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Ascophyllum nodosum-Based Biostimulants:

REVIEW ARTICLE

Front. Plant Sci., 29 May 2019 | https://doi.org/10.3389/fpls.2019.00655

Ascophyllum nodosum-Based Biostimulants: Sustainable Applications in Agriculture for the Stimulation of Plant Growth, Stress Tolerance, and Disease Management

  • 1Marine Bio-products Research Laboratory, Department of Plant, Food and Environmental Sciences, Dalhousie University, Truro, NS, Canada
  • 2Research & Development, Acadian Seaplants Limited, Dartmouth, NS, Canada

Abiotic and biotic stresses limit the growth and productivity of plants. In the current global scenario, in order to meet the requirements of the ever-increasing world population, chemical pesticides and synthetic fertilizers are used to boost agricultural production. These harmful chemicals pose a serious threat to the health of humans, animals, plants, and the entire biosphere. To minimize the agricultural chemical footprint, extracts of Ascophyllum nodosum (ANE) have been explored for their ability to improve plant growth and agricultural productivity. The scientific literature reviewed in this article attempts to explain how certain bioactive compounds present in extracts aid to improve plant tolerances to abiotic and/or biotic stresses, plant growth promotion, and their effects on root/microbe interactions. These reports have highlighted the use of various seaweed extracts in improving nutrient use efficiency in treated plants. These studies include investigations of physiological, biochemical, and molecular mechanisms as evidenced using model plants. However, the various modes of action of A. nodosum extracts have not been previously reviewed. The information presented in this review depicts the multiple, beneficial effects of A. nodosum-based biostimulant extracts on plant growth and their defense responses and suggests new opportunities for further applications for marked benefits in production and quality in the agriculture and horticultural sectors.

Introduction

The global effects of negative climatic changes have manifested as desertification, increased atmospheric CO2and temperature, soil salinization, and nutrient imbalances (e.g., mineral toxicity and deficiency) and have caused dramatic effects on agricultural production and the quality of crops (dos Reis et al., 2012). Such abiotic stresses have reduced the growth, development, productivity, and quality of plants and, in extreme conditions, resulted in death and local extinction of species (Matesanz et al., 2010Anderson et al., 2011). Abiotic stresses are reported to have led to an average yield loss greater than 50% in most crops (Boyer, 1982Vinocur and Altman, 2005). Rice yields declined 15% per 1°C rise in mean growing season temperature, measured from 1979 to 2003 (Peng et al., 2004). Additionally, changing climatic conditions can increase plant susceptibility to pathogens (West et al., 2012Elad and Pertot, 2014), further increasing adverse growing conditions for plants.

The global amount of cultivable land available for agriculture is continuously shrinking due to urbanization and the adverse effects of climate change. In order to meet the ever-increasing demands of the growing human population, world food production must double by the year 2050 (Qin et al., 2011Voss-Fels and Snowdon, 2016). To address the pressures associated with increasing agricultural productivity to subsequently meet the rising demands for food, producers have turned to excessive applications of synthetic (chemical) fertilizers and pesticides. These harmful chemicals pose both short- and long-term threats to the health of the entire biosphere (Damalas and Koutroubas, 2016). Therefore, an effective, biological-based alternative is required in order to reduce dependency on synthetic fertilizers and pesticides. Plant biostimulants are a new class of crop input, offering a potential alternative to traditional, agro-chemical inputs, and, in most cases, can reduce the application rates of synthetic fertilizers and pesticides by enhancing their efficacy (Calvo et al., 2014Van Oosten et al., 2017Yakhin et al., 2017).

According to the European Biostimulants Industry Council (EBIC), “plant biostimulants contain substance(s) and/or micro-organisms whose function when applied to plants or the rhizosphere is to stimulate natural processes to enhance/benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stresses, and crop quality”1. The concept of biostimulants has been researched since 1933 (Yakhin et al., 2017) but has gained attention in more recent years as a potential solution to mitigate the negative impacts of a changing climate on agriculture. It should be noted that seaweed extracts are but one of the inputs that are classed as biostimulants.

Seaweeds are multi-cellular, macroscopic organisms found in coastal, marine ecosystems and are a rich source of polysaccharides, polyunsaturated fatty acids (PUFAs), enzymes, and bioactive peptides among others (Courtois, 2009De Jesus Raposo et al., 2013Ahmadi et al., 2015Shukla et al., 2016Okolie et al., 2018). In particular, inter-tidal seaweeds may be exposed to unfavorable conditions including extreme variations in temperature, salinity, and light. Seaweeds, as compared to terrestrial organisms, produce different stress-related compounds that are essential for their survival in these environments (Shukla et al., 2016). As such, selected seaweed resources are important sources of plant biostimulants and are widely used to promote agricultural productivity (Khan et al., 2009Sharma et al., 2014du Jardin, 2015Van Oosten et al., 2017). The most widely researched seaweed, used as a source for industrial and commercial plant biostimulants, is the brown, inter-tidal seaweed Ascophyllum nodosum. Various commercial extracts from A. nodosum have been demonstrated to improve plant growth, mitigate some abiotic and biotic stresses while also improving plant defenses by the regulation of molecular, physiological, and biochemical processes. Of all sources of seaweed-based biostimulants, those manufactured from A. nodosum are perhaps the best studied with various modes of action being proposed (Figure 1). This review focuses on accumulating current knowledge of the bioactive compounds presents in A. nodosum extracts and their modes of action in promoting plant growth in the presence of abiotic and biotic stresses.

