Plant science and breeding technologies have been at the forefront of the last century’s new yield plateaus, particularly in corn. First was the development of hybrid seed production along with research that led to an understanding of fertilizer and then weed control. Most recently, biotech innovations have provided enhanced pest and weed management opportunities.
Amidst the growing interest in soil health and soil-plant interactions is a fledgling collective of newer concepts and products, from biologicals to nanotechnology to genetic editing, all of which hold the potential for driving yields higher still. Somewhere in that group, plant growth regulators (PGRs) — also known as plant hormones — are beginning to test the market, looking for cost-efficient means of pushing yields to new limits.
Oddly enough, plant growth regulators are not exactly a new development, at least not in the widest sphere of the agri-food industry. Apple growers in North America have been using Promalin on Red Delicious apples for roughly 50 years while grape growers have been using a product called Pro-Gibb for more than 55 years. Compared to that, PGRs in the field cropping sector are absolute newcomers.
But hormones themselves are in our crops’ genes, literally.
Without the primary five hormones present and functioning in plants, there could be no corn, wheat, soybeans or any other crop. Auxins, gibberellins, cytokinins, ethylene and abscisic acid are vital components in plant production, working independently or together.
Auxins were discovered in the late 1920s but it was the discovery of gibberellins in the 1930s which has had the more profound effect on agriculture. Gibberellins are the active ingredient in Promalin and Pro-Gibb.
Two decades after the discovery of gibberellins (isolated and named after the fungus infecting rice in Japan), researchers from the U.S., Britain and Japan determined the gibberellins were GA3, GA4 and GA7. The numbering simply followed the order of their discovery. But it’s GA3 that has had the largest impact on agri-food production. Part of the challenge in moving from horticultural usage to row crops has been waiting for changes in the fermentation process, with newer technologies allowing for reduced costs of the compounds.
“A lot of those early products were advertised as having gibberellins (or cytokinin or abscisic acid) and they did have those ingredients, just not at concentrations that would have any impact on plant growth,” says James Ferrier, technical services manager for Nufarm Agriculture. “For that reason, a lot of growers tried products in the past and had no positive yield response.”
The more costly fermentation processes of the past also put the use of PGRs on a slower trajectory than Bt corn or variable-rate planting. Plus it’s also taken roughly 70 years for growers to adopt the huge array of technologies that they’ve been asked to put to work on their farms, with everything from hybrids and fertilizers to weed control and biotechnology.
In a sense, however, the ground is now ready for the plant growth regulator’s “seed” to be planted.
“Gibberellin plays a key role in seed germination, in flower induction and fruiting development,” says Ferrier, citing its complex interactions, which are still being determined. “Much of the effort has been specialized, to the point where it’s difficult to gauge an overarching perspective on its impacts. Some people are working on the role in seed germination while others might be focused on fruiting development.”
In any plant species, gibberellins can have three primary impacts on plant growth, including cell elongation and division in different parts of the plant, as well as fruit set, and lastly, stimulation of aleurone cells which may influence root development.
Elongation promotion can also affect shoots, stems and leaves, including increasing the length of the stalk between internodes. Long-term research in the U.S. has found a roughly 9 bu./ac. increase in corn and similar levels in cereals.
“One of the more interesting things we’ve seen is that as we move north and get into lower heat-unit hybrids and varieties, we start to see more yield potential and more yield increase,” says Ferrier.
Research into the use of gibberellins is also trying to determine the optimum timing for supplementary and external applications. Existing research suggests early vegetative stages to be the preferred crop stage. Other research is trying to determine its viability as a seed treatment.
For Valent BioSciences scientists Regina Rieckenberg and Peter Petracek, the development of GA3 for field crops, particularly in corn, is something that will happen. It’s no longer just a “might.”
The challenge now is to develop practical, cost-effective products.
“You always have to do local, commercial-scale research to convince farmers, and I think that’s a good thing,” says Rieckenberg, who manages the global PGR business for Valent. “One of the big issues has been separating the ‘silver bullets’ from the real science.”
According to Petracek, the company has conducted a “data dive” pulling together research from work carried out in North America, South America, Europe and South Africa. They’ve tested corn at roughly two to three feet in height with the goal of understanding when gibberellic acid works and when it doesn’t.
“Most of the time, we can see with the application of GA3 on to the corn, we do get an increase in growth and a general improvement in vigour,” says Petracek. “If you put (GA3) on during the vegetative stage, you get bigger plants that intercept more light.”
Larger plants means more biomass, and potentially, faster plant establishment, better root and shoot growth. They’ll be quicker to tassel too, with potentially better drought tolerance as well.
Rieckenberg says that much of Valent BioSciences’ business has come in the horticulture sector where gibberellic acid expands plant cells and stimulates fruit to set. Knowing this, they thought to look at applying it on soybeans when they’re in bloom, leading to higher yields, with research underway in Brazil.
Still, Rieckenberg, Petracek and Ferrier warn that PGRs will likely be of greatest benefit to producers who are already getting the best from their crops.
In other words, PGRs won’t be magic elixirs that can save a crop after it has gone wrong, and Ferrier makes the point that first adopters of products containing GA3 tend to be growers who understand the biology of their crops and don’t shy away from new products and opportunities.
Rieckenberg notes that in the fruit sector, her team would never advise an apple grower to apply products on a poor block of an orchard. The same will apply to row crops: only the best areas of a field should be treated since it’s there to help optimize that genetic potential.
‘The Big Five’ in plant hormones
Plant hormones are chemical messengers in any plant, usually present in relatively small amounts. Although there are many different hormones, five major types — auxin, gibberellin, cytokinin, ethylene and abscisic acid — are the primary sources of interest in plant growth research.
In the case of GA3, it’s one of three gibberellic acids to be isolated and tested for potential farm use.
AUXIN — was discovered in the late 1920s and was found to be a growth stimulant without which a plant cannot survive. It’s involved in cell growth and expansion, and is located in the highest concentrations in the upper tips of a stem. Auxin’s effect is evident as a house plant bends towards the sunlight.
GIBBERELLIN — was discovered in the 1930s and has been a fixture in the horticulture sector since 1962. The hormone promotes step elongation between nodes on the stem, a valuable development in the vegetative stage of any crop.
CYTOKININ — is involved in cell division and along with auxin in the creation of new plant structures such as roots and shoots. If concentrations of the two are equal, cell division will occur: if auxin concentration is higher, roots will form and if cytokinin concentration is higher, shoots will form. Cytokinin delays senescence in a plant and is also involved in cellular repair.
ETHYLENE — is the hormone responsible for maturation and ripening, existing as a gas. It can be produced in virtually any part of a plant and it can pass through plant tissue, and continue outside of the plant potentially affecting other plants nearby.
ABSCISIC ACID — controls the “thirst signal” in a plant. It actually influences the stomata, i.e. the pores on the surface of leaves (with more on the underside of the leaf than its upper surface). It’s through the stomata that gas enters and leaves the leaf as part of photosynthesis. Unfortunately, this is also the point of exit for water. To prevent water loss in times of drought, abscisic acid signals the closure of the guard cells flanking the opening of a single stoma.
Other lesser hormones include brassinosteroids, salicylic acid, jasmonates, plant peptide hormones, polyamines, nitric oxide, strigolactones and karrikins.
This article was originally published in the Sept. 2018 issue of the Corn Guide.