Biostimulants and Plant Hormones
Growers are no stranger to crop inputs, such as fertilizers and crop protection products. However, there is another, less widely known category of inputs that is gaining popularity on performance-driven farming operations.
Agricultural biostimulants are organically or synthetically derived inputs that stimulate natural processes within a plant. They aid in plant nutrient uptake, nutrient efficiency and crop quality – while increasing tolerance to abiotic stress.
These biostimulants are composed of various compounds, substances and/or micro-organisms. Often, they are packaged with complementary nutrients. Biostimulants benefit the plant at all growth stages – from seed germination to maturity.
Long used by fruit growers, biostimulants are now being incorporated into agricultural management systems to improve production in mainstream crops such as cereals, oilseeds, corn, soybeans and more.
How hormones work.
To get a better handle on how biostimulants work, it is helpful to understand the role and function of various plant hormones.
Both plants and animals produce hormones through their glands. These hormones are responsible for sending chemical messages between organs and tissues that initiate key activities associated with growth and development. This includes digestion, metabolism, respiration, tissue function and reproduction.
The “Big Five” plant hormones.
There are five key hormones in plants that are of particular interest from an agronomic perspective. Three promote/stimulate plant growth, while two inhibit growth.
Today, we will focus on three of these hormones – auxin, cytokinin and ethylene – and their relationship to biostimulants
Auxin is a class of hormone responsible for cell growth, cell division and cell expansion in the actively growing parts of the plant. Higher auxin concentrations will cause roots to form. These hormones ensure the primary stem is dominant, which facilitates upward growth (known as apical dominance).
While auxin is active, the plant will limit lateral growth in order to expend resources on vertical growth. Another hormone, cytokinin, is responsible for lateral growth (see below). Pruning a tree at the top of its stem illustrates how you can cut off auxin at its primary source, which in turn causes the plant to branch out (disrupting the apical dominance).
Auxin plays an important role in helping plants maximize exposure to the sun. How? Auxin accumulates on the shady side, where it signals cells to elongate. The cells on the sunny side remain the same size, forcing the plant to bend toward the sun – a phenomena called phototropism.
Cytokinin is the Ying to auxin’s Yang. The two hormones work in tandem to orchestrate growth. Where auxin signals the plant to grow upward, cytokinin sends the message directing the plant to start growing laterally. This family of hormones actively encourages cell division, which allows the plant to create new organs such as roots and shoots.
This fascinating hormone also delays ageing and processes that inhibit growth in the plant. It does this by increasing the production of new proteins while limiting the destruction of older proteins.
In this way, cytokinin helps fight abiotic stress that can be caused by drought, excessive moisture and extreme temperature. It does this by delaying the natural ageing process (senescence) that is triggered by the release of yet another hormone: ethylene (see below).
Ethylene is a gaseous hormone – responsible for stimulating or regulating functions such as the ripening of fruit, opening of flowers and abscission (shedding) of leaves.
While it plays an important role in maturation, ethylene is also released when the plant is under abiotic stress – stimulating senescence and causing the plant to prematurely ripen or rot. It is often described as the plant’s self-destruct mechanism.
You can see the effect of ethylene when you place a ripe banana next to a bunch of green bananas. The green bananas will ripen and turn yellow faster than they would have normally. This happens because the brown banana is releasing ethylene, which signals the green bananas to accelerate the ripening process.
The relationship between biostimulants and plant hormones
Biostimulants have been created to initiate or suppress natural growth processes that are known to improve production.
Many are made from organic products or derived from the symbiotic relationship between a plant and soil microbes, such as kelp, amino acids or organic acids. Others are meticulously formulated in laboratory conditions.
Intrinsic™ is an interesting example of a synthetic molecule that has been developed in Canada by researchers at Active AgriScience. This biostimulant works by upregulating (inducing) and downregulating (inhibiting) hormonal activity. It can be used to promote plant development at different crop stages in wheat, canola, soybeans, peas, corn and a variety of other agricultural crops.
Intrinsic™ helps to regulate the activity of the hormones we discussed above. It increases auxin levels in tissue to promote early season root growth. It also facilitates the production of zeatin, a cytokinin that promotes lateral shoot growth and prevents senescence.
One of the real advantages that a biostimulant such as Intrinsic™ brings is the ability to help the plant withstand abiotic stress. It does this by inhibiting production of ethylene, which, as we learned, can speed up senescence and cause the plant to prematurely wilt and die.
A biostimulant’s ability to combat the effects of stress can be demonstrated through a simple test. When a plant is pulled from a treated field and placed side-by-side with a plant pulled from the untreated check, the plant with the biostimulant will take noticeably longer to wilt. This is due to its ability to inhibit ethylene production.
Stress test: Two plants a day after being pulled from a field to simulate stress. The untreated plant (l) from the check strip wilts faster than the plant treated with Active Build™ (r). Active Build contains Intrinsic™, a synthetic biostimulant.
Choosing a biostimulant.
As with anything, it is important to choose a biostimulant that has credible research behind it.
Secondly, pick a product that offers consistent results. While there are biostimulants that are made of kelp and other natural ingredients, synthetic formulas that are manufactured according to a set specification can be produced exactly the same each and every time – ensuring greater consistency.
By adding biostimulants to the agronomic toolbox, agricultural growers are making new strides in closing the gap between actual yield and the crop’s true yield potential.