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5 Types of Plant Growth Regulators (and When to Use Each)
Because applying plant growth regulators can help diminish reliance on fertilizers and pesticides, they are popular for sustainable agriculture.
In this post, we’ll cover:
What Are Plant Growth Regulators?
Plant growth regulators (PGRs) are chemicals and hormones that impact plant development and growth. From the plant embryo to the yellowing of its leaves, they are involved throughout the life cycle of the plant. These plant hormones, also known as phytohormones, manage:
Seed development
Plant size
Branching of roots and stems
Flowering
Firmness of the fruit skin
Fruit ripening
Fruit quality
In addition to controlling plant growth and maturity, plant growth regulators also help a plant tackle stressors in the environment.
PGRs naturally occur in low concentrations. However, farmers can also apply synthetic plant growth regulators. There are five types of plant growth regulators, discussed below, that serve the plants in different ways.
The PGR market is surging, thanks to its ability to boost crop yield by up to 15 million tons annually. Global PGR sales grew from $1.5 billion in 2015 to more than $2 billion in 2022.
5 Types of Plant Growth Regulators
Plant growth regulators are often broken out by whether they promote or inhibit plant growth and development.
Growth Promoters
Auxin
Cytokinins
Gibberellins
Growth Inhibitor
Abscisic Acid
Growth Promoter and Inhibitor
Ethylene
Let’s explore each of these in more detail.
1. Auxins
A type of growth promoter, auxins are plant hormones that are important for helping plants weather various environmental conditions. They are located initially in the roots and stems before moving to other parts of the plant.
Auxins functions include:
Promoting cell growth elongation
Engendering root growth
Aiding in xylem (transport tissue) differentiation
Creating apical dominance, meaning the central stem is stronger than side stems
Bringing about flowers and fruit
Restraining fruit from dropping too early
Keeping leaves from falling earlier than they should
Synthetic Auxins
Auxins represent the second-largest market share of PGRs, at $318.5 million in 2023. It’s expected that, with the growing demand for medicinal plants as well as the proliferation of the landscaping industry, this sector will increase. Farmers often use other types of synthetic auxins as herbicides.
Auxins example:
2. Cytokinins
The cytokinins (CK) are plant hormones responsible for promoting cell division and cell differentiation. They are synthesized in roots before moving up the xylem and into the leaves and fruit.
Cytokinins functions include:
Regulating the apical dominance brought on by auxins
Engendering axillary or lateral bud growth
Promoting chlorophyll synthesis, which turns plants green and is important for photosynthesis
Boosting the development of chloroplasts, which is where photosynthesis takes place
Fostering nutrient mobilization
Preventing yellowing
Synthetic Cytokinins
Cytokinins are the top-purchased PGRs, comprising 35% of the market share. The market was valued at $1.9 billion in 2023. Farmers use synthetic cytokinins to help their crops stay green longer.
Cytokinins example:
6-furfurylaminopurine (kinetin)
3. Gibberellins
There are more than 100 known gibberellins (GAs). This type of growth promoter is involved in a wide variety of processes pertaining to plant development, with the most well known pertaining to moderating seed germination, plant height, and flowering.
Gibberellins functions include:
Engendering seeds to germinate
Elongating plant stems
Controlling flower organ expression
Bringing about flowering and cone induction
Encouraging bolting, a survival mechanism in which a plant rapidly shoots upward to set seed earlier than harvest, which results in bitter-taste in some vegetables
Developing fruit without fertilization, which makes fruit seedless
Slowing plant death
Synthetic Gibberellins
Gibberellins had a market value of $736.7 million in 2023. Farmers apply them as seed treatments and foliar sprays to boost growth, increase yields, and enhance quality.
Gibberellins examples:
Gibberellin acid (gibberellin A3 or GA3)
4. Abscisic Acid
An antagonist of gibberellins, abscisic acid (ABA) is a growth inhibitor. It is referred to as a “stress hormone,” because this phytohormone manages a plant’s stress response by inhibiting cell division to help it survive fluctuations of water.
Abscisic acid functions include:
Inhibiting germination
Generating storage protein synthesis
Encourages stomata to close under osmotic stress
Bringing about abscission (leaf shedding) and senescence (deterioration)
Synthetic Abscisic Acids
Abscisic acid is projected to reach a global market value of $649 million by 2028. This group of plant growth regulators is useful for farmers because inhibiting germination allows them to store their seeds longer. It can also be used to help ripen fruit. Abscisic acid is also important to the textile industry because of its use in producing cotton.
Abscisic acid examples:
Abscisic acid
ABA-glucose ester
5. Ethylene
Ethylene hormone works as both a growth promoter and growth inhibitor. One of the most widely used plant growth regulators, it occurs in the form of a gas.
Ethylene functions include:
Promoting root and shoot growth
Breaking seed dormancy
Managing epinasty (the downward turning of leaves)
Bringing about abscission and senescence
Developing female expression
Quickening fruit to ripe
Modulating defense response
Ethylene examples:
Ethylene
Ethephon
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