a Euonymus cutting.
Apical dominance suppresses
the growth of lateral buds
Once the apical bud is removed,
buds in the axils start to grow
- Auxins promote root initiation, that is why synthetic auxins (for example α-Naphthalene acetic acid or NAA and IBA or Indole-3-butyric acid), or willow water, are used to help rooting.
- Because of the way they make cell grow, some synthetic auxins (for example 2,4-Dichlorophenoxyacetic acid or 2,4-D for short and dicamba - 2-Methoxy-3,6-dichlorobenzoic acid) are used as broad-leaf herbicides: the auxins cause the plant to grow abnormally and consequently die. Sigh.