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Re: Gib vs Auxin

Hello Rob,

>"Auxin must be a germinating hormone as well as a rooting hormone
>because that is the first thing that seeds do when they germinate, make
>a root, and conversely, therefore, gibberellin must be a rooting
>hormone" 
> 
>Has anyone EVER heard of artificially-applied auxins, rather than
>gibberellins having any positive effect on germination? Why might this
>be unlikely, technically? Might there be life-forms in the soil which
>manufacture or excrete auxin? 


I'm afraid that kind of reasoning and extrapolation can be dangerous.  My
feeling would be that auxin as a germination hormone would not be such a
good idea. And the use of Gibberellin as a rooting hormone would be
useless, if not detrimental.  Here's why:

To my understanding of plant growth regulator physiology, the effect of
gibberellins on seed germination has nothing to do with the roots, but
rather with the lifting of seed dormancy.  Giberellin "only" helps dormant
seeds "wake up" and start consuming their stored reserves in order to
germinate.  Its mode of action is complex and is also affected by the
amount of Abscissic acid present in the seed (another plant hormone). Never
have I read anything relating a positive effect of gibberellins on plant
root formation. However, gibberellin does have a pronounced stimulating
effect on stem elongation. It is one of the hormones responsible for the
bolting of plants with a "rosette" growth habit, such as cabbage and the like.

Auxin, as a rooting hormone, contributes to root INITIATION, but when
present in too large concentrations, actually INHIBITS the elongation of
the primary root on a plant.  The reason auxin is used as a rooting hormone
for cuttings is that it stimulates the initiation of LATERAL and
ADVENTITIOUS (i.e. newly-formed) roots.  

Using artificially-applied plant hormones is touchy, in the sense that
their effect on plants can vary with the concentration applied, the
presence, absence or relative concentration of other (endogenous) growth
regulators, the place on the plant it is applied, as well as the type of
material you treat (age, species, etc.).  Too low concentrations have
little or no effect, and too high concentrations, or the use of the wrong
substance, can do more harm than good.

In our nursery, we produce stem cuttings from young spruce trees.  We have
experimented repeatedly with the use of auxins as rooting hormones, only to
arrive to the conclusion that in our case, it was an unnecessary step.
Because the cutting donors were young, they had sufficient levels of
endogenous auxins to root adequately.  A low-concentration of auxin had
absolutely no effect on rooting percentage.  At high concentrations, auxin
did slightly increase the NUMBER of roots per cutting, but at the same
time, DECREASED the rooting percentage... so that was no good either.

What works for some species may not be useful to you.   Then again, it
might. If you want to use hormones to enhance your garden, always compare
your treated plants to untreated ones, to make sure you are getting the
benefits you hope for.  If possible, try out different products and
different concentrations. Better be safe than sorry... and no use working
for nothing, right?

Denise Tousignant


********************************************************
Denise Tousignant, M.Sc., Conseillere scientifique
Pepiniere forestiere de St-Modeste, 
410 rue Principale, St-Modeste, Quebec GOL 3W0 Canada
tel.: 418-862-5511  FAX: 418-862-0564
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