What are Plant Growth Hormones?
Here’s a crash course on the five different classes of plant hormones, and why knowing a bit about them and their uses can improve your yields.
I am sure you’ve heard the old saying about how one bad apple spoils the bunch? Well, it’s true, and the cause is a hormonal imbalance.
In nature, the first ripe apple of the season drops to the ground and begins to decompose. During the decomposition process, the apple releases a gas called ethylene. Ethylene is a plant growth hormone, or phytohormone, that triggers nearby apples to ripen and fall to the ground.
The sweet smell of all those decomposing apples attracts foraging animals, who eat the apples and spread the seeds far and wide. Ethylene and other plant growth hormones are vital to all aspects of plant growth and development, and a little knowledge and understanding about them and their uses can improve your yields.
When a plant sets flowers, the direction it stretches, the size of its fruits, when it drops them to the ground and virtually every other aspect of plant development is controlled by hormones.
Environmental signals trigger production of these powerful compounds. The hormones are produced either in the leaves, flowers, shoots, roots or fruits, depending on which hormone is in production.
They are made on the smooth endoplasmic reticulum within plant cells, then transported through the cell walls and circulated throughout the plant. Whether part of the normal life cycle or in response to stress, the interacting plant hormones are responsible for all growth changes.
Plant hormones are grouped into five classes depending on their chemical makeup and what they cause to occur, or prevent from occurring: abscisic acid, auxins, cytokinins, gibberellins and ethylene.
The plural is used when listing three of these agents of change because there is not one single molecule, but a group of them that have similar functions and molecular compositions. Let’s look closer at each class of hormone.
Abscisic acid (ABA) is Mother Nature’s natural timer. ABA builds up in developing seed coats, so when a seed falls to the ground, the slow dissipation of abscisic acid causes the seed to break dormancy. This is vital in frozen climates because if a seed were to germinate too early, set roots and begin to grow, it would perish.
Certain plants, such as pines, have high amounts of ABA stored in their seed coats and need to be stratified (forced germination by mimicking winter conditions) for several months prior to germination. Some short-lived annuals, on the other hand, have low levels of ABA and can pop overnight.
ABA also helps regulate the respiration process during times of drought. In a back-and-forth communication between roots and leaves, ABA is produced and used to modify potassium and sodium levels in the guard cells, causing the stomata to close and the plant to save water. (See: 10 Facts on Abscisic Acid)
Next on the roster are the powerful root, shoot and fruit regulators known as auxins. High auxin levels result in cell wall plasticity, allowing growing cells to stretch out. Bigger cells mean bigger tissues, which result in bigger organs, and bigger organs result in larger fruits and fantastic flowers.
Auxins are also responsible for phototropism, or the way plants grows towards the light. By regulating which cells elongate and which don’t, the plant is able to grow directionally. (See: 10 Facts on Auxins)
Cytokinins are as important as auxins, especially considering levels of both are kept relatively even. A simplified explanation: if one level is 50%, the other level is 50%. If one level rises to 60%, the other drops to 40%. If cytokinin levels are low, the plant produces vegetative growth.
As cytokinin levels increase—and auxin levels decrease—a plant transitions into the flowering stage. Higher cytokinin levels cause plants to grow bushier, with shorter internodal spacings. There is a good chance the secret ingredient in your favorite fertilizer is a mix of auxins and cytokinins, balanced precisely to induce your garden to grow one way or another. (See: 10 Facts on Cytokinins)
The next plant hormone is ethylene, a gas produced as pectin breaks down in the cell walls of ripening or rotting fruits. The release of ethylene gas by one rotten apple triggers nearby apples to ripen prematurely, spoiling the entire bunch.
Ethylene also plays a role in phototropism, stem growth—lower levels correspond to thicker stem growth—and takes part in the initiation of leaf development. (See: 10 Facts on Ethylene)
Last but not least are gibberellins, or gibberellic acid (GA). This class of hormones has a lot of responsibilities. Gibberellins cause seeds to start growing after germination and help seedlings manage food storage while still developing photosynthetic leaves. During vegetative growth, GA also causes stretching, or large internodal spacing.
Flowering plants affected by the daylight period length are induced to flower by adjusting GA levels. Gibberellins are often used in growth-promoting products. (See: 10 Facts on Gibberellins)
Making Plant Hormones Work for You
These five classes of hormones work synergistically to trigger all the necessary physiological processes plants undergo to complete their life cycles and ensure another generation.
