The Functions, Effects, and Applications
of a Plant Growth Hormone

By Michael Place

Auxin is the first and most studied plant growth hormone in horticultural study.   For the professor or student of a plant science, auxin is a readily accepted term that can be placed to many biological plant functions.  What about the home gardener? Does the hobbyist at home know what auxin is? By understanding the science behind the art, home gardeners can generate a positive impact on their home landscapes. This article is designed to simplify the science so the hobbyist can apply it to his/her well being.
There are 4 basic questions that must be asked about auxin. They are:

1.  What is auxin?
2.  What are the major functions of auxin?
3.  What are the effects of auxin on plants?
4.  What is it’s applications?

The following sections break down these questions so that this valuable plant growth hormone can be better understood.
1.    What is auxin?
     Answer: Auxin is a plant growth hormone that can be created chemically or produced biologically by the plant. In biological synthesis, the natural hormone is represented by the symbol IAA, standing for indoleacetic acid.  Chemically generated versions of this hormone are NAA (naphthalene acetic acid) and IBA (indolebutynic acid).  The NAA and IBA synthetic hormones can be purchased at any local greenhouse in your area.

2.    What are the major functions?

       Answer:  Auxin promotes cell growth and elongation of the plant. In the elongation process, auxin alters the plant wall plasticity making it easier for the plant to grow upwards. Auxin also influences rooting formations. By applying NAA or IBA to a plant cutting from a parent plant, rooting takes less time for the new plant greatly increasing it’s chance of survival.

3.      What are the effects?
    Answer:   Auxin is responsible for apical dominance and phototropism.  Apical dominance means that a plant has the tendency to grow quickly up the central shaft without properly filling in the plant with smaller limbs and leaves.   Phototropism is a process by which auxin on the exterior of a plant is degraded to cause a slope of the plant towards light. Have you ever seen a sunflower follow the sun across the sky? That movement is caused by phototropism. This process does not have detrimental effects to a plant.

4.      What are the applications?

    Answer:   Applications can include pruning and root formations of new plants. If you want a “full” appearance to your plant, pruning greatly enhances your chances. Pruning is the process of cutting the central shaft at its top causing a plant reaction to grow more branches, hence filling in the look of your plant---more branches mean more leaves and so on. If you would like an exact replica of a current plant in your garden, cutting a piece of the original plant for growth is made much easier when treating with the synthetic version of auxin, IBA. Dip the “ground” end of your cutting in IBA powder and then place the plant in a rich nutrient soil. You’ll see noticeable rooting formations within two weeks. This process is known as cloning.

Article written by Michael Place using information from the text and lectures of Horticulture 210, instructed by Dr. Chiwon Lee at North Dakota State University.