BIOSTIMULANTS ET PRODUITS PHYTOSANITAIRES POUR LES SYSTÈMES À GAZON : TENDANCES D'UTILISATION ACTUELLES ET FUTURES

What are Biostimulants? In the purest sense the word, a biostimulant can be defined as any material when applied to a plant that stimulates “life” (e.g. bio). Numerous materials fall this simple definition and it could even be argued that applying water to a plant is one example of a biostimulant since it “promotes life”. Research and use of plant biostimulants in turf systems is nothing new and work has been ongoing for over three decades. An early pioneer in biostimulant research was Dr. Richard E. Schmidt from Virginia Tech. His program pioneered many basic and applied aspects of biostimulants for many plants. A great deal of that research focused on exogenous applications of various natural and synthetic plant hormones with the intent of helping to optimize rooting (Liu et al. 1998), superior tolerance to soil-borne pests like nematodes (Sun et al., 1997), mitigate the effects of stresses like intense ultra-violet light (Schmidt and Zhang, 2001), heat-drought (Wang et al., 2012, 2013, 2014; Zhang and Schmidt, 1999,2000; Zhang et al., 2012,2013), salinity (Yan, 1993; Nabati et al. 1994), improving winter survival (Schmidt and Chalmers. 1993; Zhang et al., 2013), optimizing nutritional efficiency (Schmidt et al., 1999; Wang et al., 2011), or enhancing recovery from routine stressful mechanical cultural management practices (Bigelow et al., 2010).
Dr. Schmidt and with Dr. Xunzhong Zhang coined an initial operating definition for describing these various plant growth substances they were exploring that promoted plant growth without being nutrients, soil improvers or pesticides. They defined biostimulants as “materials that, in minute quantities, promote plant growth”. The use of the word minute in this definition was important and intended to differentiate the fact that these substances compared to traditional nutrients and/or soil amendments elicited a measurable and beneficial response at much lower application rates. They explained the plant biostimulation by way of hormonal effects and often plant protection against abiotic stress through various antioxidant production. Later, the term “metabolic enhancers” was also used but the important aspect was that something positive was happening to the plant beyond what mineral nutrition supplied.
More recently, the definition of biostimulants has been continually refined. In 2015, Patrick duJardin published the paper, “Plant biostimulants: Definition, concept, main categories and regulation”. In this article a plant biostimulant is defined as “any substance or microorganism applied to plants with the aim to enhance nutritional efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrient content.” The main categories of biostimulants proposed were: Humic and fulvic acids, protein hydrolysates and other N-containing compounds, seaweed extracts and botanicals, chitosan and other biopolymers, inorganic compounds, beneficial fungi and bacteria. These categories were important to define not only traditional biostimulants but also including the beneficial organisms that may elicit a positive plant health response while also helping to guide policymakers who might want to regulate these materials.
In the turf market segment, particularly in the USA, research into the biostimulant was continued at a high level by Drs. Ervin and Zhang who explored numerous aspects of biostimulant materials and their effect on turf physiology from the 2000’s to present. These projects included previously explored materials while including an expanded view of how various synthetic, non-mineral nutritional materials might also promote growth. It was postulated that these materials when applied in conjunction to the naturally biostimulants may produce additive and/or synergistic effect on turf plant growth. In addition to the categories named previously, Ervin (2013) described and suggested mechanisms for a wider range of additional products and chemistries commonly being applied to turf. For example, secondary plant hormones (e.g. salicylic acid) which can induce systemic acquired resistance (SAR) in response to plant pathogens (e.g. fungi, bacteria, insects, nematodes), similarly compounds like phosphites (PO3) that may stimulate phytoalexins (stress induced antimicrobial compounds) for health/diseases suppression. Additional products that could protect tissues from ultra-violet light injury like green pigments or compounds containing titanium dioxide and zinc oxide. Acibenzolar, which mimics salicylic acid effects to induce SAR. Synthetic fungicides like propiconazole had