Bonsai Study Group Forum

General Category => Evergreen Bonsai Discussion => Topic started by: Owen Reich on October 24, 2013, 01:20 PM

Title: Growth Habit of Conifers Based on Location of Cutting Material
Post by: Owen Reich on October 24, 2013, 01:20 PM
This is a matter I learned about in my classes on plant propagation a while back but wanted John Kirby to comment about the root causes for my and other forum members' benefit. 

So in a nut-shell, vigorous apical growth often produces plants that grow more upright naturally while lateral shoots often do not.  On a micro level, when does an apical meristem or terminal bud on a lateral branch "make this decision" and why does the trait persist?  My terminology isn't spot-on but you get the idea.  Thanks.
Title: Re: Growth Habit of Conifers Based on Location of Cutting Material
Post by: JRob on October 24, 2013, 09:15 PM

Not sure this answers the question but if I remember my botany courses correctly, a meristem is a group of actively dividing plant cells. They are found as apical meristems at the tips of roots and shoots and as lateral meristems in vascular tissue (vascular cambium) and in cork tissue (phellogen). SO I think it is where they develop, tips or vascular tissue.

Title: Re: Growth Habit of Conifers Based on Location of Cutting Material
Post by: Owen Reich on October 25, 2013, 01:48 AM
I'm talking about environmental triggers, hormones, etc.  The physical structures I understand well. 

Why do cuttings from a lateral branch tend to grow more flat and wide; less prone to form a central leader?  I'd think the rooted cutting would reboot or something.  What I want to know is when does this "decision" get made and could it be reversed by wiring a lateral branch vertically.  It could have value. 
Title: Re: Growth Habit of Conifers Based on Location of Cutting Material
Post by: John Kirby on October 25, 2013, 09:22 AM
Owen, I am in a continuous work cycle, it is homecoming.......

The key term is impriniting. I will be back with more.
Title: Re: Growth Habit of Conifers Based on Location of Cutting Material
Post by: Leo in NE Illinois on November 10, 2013, 03:24 AM
As I understand it, my info could be a little out of date.

The meristematic tissue, the buds, are basically clusters of stem cells, but they are not pluripotent (meaning capable of producing the entire range of possible tissues). The meristem in horizontal branches has already undergone epigenetic changes that turn off certain regions of DNA, so that the meristem is programed to produce only horizontally growing tissue. Vertical, especially central leaders, have different sections of DNA "turned off". So essentially, by the time the seed sprouts, the various buds of meristematic tissue have already differentiated, and will only form roots, or horizontal branches, or vertical leaders.

There are several mechanisms to cause this turning on and off of the DNA, resulting in tissue differentiation.

Protiens can bind to the DNA, as can RNA bind to the DNA, these will stay in place for some period of time, some short, some even are seasonal, others are longer term. Methylation of DNA is another way that DNA activity can be blocked. Methylation can persist for long periods, a hundred years or more is possible, before reverting back to the normal state.

I am not familiar with mechanisms discovered in the last 15 or so years. I haven't kept up.

I do know that some of the 'horizontal only' cultivars of pines and spruces are largely stable, with only a rare revert back to more normal growth. Reversion back to more normal growth is rare enough that it can not be counted on to happen, even at large mass propagation nurseries. Though reversion does occasionally happen.

There are several different mechanisms for (epigenetic) blocking and re-activating the DNA of genes, and these mechanisms come in play as tissue differentiates. This means that it is very difficult to know which mechanism(s) are in play by just looking at a tree. Therefore it is not possible to make any blanket statements about exactly how long to wait, or what to do, to get a horizontal cultivar to revert to a more normal condition.

I do know that if instead of grafting, or making cuttings, you harvest bud tissues, and use it to create clones by meristematic tissue culture, because this process forces the cells to become pluripotent again, this technique often reverses that 'horizontal only' effect you asked about. The forcing the cells to become pluripotent stem cells, is usually done by adding plant hormones to the culture media. I would have to hit the books to find out which ones. So the end result, trees from tissue culture may not retain all the characteristics of the source cultivar. Some traits will revert back to the 'normal' type. Cuttings and grafts are superior to tissue culture for fidelity to the original cultivar's characteristics.

Does this help?
Hopefully someone else can add more, or correct my essay, as I pulled it out of the "Vague, but True" section of my mind, which is sometimes a bit faulty.