Cytokinins and auxins balance out the switch from vegetative to flowering growth, gibberellins and abscisic acid work together to promote heavier fruiting, and ethylene and auxins coordinate to cause the dropping of leaves.
Low doses of gibberellic acid promotes growth, while high levels inhibit it. A cocktail of chemicals is constantly flowing through a growing plant, the recipe of which is ever-fluctuating.
Speaking of recipes, how about the ones that are in some of your favorite grow products? Often the true active ingredient is not listed on the label, which is why that bottle of potassium sulfate seems to make magic happen.
Plant growth hormones are sometimes listed on the label; other times, they are not. If a hormone is made synthetically, it is called a plant growth regulator, or PGR.
Two common PGRs seen on product labels are the auxins indole-3-butyric acid (IBA) and napthaleneacetic acid (NAA), which you have likely used in rooting hormone products. (Read More: The Science Behind Root Growth Promoters)
These two PGRs mimic the natural hormone IAA (indole-3-acetic acid), and initiate the formation of a callus and then root development. Using a product with IBA or NAA will ensure that time spent taking and rooting cuttings is not wasted.
PGRs can be used like a tools in a tool box, adjusting a garden’s growth however a grower sees fit. A common problem indoor and greenhouse growers face is running out of room, which can be remedied by using PGRs that inhibit stem elongation.
You may have heard of products that use paclobutrazol, flurprimidol or trinexapac-ethyl, which stop stems from stretching by inhibiting gibberellin biosynthesis.
Depending on the timing of their use, you can keep vegetative plants shorter for longer, or make a flowering plant produce short, tight internodes with increased lateral branching. PGRs can be useful tools if used properly, but due to health and safety reasons, some PGRs are meant to be used on ornamental crops only.
If you decide to use them, make sure they are safe to use on your intended crop, use the recommended safety equipment, pay attention to re-entry periods, and do a post-harvest rinse of all produce.
If you want the benefits of a PGR, but don’t want to use synthetic chemicals, Mother Nature has it all figured out. Potent phytohormones are produced in plants, fungi and algae. They can be directly applied to the garden, or extracted and concentrated into easy-to-use liquids and powders. Several are in products you may already be using.
Willow bark powder is a great natural rooting hormone due to the high amounts of salicylic acid present in the bark, which promotes root initiation. This natural miracle worker also plays a part in inducing systematic acquired resistance, causing a plant to bulk up its entire defense system, and reducing chances of future disease or pest problems. A foliar spray of willow bark water will toughen up your plants and keep them stronger for longer.
Another natural plant growth hormone source is yeast, which produces the auxin indole-3-acetic acid. Yeast extracts are probably on the list of unlisted ingredients in products that make your garden blast off. Sprouted seed teas (SSTs) are starting to gain popularity amongst probiotic farmers.
A sprouting seed is packed with abscisic and gibberellic acids, as well as a bunch of other bioactive enzymes and beneficial proteins. These teas are made by soaking seeds, often barley or rye, in water until they sprout their radicle (the first bit of root that emerges from a germinating seed), blending the sprouting seeds into a slurry, or just collecting the water they are soaked in.
This biologically active liquid can be used in a root drench or foliar spray. Plants will respond to the hormones with root and shoot development, cell elongation and heavy flowering.
Another common source of plant hormones is kelp. Kelp products contain auxins, gibberellins and cytokinins, causing plants to grow more leaves, as well as stimulating flowering, increasing lateral branching, developing more roots and dividing more cells.
Different products will have different concentrations and ratios depending on the extraction and concentration process and the type of kelp used. High cytokinin levels cause giant kelp to grow up to 2 feet per day, though that kind of growth might get out of hand even in the largest of warehouse gardens.
Plant growth hormones are like tools in a tool chest. Using the right product at the right time allows growers to tailor their gardens how they see fit.
One can induce vertical growth and leaf development to meet vegetative growth goals, and then stop vertical growth and promote lateral branching and flower initiation to finish big, or hold clones a couple weeks longer by halting growth altogether.
Wise use of phytohormones can bring your garden to the next level—just remember to watch out for the bad apples.
Written by Luke Besmer