been previously explored by Dr. Schmidt with positive plant responses, but newer chemistries like pyraclostrobin was shown in a number of University research trials to slow down plant respiration and boost antioxidants under heat and drought stress. One interesting find was that pyraclostrobin naturally degrades to the amino acid tryptophan which is a precursor to the plant rooting hormone auxin. Amino acids like proline and glycine-betaine are often suggested for use as osmoregulators or dehydration avoidance compounds. The practice of supplementing amino acids via foliar applications in summer months is a long-standing suggested approach to improve plant health that might be experiencing energy depletion during stressful environmental conditions (e.g. the aim of mitigating seasonal summer decline for cool-season grasses). Lastly, the long standing well researched biostimulants like humic substances, auxin, seaweed and cytokinins are listed. A large fraction of biostimulant products marketed to the turf industry contain seaweed extract (SWE), or seaplant/kelp extract. SWE is naturally high in cytokinin (Crouch and VanStaden, 1993) and research by Schmidt, Ervin and Zhang has shown that SWE was similar to a synthetic cytokinin applications and that monthly application of SWE boosted antioxidant levels, less loss of root viability and improved heat/drought tolerance. Further, Ervin and Zhang showed that combining HA + SWE can provide better plant health during stressful conditions than using either alone. The turf industry contains a vast array of products that claim positive plant or soil health benefits, particularly under stress conditions. Choosing the right material for the intended specific plant benefit is truly important.

Résumé : Gérés correctement, les systèmes de gazon peuvent constituer un atout environnemental considérable. Un peuplement dense et sain de gazon offre de nombreux avantages esthétiques, récréatifs et environnementaux, notamment en maintenant les sols urbains en place et en filtrant les contaminants des eaux de ruissellement urbaines. Il s'agit toutefois de systèmes végétaux pérennes qui nécessitent un certain niveau d'apports nutritionnels et d'eau pour rester vigoureux et persister. Si l'objectif est de les gérer plus durablement avec moins d'intrants traditionnels et de les rendre plus résistantes aux contraintes environnementales, les biostimulants et autres produits phytosanitaires sont des outils pour atteindre cet objectif. La recherche des effets des biostimulants sur les gazons permet de progresser vers des approches plus durables de la nutrition des gazons. Les aspects organiques de la physiologie du gazon sont aussi importants que les aspects minéraux pour une bonne nutrition des plantes. La différence de teneur en minéraux entre les plantes d'un même cultivar ayant des réactions de croissance différentes est relativement faible. De plus, la nutrition minérale est peu corrélée à la tolérance aux stress. Cependant, le fait de connaître l'impact des matières biologiquement actives sur le métabolisme du gazon permet aux gestionnaires de gazon de mieux conditionner le gazon pour qu'il tolère mieux le stress environnemental. Une meilleure compréhension des fonctions des minéraux et de leurs effets métaboliques permettra d'améliorer les meilleures pratiques de gestion nutritionnelle afin d'atténuer les nombreux stress du gazon. L'utilisation de ces connaissances fournit un outil supplémentaire pour produire un gazon de haute qualité et résilient lorsqu'il est cultivé dans des environnements défavorables."
Compte tenu de l'attention et de l'examen de plus en plus minutieux des apports en nutriments et en eau pour la gestion du stress parfois sévère du gazon, comme la sécheresse prolongée due à des conditions climatiques de plus en plus imprévisibles, l'application de biostimulants à des zones qui ont traditionnellement reçu moins d'attention, comme les pelouses, pourrait devenir plus courante. Quel que soit le produit ou le programme de biostimulation/santé des plantes, il est important de souligner que les biostimulants ne remplacent pas un bon programme de nutrition minérale des plantes. De plus, si l'objectif est d'incorporer les biostimulants dans le cadre d'un programme holistique de santé des plantes, la recherche sur les systèmes de gazon a démontré qu'ils doivent être appliqués avant les stress environnementaux, etc. pour optimiser leurs avantages. Des innovations passionnantes se profilent dans ce domaine et les efforts fondés sur des données probantes permettront de mieux comprendre comment ces produits peuvent contribuer à maintenir et à améliorer la santé des plantes.