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Raging Hormones!

Written by Fred Nowak

Tuesday, 05 August 2008

Well, maybe that is a bit of exaggeration but it probably got your attention.  But do our trees have hormones that affect the life, growth and health of a tree?  Yep. There are many tree growth regulators and more are being discovered.  But let us look at two primary growth regulators, hormones, that affect the growth of trees and then that information can be translated into what happens to your bonsai trees. These hormones are auxins and cytokinins. AUXIN: This is one of the primary growth regulators of a tree.  It is produced primarily in the shoot tips and is transported cell to cell down the tree (in the phloem) and out to its root tips.  It flows in that one direction only.  A high concentration of auxins can initiate root growth. It is found in cell elongation and  cell division. CYTOKININS: Cytokinins are a group of chemicals that are produced primarily in the root tips and are distributed throughout the tree in the water that is being drawn up through the xylem of the tree. A high concentration of cytokinins can cause the release of lateral buds and initiate shoot growth and leaf expansion. Cytokinin production is affected by the auxin messages  received  from the shoots.  Auxin production in the shoots is affected by the cytokinin message about the root status.  In other words, one end of the tree (shoots) is communicating with the other end of the tree (roots) and these hormones try to maintain a delicate balance. Well, what does all that gibberish mean to us?  It helps explain how a tree reacts.  As an example, if we cut off a large portion of foliage off the top of our trees, the auxin signal is cut off or diminished while the cytokinin signal remains high. This ratio will help release and expand latent or dormant growing points (buds) and initiate adventitious growing points at the site of he damage.  You have seen a branch that has been lopped off and the many sprouts that can come from the stem. Regulating Tree Growth by Kim D. Coder and Sharon Lilly On the other hand, if a tree has sustained a massive root loss (root pruning), the auxin signal remains high while the cytokinins have been greatly reduced. High auxin signals and low cytokinin signals help initiate root growth. One might then expect that pruning shoots on a tree to compensate for root loss is not a good practice for maintaining a healthy tree. Attempting to "balance" the tree by green branch pruning reduces the auxin signal that was sent to initiate root growth; the one message a root pruned tree needs to respond to for its roots to grow.  Top pruning of root damaged (pruned) trees adversely affects the health of that tree even further.   So... when we severely top prune our bonsai (causing low auxin signals), the roots (cytokinins) expend much energy to cause the foliage to be replaced. When we severely prune the roots on our bonsai trees (causing low cytokinins), the auxins in the leaf tips  send a message for the roots to grow so that we have equilibrium. It's all about balance, equilibrium.  We all like to have balance in our lives and so do the trees and they do it through their raging hormones.

 

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