Materials that, in minute quantities, promote plant growth are biostimulants. Many biostimulants are on the market. With proper use, some may enhance turf stress tolerance and quality. Others do not. The question that arises is, which ones do and which ones don't?
Many suppliers formulate biostimulants with little or no scientific input. Evidently, the thinking is that if enough materials are present in the mix, something's got to work. Surprisingly, some of these complex formulations, as well as some relatively simple materials, do stimulate plant growth.
Anecdotes about the benefits of biostimulants abound. A woman from the south island of New Zealand says her houseplants grow and perform better because she applies spent tea leaves to the plants' soil. Another product that smells like wine supposedly imparts biostimulant benefits to plants.
Do such claims have merit? It's hard to know. Personal experiences should not be entirely discounted, but scientific research is more comprehensive and reliable. Thus, the best way to evaluate biostimulants is with data and research from an independent source, such as a university. Anecdotes often are unverifiable and not good predictors of product performance in other situations.
A primer in plant physiology Before you can understand the benefits of biostimulants, you must understand the role of plant hormones. Many important benefits of biostimulants are rooted in their ability to influence hormonal activity.
Hormones (in plants, we often refer to them as phytohormones) are chemical messengers regulating normal plant development as well as responses to the environment. They regulate tissue growth and differentiation, dictating how plants grow and mature. In addition, plants can sense unfavorable environments through various hormones. The following are some of the major categories of plant hormones.
* Cytokinins. These hormones are responsible for the formation of roots and buds, and promote cell division. Cytokinins tend to counteract the effects of aging and stress in plants.
* Auxins. Hormones in this group produce several growth effects in plants, especially cell enlargement (contrasted with cytokinin-induced growth, which results from cell division). Cell enlargement causes root and shoot elongation in plants and allows them to produce tropic responses, such as shoots bending toward a light source (phototropism) and roots growing downward (geotropism). Indoleacetic acid (IAA) is perhaps the best-known auxin.
* Gibberellic acid (GA). GA is another growth promoter known for producing elongation in plants. Conversely, suppressing GA levels can cause plants to become compact or stunted. GA also plays a critical role in promoting seed germination.
* Abscisic acid (ABA). ABA induces or prolongs dormancy in plants, and also accelerates abscission (the process that results in fall leaf drop). Thus, ABA is considered generally as a growth inhibitor, rather than a promoter. ABA also is involved in water regulation within plants. ABA levels rise in plants under drought stress, particularly in leaves, where ABA prompts stomates to close.
The balance of the various hormones within a plant is a complex interaction that controls its overall growth and development. Thus, materials that alter the hormonal status of a plant can exert large influences over its growth and health.
Free radicals and antioxidants Antioxidants are another group of plant chemicals important to understanding biostimulants. Apparently, some biostimulants promote antioxidant production. Research over the past decade has shown that various environmental stresses, such as drought, heat, ultraviolet light and the use of herbicides, damage plants by causing production of free radicals or reactive oxygen molecules (such as O2-, hydrogen peroxide [H2O2] and hydroxyl radical [OH]). These molecules are strong oxidizing agents and damage lipids, proteins and DNA inside cells.
Antioxidants are metabolites and enzymes that scavenge free radicals and thereby protect plant cells from damage. Antioxidants include lipid-soluble substances such as vitamin E and beta-carotene, water-soluble materials such as vitamin C and glutathione, and varius enzymes. Frequently, several antioxidants work together to suppress free-radical toxicity.
Our research at Virginia Tech shows that antioxidant activity (such as vitamin E, vitamin C, beta-carotene and superoxide dismutase) increases rapidly when turf is under drou ControlHumic acid Seaweed extract Humic acid + Seaweed extractght stress, suggesting that antioxidants are an important defense against stress. Application of biostimulants further enhances antioxidant activity of the grasses (see Graph 1, at right), magnifying the plants' own defensive system.
Additionally, grasses with high levels of antioxidants produce better root and shoot growth, and maintain higher leaf-moisture content and lower disease incidence in both normal and stressful environments.
What are biostimulants? Biostimulants are defined by what they do more than by what they are, since the category includes a diversity of substances. As the name suggests, they stimulate growth, but they do much more. Stress tolerance is perhaps the most important benefit of biostimulants-tolerance of drought, heat, UV light and even diseases. Biostimulants impart stress tolerance partly by stimulating root growth and partly by promoting antioxidant activity. However, we still have much to learn about how biostimulants work, so these and other functions will become better understood with additional research.
