Plant Growth Regulators (PGRs)
Plant growth regulators, known colloquially as PGRs, are substances that may be applied to crops to aid their growth and enhance production. Some PGRs have been shown to improve growth during bouts of environmental stress. There are many substances that can be classified as plant growth regulators, including nutrients and biostimulants. One of the most common divisions of PGRs classifies them as either traditional or new PGRs.
Traditional Classes of PGRs
Auxins are a group of naturally occurring plant growth promoting hormones containing the compounds IBA (indole-3-butyric acid) and NAA (napthaleneacetic acid). Together these compounds stimulate root formation, and in particular, promote the development of adventitious roots. This can be used to enhance the roots of crops or even reduce transplant shock.
Kinetin is a common cytokinin that is native to many plants and stimulates growth rate. It works by inhibiting root growth and using all the plants energy resources for the rapid growth of shoots and leaves. This is a helpful property to reduce transplant shock as well as increase the number and size of fruits and flowers.
Gibberellic Acid (GA3) is a very potent plant hormone that functions to control plant development. GA promotes plant and flower growth as well as increases fruit set. It aids in frost protection, overcoming dormancy, and inhibits root formation in cuttings.
New Classes of PGRs
Newer plant growth regulators offer unique plant growth regulating properties. Some of the most common of these compounds are as follows:
Brassinolide (BR) is a brassinosteroid that, when released, encourages plants to grow taller, while also offering some protection against heat, cold, water, drought and salt stress. Additionally, brassinolide optimizes the efficiency of other fertilizers and pesticides, allowing the plant to use them more efficiently.
Salicylic acid (SA) is a naturally occurring plant hormone that can improve a plant’s resistance to drought and other environmental stresses like salinity, heavy metal toxicity and extreme heat. SA also enhances hardiness, flowering and fruit yield, and aids in systemic acquired resistance (SAR).
Triacontanol (TR) is naturally found in the leaves of most plants. It acts on many growth pathways to boost yield, uptake of water and nutrients, fruit and flower number, size and quality, and essential oil production.
Jasmonic Acid (JA) and its derivatives are naturally occurring plant hormones that promote plant growth as well as protection against environmental and biological stress. Airborne jasmonates are also used as communication signals between neighbouring plants – so the surrounding crops can preemptively protect themselves