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"Hey, what are the five major groups of plant hormones?" How many times have you been at a cocktail party and this comes up? All the time, right? Well, you no longer have to live with the anxiety of friends and loved ones unexpectedly bringing up phytohormonal inquires and not being prepared.

Trees Get Hormonal Too

By Brandon Gallagher Watson

“Hey, what are the five major groups of plant hormones?” How many times have you been at a cocktail party and this comes up? All the time, right? Well, you no longer have to live with the anxiety of friends and loved ones unexpectedly bringing up phytohormonal inquires and not being prepared. A little knowledge of plant hormones and what they do can give you a better understanding how trees grow, and, perhaps even open up your business to new services.

Plant growth is affected by many environmental factors such as light, temperature, gravity, nutrients and, of course, water. Growth is also affected by internal factors such as genetics, vitamins, enzymes and plant hormones. It is generally agreed that there are five major plant hormone groups, all discovered at different times in history so their names and functions are confusingly named, but they all have a few characteristics in common. One is that hormones are effective in small amounts, and are usually produced in one part of the plant then translocated to another part where they do their thing. All plant hormones are responsible for some certain function within plants; in fact, the word “hormone” is derived from the Greek word for “set in motion,” but the function varies widely from hormone to hormone.



Auxins were the first group of compounds found within plants that were considered hormones and were shown to affect cell elongation. Research into the nature of auxins began in the 19th Century with scientists such as Charles Darwin among the first chief investigators. Early experiments with auxin showed they involved phototrophism — the tendency for plants to bend towards the light. It turns out that auxins are light sensitive and work better in the dark. This means the shaded side of a shoot will elongate at a faster rate than the sunny side, causing the plant to appear to be bending toward the light. Auxins directly affect or participate in many different functions in plants, and are also responsible for the regulation of other plant hormones.



Cytokinins are a group of plant hormones so named because of their effect on cytokinesis — a fancy term for cell division. With more than 200 distinct compounds considered cytokinins, they are found in meristems, roots, seeds, cambium and any other location of rapid growth within a plant. Cytokinins and auxins often work in concert with each other in plant development, the former promoting cell division, and the latter helping make those cells larger. These two also play a role in determining apical dominance in plants. Auxin is produced in the apical bud, which helps regulate the cytokinin produced in the lateral buds. As soon as the apical bud is removed, the cytokinins in the lateral buds are free to do their cell dividing, and we get the sucker growth we see after pruning.



Ethylene is a unique plant hormone as it is the only one that is a gas. Ethylene is primarily known for its role in fruit ripening, and, even if you were not aware of its function as a hormone, most people are familiar with ethylene’s effects. Ever hear that one bad apple can spoil the bunch? That’s true, as the one bad will release large amounts of ethylene, causing the other apples to ripen too quickly and rot. If you have ever put avocados in a paper bag to get them to ripen faster you are using ethylene. Commercially, many fruits are picked green, shipped long distances, then gassed with ethylene to ripen them to the correct color before going on the shelves. Ethylene has also been shown to be responsible for leaf and fruit abscission, which also has commercial applications. Certain types of fruits are harvested by gassing the tree with ethylene, causing the leaves and fruit to fall easily. The fruits are then harvested by shaking the tree vigorously.


Abscisic acid

The hormone that used to be thought responsible for abscission was aptly named “abscisic acid.” Although it is now known that ethylene is the main player in that function, abscisic acid plays some other interesting roles in plants. Commonly known as “the stress hormone,” abscisic acid tends to be produced by plants as a response to tough environmental conditions. Drought-stressed plants are stuck in a Catch-22 where they need their stomatas open to get the CO2 for photosynthesis, but if they are open too long the leaves lose too much water and scorch. To combat this, stressed plants produce abscisic acid to open and close their stomata throughout the day, letting enough CO2 in while not losing too much water. Abscisic acid also plays a role in stimulating fibrous root production, which further helps a stressed plant cope with adverse environmental conditions.




Gibberellin inhibitors reduce tree growth.The last major group of plant hormones is the gibberellins. Named for the fungus from which they were first isolated, there are currently about a 130 different gibberellic acid compounds found in plants, fungi and bacteria. In plants, gibberellic acid plays many different roles depending on the specific gibberellic acid in question. It affects seed dormancy and also helps trigger flowering in response to daylight length. One of the major roles it plays is in cell elongation, similar to auxin, but with a different mode of action. Agriculture has found gibberellic acid to be a useful hormone for increasing the length of seed stalks to make them easier to mechanically harvest.


Growth regulators

Plant growth regulators (and tree growth and shrub growth regulators, for that matter) are the group of products available to tree care professional that can either increase or decrease a plant hormone for a desired effect. Growth regulators are an important revenue source for many tree companies and give arborists new options for tree care. Some may be employed just to reduce growth, some to increase growth, and others may be employed for a tree health care purpose. You may already be aware of products that affect plant hormones without realizing it. Sucker Stop is a product sprayed on new cuts to reduce sucker growth that will affect the formation of cytokinins and block cell division. Garden centers commonly sell Root Stimulator products for new transplants. These are small amounts of auxin applied to make root cells elongate.

A product widely used in the arborist market is palcobutrazol (Cambistat, Profile). This product is applied to base of the tree, and when it translocates to the canopy it will block the formation of gibberellic acid. This results in a reduction of vegetative growth, which has many uses in tree care — from keeping trees out of power lines longer to keeping a suburban lawn tree at a reasonable size. It turns out that gibberellic acid is made from some of the same base compounds as other plant hormones such as abscisic acid. So by reducing the amount of gibberellic acid we are increasing the amount of abscisic acid. This slower-growing tree will be putting more energy into root production than shoot production, and with the increased abscisic acid, be more responsive to urban tree stress conditions. Treated trees show less scorch, and there are some dramatic examples of stressed trees brought back from the brink.

As with all tree health care treatments, plant growth regulators are just another tool in the toolbox. However, if employed for the right reason can be just the tool you need. Knowing about plant hormones can give you a whole new way to affect the health of trees, can give you new business opportunities, and, of course, can make you incredibly interesting at your next dinner party.


Brandon Gallagher Watson is director of communications at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).


Watson recently discussed insects and diseases that are impacting trees as part of the latest Green Media Internet Broadcast. To listen to the interview, visit www.arborage.wpengine.com and click on the “Internet Broadcast” link in the left-hand navigation bar.



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