Plant Growth Regulators (PGRs) (aka Plant Hormones, or Phytohormones)- What They Are, How They Work, and How To Use Them

They’re natural… sort of.

It is a true miracle of nature that the individual cells of any multicellular organism each have identical DNA to each other, but find a way to create a variety of very different cells in just the right places.

Of course, this doesn’t happen by sheer luck, or magic.

In plants this is regulated by a system of different chemical signals, which enable the plant to sense “up” and “down”, to start and stop growing at particular times of the year, trigger flowering, and regulate efficient and balanced growth throughout.

These chemicals are called Plant Growth Regulators (PGRs), and are also known as Plant Hormones, or Phytohormones. Since their discovery, biologists have been able to harness the power of this class of diverse chemicals to improve plant health, increase yields and decrease cropping times, in addition to various other effects.

Extracts of natural hormones or synthetic chemicals which mimic or inhibit the activity of natural hormones have become a valuable part of the toolkit used by horticulturists around the world to achieve specific effects on growing plants.

Horticulturists (or ‘Growers’) use PGRs for a variety of reasons but usually to either as a growth retardant, or as a stimulant of a particular kind of plant growth (such as bigger flowers or fruits).

Natural PGR’s

The natural phytohormones fall under a few major categories, with their own specific function:

Abscisic Acid (ABA) – This is an inhibitory chemical which prevents growth in dormant buds and seeds until the right growing conditions. ABA also functions to slow growth and transpiration when the plant doesn’t receive enough water.

Gibberellins – These act in opposition to ABA (above)  and promote growth and cell elongation. Gibberellins have a role in breaking seed dormancy, and can be used in various ways to influence the flowering and fruiting behaviours of many plants. Excessive gibberellin will cause a plant to become elongated and stretched out as if striving to reach the light. An absence of gibberellin in natural mutants causes dwarf growth habit such as in the Minitom tomato variety, and dwarf sunflower varieties such as ‘Sunny Smile’ and ‘Choco Sun’. Some PGRs which block the activity of gibberellins are used to produce dwarf plants artificially or used as a herbicide.

Auxins– These promote ‘apical dominance’ by preventing lateral buds from growing while the main top shoot is intact. Auxins additionally promote adventitious root growth and the growth of existing roots, as well as helping to organize cell differentiation and bud formation through a complex interaction with other phytohormones – especially the cytokinins. Auxins will encourage cells to increase in size, whereas cytokinins encourages them to divide more frequently.  Auxins govern the gravitropic and phototropic sense of plants and help them find the way to the best levels of light. Auxins have a role in fruiting and seed production, as well as numerous other plant functions – often as a part of an equilibrium shift in regards to the auxin-cytokinin balance. Synthetic auxins are used to produce rooting hormone powders, in tissue culture, and some are used as herbicides. Application of auxin at flowering can encourage fruit set in some plants.

Cytokinins – This is a class of compounds which cause cells to divide more rapidly, and stimulates the growth of lateral buds. Used in tissue culture to produce cell cultures for micropropagation. These work in balance with Auxins to define growth types and are vital for the production of fruit.  Two types of Cytokinins are generally recognized, the adenine-type and the phenylurea-type. Phenylurea types are synthetic and are not generally found in plants naturally – they work in a different way to the adenine-types and typically stimulate flowering and maturation. Adenine-types are more widely used, and have the function of promoting cell division and growth, enhancing yields and shelf-life of harvested crops, and promoting resistance to some pathogens.

Ethylene– This is a maturation hormone, and is a way for plants to signal to each other as well as between the cells within a single plant. As it is a gas it can travel beyond the source organism and effect the plants around it. It is used to stimulate flowering in Bromeliads, but it can delay the flowering of other crops. Ethylene is used to signal a state of maturation and to promote the maturing process of fruit. The effect of ethylene is easily observed when bananas are stored in a paper bag with unripe fruits. The bananas, which emit high amounts of ethylene as they ripen will cause the ripening of the other fruits – useful if you have hard avocados. Ethylene build-up can be problematic, causing premature leaf drop, short flower life, and early senescence.