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The excellent benefits of seaweed on all sports surfaces

Seaweed is classed as a soil conditioner in the gardening world, but its benefits are so extensive that it really should be given top billing when discussing the care of soil and growing of plants, as it contains over sixty minerals and trace elements, along with bio stimulants.

 

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In Ireland, many links golf courses used to take seaweed from the adjacent beaches and cover greens with it during severe winters, which both protected the grass from the cold and provided a range of nutrients as it weathered down. When the weather improved the seaweed was removed and composted with sand to provide a very high quality topdressing material. Now, play continues all year round on most courses, so this practice is not feasible but, if there is seaweed available, then it is most certainly well worth harvesting and composting to provide your own supply of high quality topdressing material.

 

Seaweed contains a wide range of nutrients and trace elements which combine in such a unique manner that, when seaweed is added to heavy clay soils, it breaks down the heavy soil into a friable crumb structure, thereby providing a vastly improved growing medium. This, of course, does not happen overnight, but is one of the greatest benefits of long term applications on these types of soils.

Liquid seaweed is the best option, as it is an excellent foliar feed and can be combined with other nutrients as and when require.

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Why use Seaweed as a fertilizer?

Soil is a vital importance as a worldwide resource as well as the fact that we treat it so poorly. I will explain why we should consider using seaweed-based fertilizers.

An ideal soil has approximately 50% of its volume filled with solids and the other 50% with water and air.  Ninety percent of the solids should be minerals, basically eroded rocks, and 10% should be organic matter such as decaying leaves. The spaces between the solids accommodate water and allow air to reach plant roots, a vital step in plant growth. Healthy soil is the most biologically productive environment on Earth. A single gram of soil can contain up to a billion organisms, representing over a thousand species.

Soil contains approximately 70 different minerals. Thirteen of these are known to be essential for plant growth: nitrogen, phosphorous, potassium, sulfur, calcium, magnesium, iron, boron, manganese, copper, zinc, molybdenum, and chlorine. The other 50 or so, including things like cobalt, iodine and selenium, often referred to as micronutrients, are likely to be important to plant growth even if the mechanisms are not fully understood. For plants to be able to utilize these minerals efficiently, the soil environment must have proper moisture, pH, and organic content. In particular, when soils become deficient in organic matter, the ability of plants to absorb minerals from the soil drops precipitously.

In a similar fashion to plants, humans need a wide array of minerals in our diet to maintain our health. With the exception of taking vitamin supplements, a practice which is less effective than you might think, we get the vast majority of our minerals from the soil, either by eating plants that have extracted them from the soil for us, or by eating animals that have eaten plants. Seafood provides another important source of minerals

As farmers harvest plants, minerals which had formerly been in the soil are removed. Unless these minerals are replaced, the field will quickly lose productivity. Replenishment of minerals can be accomplished by the application of fertilizer.

Generally speaking, the industrial agriculture systems used worldwide are not particularly effective at replacing soil minerals. Most fertilizers applied on large agricultural operations include only nitrogen, phosphorous, potassium and, at best, a small handful of other minerals. To make matters worse, over-tilling and insufficient application of compost result in a reduction of the soil’s organic content. Therefore, the absorption of even the small subset of minerals applied to the fields is inefficient. To overcome this inefficiency, farmers increase the amount of fertilizer that they apply, which then results in the run-off of excess nitrogen and phosphorous into the surrounding watershed, which creates a number of additional problems.

The United States Department of Agriculture has been tracking the impact of the depletion of our soils since the 1950s. Every year, they analyze the vitamin and mineral contents of approximately forty common foods, including carrots, apples, wheat, and chicken. As the decades have passed, the vitamin and mineral content of these foods has dropped in the range of 10-30%. This slow erosion of food quality in the U.S. is a key, but little discussed, underlying cause of many of our public health challenges.

So what can we do to reverse this trend? We can improve our soil management practices and we can use fertilizers which contain a broader array of minerals by looking to the oceans.

Since the oceans of the world are downstream from everywhere, they are not depleted of minerals. The cobalt, iron, and selenium content of the oceans are roughly the same as they were when our ancestors first crawled up on the sand. Therefore, plants which live in the ocean can, and do, absorb up to 50 to 60 different minerals, including the full array of micronutrients. This characteristic is key to the attractiveness of seaweed both as a fertilizer and a health food. Japan utilizes seaweed extensively in both of these capacities.

Given our long coastline with its variety of inlets and bays, Ireland has the potential to develop a successful seaweed farming industry. To me, there is a certain beauty in the idea. Minerals from our fields find their way from the soil, to our food, to us, and then to the ocean (I’ll let you puzzle out the mechanisms and pathways on your own). Then the seaweed can collect these minerals for us so that we can harvest them and bring those minerals back to our fields.

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