Auxins are plant hormones that help guide plant growth. They work in conjunction with but in opposition to, cytokinin hormones. They are both required, and the ratio of one to the other will either promote root growth (if the ratio has more auxin), or promote shoot growth and inhibit root growth (if the ratio has more cytokinin).
IAA (indole-3-acetic acid), and IBA (indole-3-butyric acid) are the two primary natural plant auxins. Two other natural plant auxins used by some plants are 4-Cl-IAA (4-Chloroindole-3-acetic acid), and PAA (2-phenylacetic acid). The primary (but not the only) flow of IAA is from the growth tips down through the phloem (transport tubes in the stem) toward the root tips. The effect is additive, so the effects of the auxins are least at the top of the plant, and most at the root tips. The apical (top) budding site generates IAA, but doesn’t get any from above itself, so is the least affected by it. This causes the top node to grow unhindered, and therefore generally larger. Anything below it is in part influenced by the IAA the apical site produces. You can see a similar effect on the last node on a branch, which tends to be larger than other sites along that same branch.
A common tactic in pruning is the removal of the top growth tip or bud, which also removes the IAA it was producing. This gives the bud nodes directly below it the advantage of not being hindered, allowing them to develop larger. This is commonly known as pinching, because the tips are “pinched” off, although in practice using something sharp such as scissors is usually preferred.
In tall plants, the normal growth pattern is often a large central top. By pinching off the top growth tip, the next highest buds will tend to increase in size, and they, in turn, will produce IAA to inhibit growth below themselves. The energy that would have gone into increasing the size of the top bud is now divided between the new topmost nodes. This is often done to promote a bushier plant.
The top growing tip need not be removed to take advantage of this phenomenon, alternative methods involve simply bending the top down to the same level or lower than the next highest budding sites. Benefits to using these methods include a reduction in stress from pruning, and they prevent the loss of the budding site, but these are more labor-intensive, and they do require more ongoing maintenance.
Sometimes the normal auxin flow will get sidetracked in response to stimuli, such as light (turning the plant toward the light), or gravity (turning roots downward). It does this by concentrating in an area on the inside of the turn to inhibit growth, allowing the opposite side to grow normally, and thereby curve the stem or root in the desired direction. One way to visualize this is to think of toothpaste being squeezed from a tube. Place a fingertip on one side of the stream, that will inhibit flow (growth) on that side, and the toothpaste flow will curl toward the fingertip.
One reason cuttings need to have an exposed growth node is so that the node can send auxins down to form roots, which will send cytokinins back up to the growing tip to promote leaf, stem, and branch development. Rooting products generally add auxins near where they’re needed, namely on the stem where you want root development. Along with the natural auxins, these hormones encourage the undifferentiated meristem cells to become root cells, forming callouses first, and then roots on the stem.
Since IAA is not suitable for packaging and storage, most commercial rooting products use either IBA or a synthetic auxin, such as NAA (1-naphthaleneacetic acid). Not all auxins are good for plants. Some synthetic auxins are used as herbicides. These auxins send the plant into an uncontrollable growth spurt that the plant can’t keep up with, quickly over-driving it to death.
Broad-leafed dicot (two seed leafed) plants are more easily influenced by auxins than narrow-leaf monocot (single seed leaf) grasses. Certain synthetic auxins take advantage of this to selectively affect broad-leafed plants. An example of this is an auxin-based herbicide that kills the dicot dandelion without killing the surrounding monocot lawn grass.
Agent Orange is a 50/50 mix of the synthetic auxins 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid). They have discontinued the use of 2,4,5-T in most places due to medical concerns, but 2,4-D is still one of the most commonly used herbicides.
Some GMO crop plants are specially designed with even more resistance to the effects of such auxins, intentionally making them very well-suited for using auxin-based herbicides in the growing program.
Cytokinins are the opposites of auxins in more ways than one. Just as using auxins promotes root growth, you can use cytokinins to encourage bushy growth over root development, and they’re used in some PGR (plant growth regulator) products.
As stated above, auxins start at the growth tips and travel down the phloem towards the root tips to encourage roots to grow. Cytokinins start at the meristem cells located at the roots and travel up the xylem to encourage shoot cell division (growth). Because of this, damage to the growth tips will reduce the auxins needed for root development, stunting the roots, and damage to the roots will reduce the cytokinins needed for shoot development, stunting the growth tips.
Having an understanding of the interaction between auxins and cytokinins is helpful when pruning plants, training branches, trimming roots or applying a rooting compound. It should now be obvious why adding an auxin to a cutting is conducive to rooting, but applying a cytokinin would be counterproductive, and that it is the ratio between auxins and cytokinins that matters, so adding both at the same time is counterproductive, and potentially a waste of money. Use one or the other to promote either root or shoot development depending on the desired results.