During the past 10 years at Virginia Tech, we have evaluated many kinds of biostimulants for use in the turfgrass industry. Of the various materials we've examined, the most promising are seaweed extract, humic acid, amino acids, benzyladenine, trinexapac-ethy, propiconazole, salicylic acid and silicates. All of these products exert beneficial influences either through hormonal effects or by raising antioxidant levels.
Although biostimulants can be synthetic chemicals, naturally occurring organic materials are excellent sources of biostimulants. For example, humic acid and seaweed extract are two commonly used turf biostimulants.
* Seaweed contains various hormones, vitamins, amino acids, mineral nutrients and other components. Thus, it may affect plants in several ways. However, its stimulating influence-particularly for turfgrasses growing under environmental stresses-has been attributed to its hormonal activity, especially that of cytokinins and auxins.
* Humic acid has received increasing attention in recent years. Humic substances are naturally occurring organic materials derived from biological sources (i.e. decomposed organic matter). They typically are mixtures of several types of chemical compounds, including humic acids, fulvic acids and humins. Leonardite and peat are good sources of humic acids.
Scientists were exploring the benefits of naturally occurring soil humic acid on plants as far back as the 1940s and '50s. In the '70s, researchers found that humic substances exhibited auxin-like activity and chelation properties (chelation of micronutrients, such as iron, aids plant uptake and utilization).
In our more recent research at Virginia Tech, we've found that-in addition to increasing root initiation, nutrient uptake, chlorophyll content and photosynthesis-humic acid inhibits indoleacetic acid (IAA) oxidase (which destroys IAA). The net result is higher growth-hormone levels, which promote more growth.
* Amino acids are building blocks for proteins and enzymes. Some free amino acids, such as proline, improve osmotic adjustment and water-stress tolerance of plants.
* Benzyladenine is a highly active synthetic cytokinin which regulates plant metabolism, increases or maintains photosynthetic rates and delays leaf senescence.
* Trinexapac-ethyl is a plant growth regulator (available commercially as Novartis' Primo) that interferes with gibberellin biosynthesis and suppresses plant shoot growth.
* Propiconazole is a triazole fungicide (Novartis' Banner Maxx) with growth regulatory properties.
* Salicylic acid, the active ingredient in aspirin, seems to improve plant resistance to disease and environmental stresses.
Silicate is not usually listed among the generally essential elements or nutrients for turfgrass, but it has been shown to enhance plant growth and metabolism, regulate nutrient balance, enhance antioxidant activity and improve plant resistance to various stresses.
All the biostimulants mentioned above, including amino acid products and silicates, promote turfgrass antioxidant activity and enhance stress tolerance. Products we've used in some of our studies, aside from those already mentioned, include Macro-sorb amino acid (Nutramax, Baltimore, Md.) and potassium silicate Kasil #1 (PQ Corp., Chester, Pa.). Many other products are available that incorporate these or other ingredients, seaweed extract and humic acids being among the most frequently used.
Most biostimulants are formulated as liquid products (except salicylic acid, which is a powder). Treatments, therefore, consist of conventional spray applications-dilution in water and then application to the turf.
Biostimulants as preventive measures Turfgrass typically grow well without biostimulants when the environment is favorable. In these situations, the beneficial effects of biostimulants may not be easy to identify solely based on leaf color or other visual indices. When the plants become stressed, however, biostimulant-treated turfgrasses perform better because they have developed a better defense system, apparently due to higher levels of antioxidants.
As with mineral fertilizers, biostimulants take time to exhibit their influence on turfgrass physiology. Under the conditions of some of our studies, for example, growth stimulation was not large enough to measure until 4 weeks after treatment. After 6 weeks, by contrast, we found significant differences in leaf and shoot number, as well as shoot and root weight of treated vs. non-treated Kentucky bluegrass and creeping bentgrass. These findings suggest that biostimulants yield the best results when you apply them several weeks before a stress occurs-that is, as pre-stress conditioners.
In another case that supports this concept, our research showed that application of biostimulants in the spring significantly improved photosynthetic efficiency and reduced the incidence of some diseases in summer, such as dollar spot in creeping bentgrass (see Graph 2, page 20).
Biostimulants perform multiple functions Because biostimulants contain a diversity of chemicals, it's not surprising that the benefits associated with their use vary as well. However, in light of the preceding discussion of hormones and antioxidants, the results listed below make sense and apparently stem largely from effects on these two systems.
To some extent, it's useful to evaluate biostimulants visually (using such factors as leaf color, density and disease incidence) and with growth parameters (such as root mass, clipping weight and root strength). However, physiological parameters (photosynthetic effects, antioxidant analysis and hormone activity, for example) provide a more in-depth understanding of the influence of biostimulants.