Plus a few extras which also modulate growth and health:

Brassinosteroids– An interesting group of chemicals first isolated from the pollen of Oilseed Rape (Brassica napus). These chemicals are used by plants to stimulate pollen tube growth and have a modulatory role in cell growth and differentiation. Brassinosteroids like Brassinolide and Epibrassinolide work as a plant strengthening agent, enhancing resistance to stress (including drought and high salt levels) and against diseases. These essentially natural chemicals have an effect at quite low levels of application, and are seen to be relatively safe.

Jasmonates – Jasmonic acid and methyl jamonate are two examples of this interesting and relatively recently discovered group of phytohormones. The Jasmonates have a variety of different roles in signalling within plants, differing from species to species, however in most cases jasmonates function as a collective signalling mechanism between plants in the environment to communicate stress and attack from herbivore pests. Jasmonates can trigger defensive responses.

Salicylic Acid – Structurally related to asprin (acetylsalicylic acid) and wintergreen (methyl salicylate), this chemical is of primary interest as a stress-signalling chemical which encourages systemic   acquired resistance to pest or pathogen attack.

Triacontanol (Melissyl Alcohol) – A potent growth-booster which is active at relatively low quantities. This chemical is found in relatively high levels in Alfalfa and Beeswax. Used to encourage basal shoots in roses, this powerful growth stimulant is used to safely speed up harvest maturation, increase the number of flowering shoots and promotes healthy growth.

Commonly Used Commercial PGR’s (and What  They Do)

Most Commercial PGR’s are synthetic and closely mimic the natural chemicals used by plants – in some cases they are identical.

Paclobutrazol (Bonzi and similar brands) – A widely used growth retardant which is also antifungal, being a triazole fungicide. Paclobutrazol acts by inhibiting the function of gibberellins in the plant, restricting the upwards growth and encouraging growth of roots. It is a potentially dangerous and toxic chemical that must be handled carefully to minimise exposure. If used correctly it can be totally safe for those handling the chemical, and for the end user, however it must not be consumed so it is used generally on ornamental (non-edible) crops only.

Daminozide – (B-Nine and similar) Works to inhibit gibberellin and reduce the height or ‘stretch’ of plants. Withdrawn from use on edibles due to cancer risk concerns, still used on ormamentals, used only as foliar application.

Ancymidol– (Abide and similar brands) Growth retardant. Handle with extreme caution and do not use on crops to be consumed.

Chlormequat– (Citadel, Cycocel and similar) Growth retardant. Handle with extreme caution and do not use on crops to be consumed.

Ethephon – (Florel, Etherel, and similar) Used to promote ripening, maturation and lateral growth as well as flowering in Bromeliads and fruit crops including Pineapple. This is a form of ethylene, a natural phytohormone which is relatively safe when handled correctly.

Thidiazuron– (Dropp, Revent) A synthetic cytokinin which has the unusual trait of speeding up the maturing process and can speed up harvests. The safety of this chemical is dubious and it is banned in the EU.

Forchlorfenuron-(Prestige) A synthetic phenylurea-type cytokinin which is used to speed up harvests and increase fruit size. Blamed for the famous exploding watermelon disaster of 2011 in China. Approved for us in EU (as of time of writing), this chemical is widely used for promoting the timely harvest of Kiwi Fruit, producing large and firm table grapes, and the production of speedy (and hefty) Apples, Pears, Pumpkins and Melons.

Benzyladenine – (BA, Benzylaminopurine, 6-BAP) This is a well-known cytokinin which is used to keep plants greener for longer and promotes a bushy habit. Used to increase flower size and to stimulate growth, especially when used in conjunction with gibberellins.

Gibberellic Acid – (Progibb, GA3) Used to promote height or ‘stretch’ in ornamentals, and used to modify growth on seedless grapes, in addition to many other niche uses. Widely used in the ornamental tree industry, and by horticulturists germinating dormant seeds. Relatively safe when handled correctly.

Safety note: As with all Horticultural Chemicals, these are generally deemed to be safe for use on ornamental crops – only when used correctly and according to the safety instructions from the manufacturers. Different formulations may include other chemicals intended to stabilize or improve efficiency, but these chemicals may be much more dangerous than the PGR’s themselves. No PGR should be used on a crop intended for consumption, unless it is specifically formulated and marketed for this purpose. Always read the label!

Useful Links:

Information on how to use garden chemicals safely, from the RHS (UK)

A list of Withdrawn Chemicals which are no longer allowed in garden use, also from the RHS.

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