You may have seen a lot of graphics on the internet regarding cannabis and the use of PGRs, or plant growth regulators that look like this:
These graphics are extremely misleading. Genetics and environment often play a large role in what finished flower looks like. Genetically some cultivars are simply made to look either way, and neither is an indicator of use of PGRs.
So, what are plant growth regulators?
Plant growth regulators are chemicals produced naturally by plants that regulate its growth and development. Natural forms of PGR’s are also called phytohormones and are active at very low concentrations. They can be gases, terpenes, or carotenoid derivates), and be classified in five major groups: auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
Auxins are present at the tips of roots and shoots and promote growth and elongation. Indole-3-acetic acid (IAA) promotes cell division, elongation, and differentiation. At high concentrations auxins can also inhibit lateral shoot growth and can be used as an herbicide.
Gibberellin plays a role in lateral shoot growth and formation of floral organs. It also contributes to the maintaining the metabolism of plants and efficient nutrient uptake. It can also increase leaf area and fruit yield. Gibberellins can also help protect the plant against stress and increase in osmotic components in plants exposed to salt stress.
Cytokinins (CK) are hormones involved in the maintaining of cellular proliferation, differentiation, and the prevention of senescence. Senescence is an aging mechanism that contributes to the loss of leaves in deciduous plants among other things. They also promote cell division, nutrient uptake, phyllotaxis (the arrangement of leaves on a stem), and embryonic development.
Abscisic acid (ABA) regulates growth and can improve stress responses and adaptation. It can help resist stressors such as cold, high salinity, cold tolerance, heat stress, and drought. It also aids in developmental processes such as seed and bud dormancy. It is also responsible abscission in some plants.
Ethylene is a gas molecule that is one of the most important hormones in plants. It is involved in seed germination, shoot and root growth, formation of adventitious roots, abscission of leaves and fruits, sex determination, flowering, and senescence of flowers and leaves. Like ABA, it also helps to protect a plant against stress. It is most known for its involvement in the ripening of fruits such as apples, tomatoes, pears, and bananas.
How are PGR’s Used?
There are commercially used PGR’s in agriculture and horticulture. Some of these can help control shoot growth, which can keep fruit tree canopies open, regulate the shape of fruits, promote or inhibit ripening, and control fruit development times.
The use of PGR’s in agriculture dates back thousands of years where it was a common practice in the Middle East to use a drop of olive oil on figs to promote their development. The oil would break down and release ethylene, which helped to ripen the fruits. Changes in modern climate have contributed to the increase in the use of synthetic PGR’s such as Apogee and Kudos to help protect plants against increasingly extreme weather conditions such as drought and flood.
There are also some synthetic PGR’s that have been shown to be carcinogenic when used in food production. In recent years there’s been a lot of talk around the use of PGR’s in medicinal and recreational cannabis. As of now, PGRs may be used in some medical marijuana grow operations, but is not standard practice in Ohio.