Phytohormones

Phytohormones are chemicals used by plants to regulate growth. Since we all want our plants to grow, phytohormones are pretty important. Let’s learn how we can use phytohormones to our advantage in the garden.

Lack of the plant hormone auxin (right) can cause abnormal growth (William M. Gray) CC BY 2.5

Unlike our glandular bodies, each cell within a plant is able to produce its own hormones. These hormones dictate flower, leaf, and stem formation, the timing of leaf drop, seed growth, flowering, flower gender, and, the really Bog One, fruit development and ripening. These chemicals also tell which tissues to grow down (roots) and which tissues to grow up (stems), and even how long a plant will live.

​Appropriately enough, the word hormone comes from a Greek word that means to set in motion. Phytohormones, more commonly known as plant growth regulators, or PGRs, trigger certain cells to respond only at certain stages of the cell’s life. New, undifferentiated meristem tissue produces a lot of phytohormones. After the cells have used all that they need, any remaining phytohormones are moved to other locations within the plant where they are needed, or they can be inactivated and stored for later use. In some cases, phytohormones are cannibalized for their various parts, or chemically destroyed within the plant.

What do plant hormones have to do with gardening?

For one thing, it means that every time you prune out a branch or stem, you are altering the hormonal activity within your plant. The auxins that prohibit bud development further down a stem are mostly found at a growing tip. Remove that tip and hormone levels change, allowing more buds (read fruit) to develop further down the stem.

Secondly, upright, vertical stems tend to produce leaves over buds, because they contain a phytohormone called auxin. Bending a vertical stem into a horizontal position suppresses auxin development, allowing several flower-bearing buds to develop that otherwise would have remained dormant. Bottom line: more fruit.

Also, some bacteria create the plant hormones auxin and cytokinin, which can result in tumor-like crown galls.

Classes of phytohormones

There five basic classes of PGRs. Please don’t let the chemistry scare you off. After reading the descriptions, you will find a handy poem at the bottom to help you remember:

1. Abscisic acid (ABA) - growth inhibitor; closes the stoma of water-stressed plants; prevents seeds from germinating in unripe fruit, or when weather is too cold; slows growth in mature areas of the plant to promote new growth elsewhere; used by nurseries to keep plants dormant during shipping
2. Auxins (IAA; indole-3-acetic acid) - travel downward from shoot tips to stimulate root formation; encourages plants to grow toward light sources (phototropism); inhibit bud formation lower on stems; toxic to plants in large doses, especially to dicots; synthetic versions are used as herbicides, such as Agent Orange, and as root stimulants on cuttings
3. Cytokinins - move up the plant to promote promote axillary (side or lateral) bud development, cell division and cell formation; delay aging; affect internodal length and leaf development; counteract auxins (when a plant is injured, higher auxin levels increase root development/repair, while higher cytokinin levels increase stem development and repair)
4. Ethylene - a gas that is produced when plants are growing rapidly, especially in the dark (underground); at first, more ethylene gas is produced than can escape the plant, causing  shoot development; as levels decrease, leaf development occurs; this gas helps new shoots push their way through the soil; stimulates fruit ripening and rotting (which is why one ripe banana makes all the unripe ones ripen faster!) and leaf drop; some producers spray unripe fruit with ethylene to cause the appearance of ripening (but the flavor aspect never really catches up)
5. Gibberellins - mobilize nutrients within seeds to stimulate germination; cause stems and leaves to grow long; initiates bolting; stimulates pollen tube growth; counteracts abscisic acid

​See if the poem below can help you sort all this information out:

 
Germinate with gibberellins, then ethylenes break ground
Auxins push them higher, abscisic acid slows things down
Cytokinins make them longer, keep them young and stronger 
Always working in the garden, in flowers, shrubs and trees
I wish there was a cytokinin made especially for me!

Okay, so it doesn’t exactly roll off the tongue. How about trying your hand at creating an easy pneumonic and share it with your friends and fellow gardeners!