Crop Bio-Stimulants

There are a number of naturally occurring minerals, compounds and organic substances which have the ability to enhance crop growth and yields, but are usually not considered essential for plant development.

Most growers are fully aware of the list of macro and micro mineral elements and know the importance of CO2 in the aerial environment and oxygen in the root zone, but few fully understand how the use of bio-stimulants could enhance crop production.

These days we are seeing a wide range of plant nutritional and supplement products come onto the market, many of which claim growth promotant properties and are aimed at the `organic’ sector. However, it can be hard to determine if the extra cost of these additives actually pays back in terms of improved plant health, produce quality and yields. Many of the compounds we are seeing being marketed as growth promotants have a long history as plant stimulants, and have in fact been well researched and reported over the years. Some are now undergoing renewed interest as organic supplements which can assist in a number ways with crop production, others are being re examined by high tech greenhouse growers looking to push yields beyond what can currently be achieved. Whatever the reason, crop bio stimulants are something both today’s growers and those in the future should not overlook.

Bio-stimulants

A plant bio-stimulant is defined as `an organic material, that, when applied in small quantities, enhances plant growth and development such that the response cannot be attributed to application of traditional plant nutrients’. Plant bio-stimulants are a category separate from the synthetic plant growth regulators, although there can be some overlap with certain compounds. Generally synthetic plant growth regulators are those such as auxins, gibberellins, cytokinins and ethylene which have well known uses in horticulture such as root formation, fruit set, fruit ripening and others. Bio-stimulants may have similar modes of action, however many have numerous other roles involved in disease resistance, chelation of mineral ions, stress resistance, metabolic enhancers, promotion of microbial populations and regulation of certain physiological and biochemical processes in plants.
While biostimulants may seem like compounds only destined for high tech production of valuable crops, many are readily available on the market, often as additives to fertilizer products or as stand alone products for widespread crop production. Triacontanol is used as a fertiliser additive, humic and fulvic acids can be purchased as stand alone products, amino acids are becoming increasingly popular and various forms of seaweed products can be purchased for use on soils and as foliar sprays – to name a few.

