Hormonal Responses—Are They Helping Or Hurting Your Greens?

Turf Stress and Hormones

Hormone production or suppression within the plant can be triggered by numerous environmental stresses such as extreme heat, cold, drought and high salt concentrations. When stress develops, various hormones inside the plant elicit a natural self-defense mechanism to help the turf survive. Biological signals from disease-causing pathogens and insects can also cause hormone responses.

Hormone response is self-regulated by the plant and is usually an afterthought for turf managers since we can’t see inside plants and hormone responses are complex. But, understanding the basic hormonal responses is critical to avoiding unintentional stress.

Five of the major plant hormones critical to turf health and performance include auxin, cytokinin, ethylene, abscisic acid and gibberellic acid. Other hormones have been discovered and researched, but cytokinin, gibberellic acid and abscisic acid arguably have the most significant impact on turf health, so a brief description of their function is included below (Yamaguchi, et. al., 2010):

Cytokinin: Depending on concentrations inside the plant, this hormone regulates the rate of cell division, gas exchange through stomatal openings and root development. It is the primary hormone that governs the rate of growth.  

Gibberellic acid (GA): This is a growth stimulator that is important in cell growth and elongation, and breaking dormancy. High or increased gibberellic acid levels from product applications during periods where freezing temperatures are likely can affect the tolerance of turfgrass to cold and freezing temperatures. Gibberellic acid also promotes seed germination and seedling development. Suppressing gibberellic acid levels can cause the turf to become compact or stunted.  

Abscisic acid (ABA): This hormone is a growth inhibitor, commonly referred to as the stress hormone. It enables the plant physiology for self-defense. It prolongs dormancy, increases water conservation via stomatal closure and inhibits cell division, elongation and growth. 

Plant Protectants

Protecting turf from diseases and insects with fungicides or insecticides is a necessity at most golf courses. These applications are primarily intended for managing pests. However, research indicates that some products can provide additional plant health benefits via hormonal responses. For instance, strobilurin fungicides can stimulate plant hormone responses that result in decreased respiration (Debona et. al., 2016). Decreased respiration will likely result in reduced carbohydrate consumption, which should result in more energy conserved for turf growth.

With phosphonate fungicides and fertilizers, salicylic acid levels increase the host-defense system of the plant, similar to abscisic acid but through a different physiological pathway (Groves, Howard, Hardy and Burgess, 2014). Ironically, though their physiological responses are similar in plants, salicylic acid and absicisic acid can be antagonistic, causing a loss in host defense to pathogens (Jiang, Chang-Jie, et. al., 2010). As mentioned before, hormone responses are complex.

Plant Defense Stimulators

Plant defense stimulators – e.g., acibenzolar – are different than plant growth regulators in that their activity is not constrained to effects on growth. They either contain hormones, directly activate hormones or are hormone mimics that are physiological effectors of turf. These may or may not affect growth or plant function. Examples of these are salicylic acid and acibenzolar. Acibenzolar is marketed as a plant activator, with claims of increasing tolerance to heat and drought (Jesperen, Jingjin, and Bingru 2017). These claims are attributed to acibenzolar sold as a premix in combination with a fungicide, but the activity of acibenzolar alone can be difficult to ascertain. There are also several companies marketing salicylic acid in combination with amino acids for reducing plant stress or assisting in nutrient uptake – e.g., Green-T Impulse^®. Amino acids will be discussed in more detail below.

Biostimulants

Biostimulants encompass a huge array of different products that are becoming increasingly marketed and used in golf turf management. These products are often marketed as soil amendments or added into traditional plant protectants or fertilizer products. By definition, a biostimulant is something that temporarily increases the activity of some physiological plant process. Researchers have found various plant health benefits from biostimulants, although these are sometimes difficult to document in the field. The interactions between biostimulants, plant hormones and turf response is not well researched.

Seaweed extracts and humic acids appear to be the most widely used biostimulants in the golf turf market. Both contain hormones and amino acids that exhibit cytokinin and auxin hormone responses. These products are likely to be most beneficial when applied prior to stresses caused by drought, heat, salinity and potentially other issues that adversely impact turf health.

Molasses extract from sugar is sold in the golf turf market as a carbon source of amino acids. There are a multitude of claimed benefits associated with molasses applications, such as increasing populations of beneficial organisms like bacteria or fungi. In theory, if beneficial nonpathogenic bacteria or fungi populations are increased in the soil, there would be lower populations of pathogenic fungi in comparison. Molasses is also known to contain amino acids that may elicit a hormonal response, but research is inconclusive in this area. As a result, the specific hormone response is unknown.

Lastly, there are many bacteria and fungi that produce compounds that can either produce or elicit a hormonal response in plants. Hormones within plants – such as gibberellic acid, auxin, cytokinin and ethylene – can be directly or indirectly triggered by these microorganisms. In the case of plant-growth-promoting Rhizobacteria, root-inhabiting bacteria can produce hormones, metabolize them or directly affect a plants’ hormone production. Mycorhizal fungi are also known to produce hormones such as gibberellic acid and cytokinins (Barea and Azcòn-Aguilar, 1981).  These are being commercialized and used to reduce reliance on synthetic fertilizers and pesticides in agriculture.