IAA's effects on plant growth

What causes a plant to “grow-up,” roots to grow down, or plants to grow towards a light source?  The answer is plant hormones or plant growth regulators as they are sometimes called.  In this particular case the hormone involved is the naturally occurring plant substance called indole-3-acetic acid or IAA for short. 

 IAA’s primary function in life is to stimulate cell growth especially near growing points.  In stems the more IAA present the more cell elongation occurs.   If you place a living, growing stem on its side it will naturally bend to return to growing in an upright position. Here’s how it works in our horizontal stem example.  In response to basic gravity IAA accumulates in the bottom portion of the stem.  This increased concentration causes the cells at the bottom of the stem to elongate (grow) at a faster rate then those cells at the top causing the stem to naturally curve upwards.  In roots IAA has the opposite effect.  The more IAA presents the slower the growth rate causing a natural downward growth.  Phototropism, or the growth of plants towards a light source, is also under the control of IAA.  IAA is degraded by sunlight and thus accumulates in the shady side of stems causing cells on the shady side to elongate at a faster rate then those on the sunny side curving the stem to naturally grow towards the light.  Amazing!

It's all in the timing

Plant phenology is the study of when plants do some visible act in relationship to the calendar or clock. For example, spring snowdrops usually come into bloom in late March or early April.  During Victorian times plant phenology was taken to its highest levels, almost becoming an art form. As Victorian era gardeners planted massive carpets of annuals in intricate designs, they also began a tradition of planting floral clocks; literally a planting of flowers which could be used to tell the time (or at least approximate the time).

Going back to phenology, we’ve all noticed that certain plants open their flowers at various times through out the day.  For example:  morning glories, moon flowers, four-o-clocks, evening primrose and daylily to name just a few.  It’s this phenological characteristic that was used to create these intricate and surprisingly accurate floral clocks.

Now we may no longer plant flower clocks and our busy schedules may not allow us the luxury of keeping our own phenological records, but plant phenology can still play an important role in our industry.  Close observations of when certain events occur are becoming more and more useful in the control of many plant pests.

Let me give you just one example.  Bronze Birch Borer (a widespread and possibly fatal insect of paperbirch) is best controlled by certain chemicals when the lilacs are in bloom. Applications of these same chemicals at other times to control the borer are mostly ineffective.  Lilac time is a simple observation which can allow you to better control a pest problem and lessen useless applications of hazardous materials into our environment.  So next time you “stop to smell the roses,” maybe take a few notes or observations as well.