Proven Biostimulants

Humates – humic and fulvic acid

Humic and humic derived acids occur widely in mineral soils, peats, compost as well as other sources such as high grade coals. Leonardite has the highest humic acid content of any natural source and has been used as a source of humic acids for crop production since the 1960’s. All humic acids contain carbon, hydrogen, oxygen and nitrogen with small amounts of sulfur and phosphorus and are from a larger family of organic compounds which have similar characteristics. The soluble humic acids have 3 major divisions, humic acid, ulmic acid and fulvic acid. Fulvic acid is a short chain molecule, which is yellow in colour and soluble. In horticulture it is the humic and particularly the fulvic acids which are the most effective in stimulating plant growth.
The beneficial effect of adding the correct humic substance on plant growth has been known for a number of decades. These compounds are not only effective in soil grown crops, but also in various potting mixes and hydroponic systems. Application of humic substances to soils naturally low in organic matter or soilless systems seems to produce the most significant growth enhancement effects. Humic compounds can be absorbed by plant roots and transported to the shoots, thus enhancing the growth of the whole plant.
In the early 1970’s the effect of humic preparations on root formation in cuttings was examined and this revealed that humic and fulvic acid posses properties that are extremely beneficial to initiation and growth of roots on cuttings. Early on, it was shown that root formation of bean seedlings was stimulated and maximized by applications of fulvic acid solutions. In hydroponic tomato plants, humic acid application resulted in higher root fresh weight and dry weights and higher levels of certain mineral elements in the shoots and roots than those plants grown in nutrient solutions with no humic acid. This same effect has been found in other hydroponic crops.
Studies on the effects of humic acid on plant growth under conditions of adequate mineral nutrition consistently show positive effects on plant biomass. Humic acids increase soil and plant productivity by enhancing the uptake of essential nutrient elements through chelation and increasing plant growth through their effect on root enzymes and the soil. Both increases in root length and stimulation of the development of secondary roots have been observed for humic acid in nutrient solutions, soils and growing mixes. The stimulating effects of humic acids has been correlated with enhanced uptake of nutrients. Humic acid has also been found to increase photosynthesis in some plant species, thus enhancing the growth of the whole plant. Humic acid has been widely proven to have beneficial effects on plant growth over a wide range of concentrations with application as a soil/media drench, seed soak and nutrient solution additive.
Fulvic acid, is one of the many chemical compounds that qualify as humates and appears to be more biologically active than humic acid. Fulvic acid has direct effects on membranes resulting in improved transport of nutrients, enhanced protein synthesis, possible plant hormone like activity, enhanced photosynthesis and indirect effects such as solubilisation of microelements (Fe, Zn, Mn), and some macro elements (K, Ca, P), reduction of active levels of toxic elements and enhancement of microbial populations.
In a trial carried out in New Zealand, the effects of addition of fulvic acid to an organic nutrient solution were examined. Nutrients used in such organic liquid fertilizers often require processing by micro organisms before plants can take these up. Furthermore organic systems can’t make use of synthetic chelation agents such as Iron EDTA, and often suffer from deficiencies as a result. Humic acids have been proven in other studies to promote the conversion of a number of elements into a form available for plants and more importantly, act as natural chelation agents for many nutrient ions. In this trial, dwarf green beans were grown in a coconut fiber media and supplied with an organic nutrient solution with and without the addition of fulvic acid at the recommended concentration rate. At the time of harvest, both bean weight and the weight of plant tops (stems and leaves) was determined. The fulvic acid treatment resulted in a 36% increase in bean weight at harvest and a 36.5% increase in the above ground portion of the plants. This represents an overall increase in plant growth as well as in harvestable yield. The plants treated with fulvic acid flowered on average 4 days ahead of the control plots. Apart from the effects on the nutrient and uptake of minerals, the addition of fulvic acid is also likely to have had a stimulatory effect on plant growth – as this has been proven in systems using inorganic nutrient sources. Humic and fulvic acids have been widely proven to increase the rate of development and length of root systems and to accelerate cell division, this effect would also have been contributing to the increase in foliage and bean yield obtained from this trial.
While the addition of humic and fulvic acids to both soil and soilless systems has in the past proved to be beneficial to crop growth and development, it also appears to have considerable advantages to organic production systems where a complex range of micro organisms, organic compounds and substances need to co-exist for the system to be successful.
Humate products are readily available for horticultural use, both as stand alone products and as additives to various crop stimulants and fertilizers products. There is evidence to suggest that combining humates with other crop bio stimulates such as seaweed extracts or amino acids can provide further growth benefits, although this is highly dependant on the method of application, concentrations and crop species grown.

Triacontanol

Triacontanol is a naturally occurring compound synthesized by plants and also found in bees wax which exhibits many characteristics of a plant hormone. Typically, triacontanol is considered to be more of a secondary plant growth substance rather than a plant hormone. This compound was first identified in alfalfa hay in the 1930s. In the 1970’s researchers found that incorporating chopped alfalfa hay into the soil below and to the side of crop seedlings increased growth and yield due to the presence of 1-tricontanol in the hay. Triacontanol has been found to regulate several physiological and biochemical processes and to increase the yield of a number of crops. However it appears to be most beneficial when applied via the nutrient solution in hydroponic production or as a foliar spray. It is less effective if used as a soil or media drench as it appears to loose its activity within growing substrates, possibly because many compounds have an antagonistic effect on the response of plants to triacontanol. While it is possible to buy pure triacontanol, it is expensive and is mostly supplied in a waxy form which will not dissolve directly into water. The most effective results with this compound have been at concentrations of 0.5 – 10.0 micrograms per litre of water, sprayed onto the crop under warm conditions such as in the late afternoon. For fruiting vegetables, application at the time of flowering has been most effective, while for leafy vegetables the greatest bio stimulant effect will occur when vegetative growth is most rapid. Studies on hydroponic lettuce have found that triacontanol applied as a foliar spray on young, 4 day old lettuce increased leaf fresh and dry weight by 13 – 20% 6 days after application, relative to plants treated with only water.

