How Bonsai Grow: Secondary Growth

Its been a busy week at the day job, dealing with a quick turn-around database job, reviewing Iraq S&T proposals, keeping the Plone sites spinning along, and handling some political issues surrounding WACSI. Somewhere in there were repairmen all over the house, keeping an eye on Terrance, watching the new transplants in the gardens what with 97 F weather, and the usual groceries and laundry. No matter, I still had time to update the Travel Schlepp website, moving all the podcasts onto a Comcast filespace and fixing the permalinks and xml.

So here it is 1:30 AM and what better time to continue our discussion of secondary growth in woody dicots. Since the last installment, I came across an excellent PowerPoint reference for those who would like a well illustrated, basic explanation of how plants grow.

On a more technical side, Groove and Robischon at the USFS have published a detailed review of the current state of knowledge. Three key areas are identified plus the interactions among them:

• Transcriptional regulation (microRNAs, differential gene activity, etc.)
• Phytohormones (auxins, cytokinins, and gibberellins)
• Cell wall regulation (hydrolysis, Xylogen induction, etc.)

Cytokinin is primarily produced in the roots, although probably all meristems generate some. From tissue culture experiments we know that low cytokinin to auxin ratios promote root growth; high C/A ratios promote shoot and lateral meristem growth. Considering that auxins are produced in the apices, this makes perfect sense. One important observation is that over-expressing a cytokinin gene reduces xylem formation.

From the point of view of growing a thick trunk on a bonsai, one would probably want to keep cytokinin production relatively low. However, to enhance the above ground portion of the tree, we would like to see a high C/A ratio. That means reducing the auxin supply as well.

When you consider that managing a bonsai involves tip pruning as well as root pruning, this begins to come together. When I keep the rootball small and vigorously nip the apices of my junipers, I not only am reducing apical dominance and getting a bushier tree, I am probably controlling the C/A ratio to enhance secondary thickening. By maintaining the plant in a shallow pot, we further spread out the surface roots and develop the much sought after nebari.