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apical_dom.jpgPlant Cut-Backs

Pruning is one of the most important cultural practices for maintaining woody plants, including ornamental trees and shrubs, fruits and nuts. It involves both art and science: art in making the pruning cuts properly, and science in knowing how and when to prune for maximum benefits.” (from ref. 1 below)

Briefly, the main effect of pruning a plant is to stimulate the growth of axillary buds (a.k.a., lateral buds).

But why is the growth of axillary buds stimulated by cutting off the terminal (or apical) bud?

The most common explanation to this question, dear readers, is the long-known, and somewhat confounding, phenomenon called “apical dominance”.

The Curious Case of Apical Dominance

Apical dominance is defined as the control exerted by the shoot tip on the outgrowth of axillary buds, whereas correlative inhibition includes the suppression of growth by other growing buds or shoots. The level, signaling, and/or flow of the plant hormone auxin in stems and buds is thought to be involved in these processes.” (from ref. 2 below)

But hold on a second, I thought auxin stimulated plant growth.

Apical_Dominance.jpgThis contradiction was noted way back in 1933 by the eminent plant physiologists Kenneth Thimann and Folke Skoog – “…it seems paradoxical that a substance promoting cell extension can also act as an inhibitor….” (from ref. 3 below)

New Evidence

In a more recent post, I report on a proposed new class of plant hormones called strigolactones.

These compounds, derived from carotenoids, have been shown to inhibit shoot branching.

Research on strigolactones, especially as they relate to apical dominance, has yielded evidence that auxin may indirectly stimulate the production of strigolactones (see ref 4 below).

So, does reducing auxin in the plant by removing the apex via pruning also reduce the levels of strigolactones, which, in turn, may increase the growth of axillary buds?

It’s likely not that simple an answer. Other factors such as the plant hormone cytokinin are also involved.

Bottom Line: Pruning works by disrupting a natural process we call apical dominance, which is likely the result of complex interactions among two classic plant hormones (auxin and cytokinin) and one new one (strigolactones).

References

1. Wade, G.L. and R.R. Westerfield “Basic Principles of Pruning Woody Plants” The University of Georgia Cooperative Extension.

2. Ferguson, B. J. and C. A. Beveridge (2009) “Roles for Auxin, Cytokinin, and Strigolactone in Regulating Shoot Branching” Plant Physiology vol. 149, pp. 1929-1944.

3. Thimann, K. V. and F. Skoog (1933) “Studies on the Growth Hormone of Plants III. The Inhibiting Action of the Growth Substance on Bud Development” Proc Natl Acad Sci (USA) vol.19 pp.714–716.

4. Hayward, A., P. Stirnberg, C. Beveridge, and O. Leyser (2009) “Interactions between Auxin and Strigolactone in Shoot Branching Control” Plant Physiology vol. 151, pp. 400-412.

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