Plant Stem Growth
If you cover just the leaves on a plant the stem will
elongate rapidly and eventually the leaves will fall off. What plant
hormones/chemicals will cause the rapid growth in the stem?
This effect is known as etiolation.
To be perfectly honest, we don't have a good understanding of the
hormonal regulation of this phenomenon. We do understand the
biochemical mechanism by which the light level is sensed. Higher plants
have a pigment known as phytochrome which senses the relative
proportions of red (660nm) and far-red (730nm) light in their
environment. This allows plant seedlings to sense, firstly, whether
there is any light present and secondly, the degree to which they are
shaded by neighboring plants. Unfortunately, we don't really have a
clear understanding of how phytochrome brings about different growth
responses. I personally suspect that the hormones auxin (indole acetic
acid, or IAA) and cytokinin (a variety of different related compounds)
may be involved. A few years ago I did some experiments looking at
hormone levels in plants that were shaded compared to those in full sun.
I'm sorry to say that I never managed to find anything conclusive.
Plant hormone work is notoriously difficult, although a lot of progress
has been made fairly recently. This is one of those areas of biology
where researchers are still scratching their heads.
Auxins are plant hormones that cause stem elongation.
Anthony Brach Ph.D.
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Update: June 2012