The most important nutrient for optimal plant growth is nitrogen (N), but it is known that in intensive agricultural systems there is an environmental risk caused by inadequate management of N fertilization (Neeteson and Carton, 2001). Processes such as nitrate leaching and nitrous oxide emissions produce major environmental problems in water and the atmosphere (Guo et al., 2010; Ramos et al., 2002). On the other hand, other agricultural inputs such as pesticides can be harmful to nontarget organisms and can produce adverse effects on human health and the environment if used negligently. Regulations set by the European Union for the sustainable use of fertilizers and pesticides to reduce risks and to lessen the impact on human health and the environment (European Union, 1991, 2009, respectively) may increase the use of biostimulants as replacements for many ingredients that are being rejected for use in turfgrass. However, the term “biostimulant” is wide and loosely defined (Mueller and Kussow, 2005). The term includes an array of products, from seaweed to cultured living microorganisms and various natural chemicals and compounds (du Jardin, 2015; Karnok, 2000; Van Oosten et al., 2017).
Calvo et al. (2014) extensively reported on the agricultural uses of plant biostimulants. Specifically, biostimulants have already been positively tested in horticulture. Battacharyya et al. (2015) reviewed the effects of seaweed extracts on vegetable, fruit, and ornamental crops—including tall fescue (Festuca arundinacea) and creeping bentgrass (Agrostis stolonifera)—and concluded that there are a number of questions that still need to be addressed for the better use of seaweed resources and their extracts in horticultural crops.
Another review of the effect of biostimulants based on humic and fulvic acids on 30 horticulture crops concluded that the use of such biostimulants in horticultural crops is a key sustainable technology that can make cropping systems more productive, efficient, and less harmful to the environment (Canellas et al., 2015). Protein hydrolysates (a mixture of peptides and amino acids) have also been positively used as biostimulants for making horticultural crops more sustainable (Colla et al., 2015, 2017).
Finally, the use of soil microorganisms (bacteria or fungus) as biostimulants for increasing the nutrient and water-use efficiency of horticultural crops has also been successfully tested (Acikgoz et al., 2016; López-Bucio, et al., 2015; Rouphael et al., 2015; Ruzzi and Aroca, 2015; Sahin and Turan, 2013). Yadav et al. (2017) reported that the use of plant growth promoting bacteria (Azotobacter, Bacillus, and Pseudomonas, and other genera) may prove useful in developing strategies to facilitate plant growth under normal conditions, as well as under abiotic stress. Another area of potential enhancement of N nutrition in turfgrasses and adaptation to environmental stress involves associative N fixation by bacteria and mycorrhizal fungi (Duncan and Carrow, 1999). Contributing to other positive roles, Azotobacter vinelandii, for example, can be used as soil contaminant detoxification bacteria (Ehaliotis et al., 1999).
Due to the success of biostimulants in vegetable and fruit crops, the biostimulant industry is focusing on turfgrass species, as there is a considerable business opportunity due to acreage and pesticide reduction regulations in this sector. However, many biostimulants marketed for turfgrass include mineral fertilizers that mask the effect of biostimulants on turfgrass, and which together with the large number of available products is making the use of biostimulants confusing for scientists, greenskeepers, and householders.
One of the most important turfgrass species in the temperate regions of the world is perennial ryegrass (Lolium perenne). This grass is suitable for home lawns, parks, cemeteries, roadsides, golf courses, and athletic fields (Wu et al., 2005). Perennial ryegrass is a cool-season turfgrass species, and its growth often is limited by high temperatures during summer months in warm climates (Jiang and Huang, 2001). A perennial ryegrass stand stressed by heat and/or other abiotic factors is more likely to be infected by diverse microorganisms, and the lack of pesticides is leading groundskeepers to apply biostimulants with little knowledge of their mechanism of action or how to integrate application procedures with other management practices (above all mowing and irrigation).
Little research has been conducted on the effect of biostimulants on perennial ryegrass. Kauffman et al. 2007 demonstrated that amino acid containing biostimulants when applied sequentially to perennial ryegrass foliage could positively affect turfgrass heat stress tolerance. Botta (2013) reported that an amino acid-based biostimulant applied on heat-stressed perennial ryegrass showed superior photosynthetic efficiency and maintained higher levels of chlorophylls and carotenoids. Another amino acid (gamma aminobutyric acid) was foliar applied on drought-stressed perennial ryegrass, demonstrating that it was effective in mitigating the physiological response of drought stress damage (Krishnan et al., 2013). Acikgoz et al. (2016) conducted a study on perennial ryegrass and tall fescue that resulted in an increase of color ratings and clipping yields when treated with plant growth-promoting rhizobacteria. Several studies have been performed on perennial ryegrass for salt tolerance enhancement by applying biostimulants: Hu et al. 2012 found glycine betaine treatments useful in this type of stress and Sun et al. (2015) and Wu et al. (2017) suggested that a 24-epibrassinolide treatment (a plant hormone) might improve perennial ryegrass salt tolerance.
Biostimulants have also been tested on other important turfgrass species. Elliot and Prevatte (1996) reported no positive effects for seaweed extracts on hybrid bermudagrass (Cynodon dactylon × C. transvaalensis) growth or quality. In tall fescue, seaweed extracts improved post-transplant rooting and quality of tall fescue sod (Zhang et al., 2003a). In creeping bentgrass, Aamlid et al. (2017) reported a faster grow-in on plots receiving amino acid-based biostimulants than on plots receiving mineral fertilizers. Xu and Huang (2010) reported improved turfgrass quality with sea plant extracts and microorganisms. However, a previous experiment (Aamlid and Hanslin, 2009) indicated that, on average, none of the tested biostimulants caused significant improvements in the overall impression of turfgrass [creeping bentgrass, perennial ryegrass, and kentucky bluegrass (Poa pratensis)] compared with control mineral fertilizer treatments. Only few improvements in foliar N uptake were reported by Stiegler et al. (2013) when single amino acid-based biostimulants were applied and compared with mineral fertilizers on creeping bentgrass. These last authors also reported that glycine was better absorbed than potassium nitrate, but none of the studied amino acid-based biostimulants exceeded urea N absorption. Another negative effect on creeping bentgrass due to a microbial inoculant biostimulant application was a delay in germination (Butler et al., 2007) or a N leaching increase when establishing a new golf green (Butler et al., 2012). Zhang et al. (2003b) and Zhang and Ervin (2008) reported that seaweed extracts and humic substances may be beneficial supplements for reducing standard fertilizer and fungicide inputs, while maintaining adequate health for creeping bentgrass. Zhang et al. (2013b) indicated a positive role for foliar amino acids in creeping bentgrass N summer fertilization programs.
Hence there is a need to improve our understanding of the function of biostimulants so that the efficacy of these materials can be improved and industrial processes can be optimized (Brown and Saa, 2015). The main research trend dealing with biostimulants on turfgrass science is in demonstrating that biostimulants are useful when applied before stress situations (heat, drought, disease, and nutrition) to reduce polluting agricultural inputs such as fertilizers and pesticides. A priority for many turfgrass managers is to maintain high-quality turfgrass while minimizing their environmental risk.
The objective of our research was to test three commercial biostimulants on perennial ryegrass in greenhouse conditions considering two hypotheses: 1) the biostimulants increase turfgrass quality, and 2) the use of these biostimulants under nutritional stress reduces mineral fertilization in a turfgrass fertilization program.
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