Mac Caruthers - How Trees Adapt & Respond: The Growth Cycle Via Hormones

Interesting reading about hormones affecting growth of bonsai trees. I liked it very much, so if you have some time I recommend you to read it. Author is Mac Caruthers and article was inspired by the work of Brent Walston. Brent was one the first Bonsai writers to introduce “Hormone Growth Regulation” into the Bonsai literature. You can download full article here form website of Florida Bonsai Society or here in bit different form.
Two small samples from article about auxin:
Auxin as part of the “Growth Control Field”:
Auxin is a hormone produced by any growth activity in the top of the tree. The strongest level of Auxin is produced by the apical buds. Auxins‟ strength varies in direct ratio to the growth of the buds/limbs/branches and the vertical orientation of these limbs/branches. You will learn more on this later. (Ref: 12, 24)
Auxin like the other hormones serves many different functions. This article will only discuss the ones that are of most importance to the understanding of the person studying Bonsai.
1) Auxins hormones are involved in plant-cell elongation
2) Auxin creates apical dominance by suppressing bud break behind the apical bud
3) Auxin stimulates root growth.
4) Auxin hides from sun light; it concentrates on the shady side of the tree. (Ref: 15)
Apical dominance means that the Auxin produced by the buds at the apex, (end of the branches) tries to suppress the growth of any bud break behind it; these other buds are called latent or adventurous buds.
The presents of Auxin causes cells to elongate. It does not cause them to divide but they do grow in length.
Auxin travels down the phloem of the tree to the roots. When the ratio of Auxin becomes higher than that of Cytokinin, it signals the roots to grow.
The flow of Auxin is driven by two systems. One is gravity the other is the proton pumps within the cells, (Ref: 28). The flow of Auxin is strongest when the two systems are working together at their maximum efficiencies. This occurs when a limb is growing absolutely vertical. The less vertical a limb grows the more the effects of gravity are reduced, thereby reducing the flow of Auxin. In the case of limbs bent below 45 degrees the flow of the proton pumps is also some what reduced due to the resistance of pumping up hill to get the Auxin to the main trunk.

Dilemma (4)
I have just brought a nice plant from the nursery. It is about 8 inches tall. If I put in a bonsai pot how long before it will be a beautiful Bonsai like I see in the books?

Answer: I take it from your question that you do not want to keep this tree the size as it is now. I assume you want to grow into something that gives the appearance of an old tree of great character with a large trunk. If that is the case then the answer is never. This is one of the most general misconceptions of people just staring out in Bonsai. Putting a tree in a shallow bonsai pot is almost the final act that you will perform on a tree after it‟s‟ development and training period. Lets review some or our growth regulation theory.
1. Auxin signals root growth.
2. Root growth produces Cytokinin
3. Cytokinin in turn signals for more top growth.
For roots to produce Cytokinin they must grow. By putting the tree in small bonsai pot you have severely restricted its‟ root growth. The strength of the growth cycles between the tops and the roots will get smaller and smaller over time. There will reach a point where the tree is so root bound that there is practically no growth. The Auxin and Cytokinin signals are diminished and roots are taking in only small amounts of nutrients and water. Much smaller internodes and leaves are produced therefore much less food is produced. Putting the tree in bonsai pot is the primary way to slow its‟ growth so that it stays virtually as it is. At that stage it is considered a finished bonsai. It is not uncommon for a tree to need to be developed, (called training), for 15 to 25 years before it is put into a Bonsai pot. See developing a Bonsai below. Yes, there are all ways exceptions