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: 

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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