Biostimulants: PGRs and PGSs
Plant growth regulators (PGRs) and plant growth stimulators (PGSs) are naturally-occurring or synthetic compounds that, when applied to plants, modify their physiological processes, and growth and development habits. In plant cells, they stimulate specific enzymes or pathways and help regulate metabolism.
The term “PGR” is widely used in Western Canada to refer to products that are sprayed on wheat to shorten the height of the crop, while PGSs are designed to be incorporated into a well-balanced nutrition program to achieve a better uptake of nutrients, enhanced growth and development, better seed, fruit or tuber set, improved crop standability and more. Producers looking to preserve yield and profitability are encouraged to take a closer look at PGSs.
Although PGRs and PGSs are relatively new tools in the Canadian grower’s toolbox, they have been around since the 1930s. Initially used only to change the size and shape of a plant, they have been further developed to meet a range of desired outcomes, including building lodging tolerance, initiating and extending flowering in horticultural crops, increasing branching and enhancing fruit ripening, amongst others.
Most PGSs will fall into one of the following categories:
- Auxins (AUX) – primarily control growth through cell elongation and by stimulating cell differentiation. Auxins move downward only and promote root growth, and may act differently depending on the part of the plant they are applied to.
- Cytokinins (CKs) – enhance the processes of cell division and cell enlargement, and the transport of amino acids and nutrients. They are involved in branching and stimulating bud initiation.
- Gibberellins (GA) – Control cell elongation and division in shoots. They are known to stimulate ribonucleic acid and protein synthesis in plant cell, and are also involved in regulating dormancy.
- Abscisic Acid (ABA) – primarily regulates leaf abscission and seed dormancy. ABA plays an important role in stomata opening/closing and tolerance to a wide variety of biotic and abiotic stresses.
- Ethylene (ET) – promotes senescence and maturity while enhancing respiration rate during fruit ripening. It also increases root growth and root hair formation, thus helping plants to increase their absorption surface area. Ethylene, along with jasmonic acid, plays an important part in signaling and triggering of plant natural defense responses.
- Jasmonic Acid (JA) – is a key signaling molecule that often gets triggered in the plant after infection with certain pathogens or pests. It is the trigger for the shikimate pathway.
- Salicylic acid (SA) – is an important signal molecule for plants infected with pathogens to trigger the so-called Systemic Acquired Resistance (SAR) or plant immunity.
Lodging can be challenging to growers across the prairies. Yield loss severity due to lodging is dependent on increased fertilization, varietal susceptibility to lodging, plant growth stage, wind and rain intensity and seeding rates. Additionally, occurrence of lodging during ripening can downgrade the grain quality and increase grain sprouting.
Only a few PGRs are registered for use on cereal crops to manage lodging risk by reducing plant height through the suppression of gibberellin production or by eliciting production of ethylene. The premise is that reducing plant’s height lowers its susceptibility to lodging, resulting in improved harvest ability and grain quality. These PGRs are registered through the pesticides stream with PMRA and do require MRLs (maximum residue limits); growers are advised to check with their grain buyers before applying PGRs to their crops.
PGSs, when used with key nutrients (i.e. potassium, manganese, copper), can encourage lateral growth and increase stem strength, hence preventing lodging. These PGSs are biostimulants and registered through CFIA under the Fertilizers Act. They do not require MRLs and are authorized for use without restriction by a wide variety of markets.
Stimulating Growth and Promoting Health
The use of PGSs to stimulate or direct growth represents a great opportunity for growers in the Prairies who deal with a wide variety of stresses. For instance, using gibberellins to break dormancy and stimulate germination and emergence can make a difference in terms of crop emergence and establishment early in the season. The use of cytokinins can also keep the crop greener longer, preventing early senescence and loss of yield. Auxins, on the other hand, can be combined with key nutrients to drive roots and stimulate flowering.
Some of the PGSs currently available – jasmonic and salicylic acid, for example – have the ability to trigger crop defense responses to pathogens and pests. Combined with key nutrients and pesticides, these substances can enhance disease resistance and help prevent the build-up on the farm of adaptation to active ingredients.