For greatest effectiveness, you should apply biostimulants prior to the onset of stress. Usually, you can see a physiological response 7 to 10 days after application. Frequent, low-dose applications produce better effects than less-frequent, heavy-dose applications. For example, we applied propiconazole at 0.5 ounce per 1,000 square feet at 5 to 6 week intervals beginning in May. This significantly enhanced antioxidant activity and resistance to summer stress in creeping bentgrass.
Below are some results of studies we conducted at Virginia Tech.
* Enhanced root and shoot development. Application of seaweed extract increased root-mass weight of creeping bentgrass by 66 percent. Biostimulant-induced turfgrass growth such as this may be due to the hormonal activity of these materials.
* Drought tolerance. Application of biostimulants-seaweed extract + humic acid, silicate or salicylic acid-increased the photosynthetic capacity of Kentucky bluegrass by 72, 84 and 63 percent, respectively, when the grass was under severe drought stress.
* Salt tolerance. Salinity stress reduces turfgrass growth and quality. However, our research showed that seaweed extract or propiconazole treatments enhanced turfgrass root and shoot growth of Kentucky bluegrass under salinity stress. Other recent studies have shown that the negative effects of salinity on bermuda-grass root growth were less when biostimulants such as seaweed extract and humic acid were applied (see photo, page 18).
* Disease resistance. Seaweed extract + humic acid reduced dollar-spot incidence in creeping bentgrass under both low- and high-fertilization regimes.
* UV light tolerance. Ultraviolet light damages many important cellular components and raises levels of free radicals. To see if biostimulants provide some protection against the effects of UV light, we subjected plugs of Kentucky bluegrass treated with various biostimulants to UV light for 4 weeks. Application of salicylic acid, as well as seaweed extract + humic acid treatments, significantly enhanced photosynthetic activity of Kentucky bluegrass grown under UV-light stress by limiting the influence of free radicals caused by exposure to UV light.
* Heat tolerance. One of the major problems sod producers face is the reduced shelf life of sod harvested during stressful periods of high temperature and light intensity. However, applications of propiconazole, alone or with seaweed extract or humic acid, can lessen the injury associated with sod in pallets or rolls for extended periods of high temperature (100 degrees F).
In a different study, we found that photosynthetic activity of creeping bentgrass growing under summer stress did not improve with a high rate of nitrogen fertilizer alone. However, when we added a biostimulant, it rose significantly. We associated this with high antioxidant activity that resulted from the biostimulant application.
Biostimulants help plants help themselves One of the roles of plant hormones is that of chemical messengers that tell plants when stressful environmental conditions exists. In response, plants may initiate or increase physiological processes that increase their tolerance to stress. For example, small, rapid changes in abscisic acid levels (which occur when plants are under moisture stress) cause stomates to close. Also, water stress reduces cytokinin activity. This change signals the plant to initiate a defense system to drought. Thus, another function biostimulants may perform is that they act as "switches" that turn on plants' preparations for adverse conditions by altering hormonal balances.
Because humic acid and seaweed extract exhibit cytokinin-like auxin-like activity, these and other biostimulants that contain hormonal materials may signal plants to switch on their defense system to harsh environments.
In a similar manner, salicylic acid is a factor in disease resistance. An application of salicylic acid induces a typical disease-resistance response called systemic acquired resistance, or SAR. Other materials, such as jasmonic acid, peptides, amino-acid analogs, fatty-acid derivatives, phenolic compounds and silicates also induce systemic resistance to pathogen attack.
Applications Turf's response to a biostimulant treatment may depend on several factors:
* Composition. Not all biostimulants are equal. A hormone-balanced product produces the best response.
* Stress. Responses from biostimulant treatments generally increase with the level of stress. To put it another way, plants growing under minimal stress may perform similarly regardless of biostimulants.
* Timing. Applications that begin prior to the onset of stress will better condition the grass to withstand an adverse environment. To obtain the best results, turf managers must be proactive and apply biostimulants about 4 weeks prior to the onset of stress, and then repeat applications every 5 to 6 weeks.
Our research clearly shows that biostimulants can improve turf's health and stress tolerance. Many biostimulants besides those we've tested are available to turf managers. Although our discussion here has covered substances that demonstrably benefit turf, products containing other ingredients may or may not be as useful. Turf managers should be wary of products that make boastful claims without backing from scientific studies performed by university or other independent researchers.
Drs. Xunzhong Zhang and Richard Schmidt are professors of turfgrass ecology at Virginia Polytechnic Institute and State University (Blacksburg, Va.).
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