Plant Hormones - July 20, 2005
Jeff Schalau, County Director, Associate Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

Plants need light from the sun, carbon dioxide from the air, and water and mineral nutrients from the soil. From these building blocks, plants manufacture complex compounds that are needed for other plant growth and reproductive processes. Think of each individual plant as a large factory that produces one or more primary products but also creates many intermediate products needed for the overall operation. Intermediate plant products are the building blocks: carbohydrates (sugar and starch); lipids (oils and fats); proteins (structural components, hormones, and enzymes that assist in chemical reactions); and nucleic acids (for genetic material such as DNA).

The primary plant products could be the fruit, flowers, foliage, or other agricultural/horticultural products we value. In some cases, the intermediate products are important products in and of themselves (e.g. sugars and oils). Intermediate plant products are also used to produce plant hormones such as auxins, gibberellins, cytokinins, ethylene, and abscisic acid.

Plant hormones are needed in very small quantities but they facilitate essential plant processes such as growth, dormancy, flowering, cell differentiation, and fruit ripening. Environmental factors, such as temperature, light, day length, and gravity interact with plant hormones to cause developmental responses in plants. Several plant hormones are synthesized for use in food and beverage production, herbicides, and plant propagation.

Auxins are produced in buds and seeds and promote cell elongation. Indoleacetic acid (IAA) is a naturally occurring auxin. While it is possible to extract IAA from plant tissue, it is more practical to produce a similar compounds, indolebutyric acid (IBA) and naphthaleneacetic acid (NAA), synthetically. Growers find it more effective and efficient than its natural counterpart because plants cannot break IBA and NAA down as quickly as they can IAA. IBA and NAA are common ingredients in rooting compounds and root stimulators. Another synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D), is a widely used selective herbicide that kills broadleaf plants and has little effect on grasses and other monocots.

Gibberellins are hormones produced in shoot tips, root tips, and plant embryos. In grapes, gibberellins are produced in the developing embryos and cause the berries to enlarge. Since seedless grapes do not have seeds, they cannot produce gibberellins to increase fruit size. Small amounts of gibberellins are applied to the developing berries to produce the large berries we see in the supermarkets. You may have noticed that homegrown seedless grapes always have small fruit unless gibberellins are applied. Ruby seedless grapes are an exception-they actually have embryos that produce gibberellins, but the developing seeds abort before the berries mature.

Cytokinins have been found in many species of plants and play a role in many plant processes including cell division, shoot initiation, and growth. Researchers have found the ratio of cytokinin to auxin can affect how undifferentiated plant cells (callus tissue) develop into specific plant tissues such as shoots or roots. These interactions are complex, but very useful in plant science research and tissue culture applications. Tissue culture is the process of growing and multiplying plant cells in artificial media. In tissue culture, ratios of cytokinin to auxin can be manipulated to alternately initiate root or shoot growth. Plant tissue culture is used commercially to develop new cultivars, ensure disease-free stock, and propagate large numbers of genetically identical plants.

Much is understood about the function and practical uses of plant hormones. However, new discoveries are still occurring. Recent research has just uncovered how individual plant cells detect the presence of auxin through a protein on the cell surface. New discoveries expand our understanding of plant processes ultimately creating new opportunities for agriculture and horticulture. I know this column was a little slanted toward the “techie”, but how else are you going to increase your plant science literacy? If you got this far, you were either waiting for me to make a useful point or actually interested in plant hormones. I hope it was the latter!

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: July 14, 2005
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