Amino acids

Amino acid plant bio stimulant products and additives are relatively new and not well understood by many growers. There have been conflicting results in the scientific literature as to the effectiveness of amino acid supplements on crop growth and much of this may stem from the differing composition of amino acid products on the market. Amino acids are the building blocks of proteins with the L-amino acids having metabolic activity. Unlike the other bio stimulant compounds there are a number of different amino acids, and some appear to be more biologically active than others. Most of the amino acids used as plant growth simulators are derived from hydrolysed proteins such as soy. Some of the amino acid mixtures tested contained alanine, aspartic acid, glutamic acid, glutamine and glycine, although other mixtures contained different aminos and it appears the degree of protein hydrolysis, free amino acid (AAL) content and molecular weight determined the specific activity of amino acid mixtures.
Scientific studies into the effects of amino acids on plant growth are mostly relatively new, having only been carried out over the past 3 – 4 years. Some of these have found that certain amino acids can directly and indirectly influence the physiological activities of plants. Amino acids may be foliar applied where they are absorbed through the stomas, although this is dependant on the temperature at the time of application. Amino acids may also be supplied to plants by incorporation in the soil where they benefit the micro flora and improve nutrient availability and assimilation. One recent study found that application of mixed amino acids to red pepper increased NO3 uptake by seven times higher than the untreated control. It was also found that enzymatic activities of nitrate reductase and nitrite reductase were positively affected by treatments of mixed amino acids in the roots. Other researchers have claimed that amino acids have an effect on photosynthesis, protein synthesis, lignification, resistance to abiotic stress and have a hormone like activity.

Salicylates

Salicylic acid is the main ingredient in aspirin (as acetylsalicylic acid) and was originally obtained from the bark of the willow tree. However this acid is known to be present in a wide range of plants and is considered to be a plant growth substance which has been shown to have an affect on a variety of plant processes including flowering and promotion of disease resistance. There is evidence to suggest that salicylic acid exhibits plant hormone like activity as it acts at very low concentrations and can be transported within the plant to sites of attack by disease pathogens. There have been conflicting reports from scientific studies indicating that salicylic acid can stimulate flowering on a number of crop species and the evidence on this is still inconclusive. Some studies have reported that application of salicylic acid increased pod number and yield in mung beans and increased height and grain number in millet. In combination with IAA, salicylic acid stimulated root formation in mung beans.
Salicylic acid is best known for its ability to induce disease resistance in many plants. Resistance to pathogens and the production of some `pathogenesis-related’ proteins can be induced by application of salicylic or acetylsalicylic acid which then provides resistance to subsequent disease infection. Despite these findings, research still continues into the possibility of using salicylic acid as a natural disease response elicitor in commercial crops.

Seaweed and naturally occurring cytokinins

As early as the 1960’s researchers found that naturally occurring cytokinins in seaweed could provide a growth stimulator effect, however the results were often unpredictable and inconsistent as different seaweed sources had varying levels of cytokinin. Stable extracts of seaweed of certain species have been shown to improve root and shoot growth and improve stress resistance in some crops by increasing the endogenous natural production of antioxidants in the plant. Seaweed products also contain a wide range of minerals, some of which, while not essential for plant growth, are known to be beneficial for growth, development and disease resistance.
Many bio stimulants applied to soil can assist with the breakdown and mineralization of soil organic materials, possibility by stimulating certain aspects of microbial populations. This often leads to an increase in soil nitrogen availability. These types of bio-stimulant products are often aimed at organic producers who need to maintain healthy microbes to convert organic fertilizer sources into plant usable nutrient ions as rapidly as possible.
In the future we can expect to see increased research into the use of various bio stimulants for commercial crop production and many new products come onto the horticultural market. Since benefits from using such products can vary depending on the environment, crop and application conditions, it is worth trialing reputable new products and assessing the results as an ongoing evaluation process which may lead to yield increases.

Dr. Lynette Morgan and her husband, Simon, are the owners of Suntec International Hydroponic Consultants in New Zealand. Dr. Morgan was a regular contributor to the print edition of The Growing Edge.
www.suntec.co.nz/consultants.htm

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