The Chemical Biology of Plant Biostimulants

Danny Geelen, Lin Xu, Danny Geelen, Lin Xu, Christian V. Stevens

• Publisher: Wiley
• Year: 2020
• Print ISBN:
  9781119357193
• Edition: 1
• eBook ISBN:
  9781119357100
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Introduces readers to the chemical biology of plant biostimulants

This book brings together different aspects of biostimulants, providing an overview of the variety of materials exploited as biostimulants, their biological activity, and agricultural applications. As different groups of biostimulants display different bioactivity and specificity, advances in biostimulant research is illustrated by different examples of biostimulants, such as humic substance, seaweed extracts, and substances with hormone-like activities. The book also reports on methods used to screen for new biostimulant compounds by exploring natural sources.

Combining the expertise of internationally-renowned scientists and entrepreneurs in the area of biostimulants and biofertilisers, The Chemical Biology of Plant Biostimulants offers in-depth chapters that look at: agricultural functions and action mechanisms of plant biostimulants (PBs); plant biostimulants from seaweed; seaweed carbohydrates; and the possible role for electron shuttling capacity in elicitation of PB activity of humic substances on plant growth enhancement. The subject of auxins is covered next, followed closely by a chapter on plant biostimulants in vermicomposts. Other topics include: exploring natural resources for biostimulants; the impact of biostimulants on whole plant and cellular levels; the impact of PBs on molecular level; and the use of use of plant metabolites to mitigate stress effects in crops.

• Provides an insightful introduction to the subject of biostimulants
• Discusses biostimulant modes of actions
• Covers microbial biostimulatory activities and biostimulant application strategies
• Offers unique and varied perspectives on the subject by a team of international contributors
• Features summaries of publications on biostimulants and biostimulant activity

The Chemical Biology of Plant Biostimulants will appeal to a wide range of readers, including scientists and agricultural practitioners looking for more knowledge about the development and application of biostimulants.

Part I
Introduction

1
Agricultural Functions and Action Mechanisms of Plant Biostimulants (PBs): an Introduction

Patrick du Jardin¹, Lin Xu², and Danny Geelen²

¹Plant Biology Laboratory, Gembloux Agro‐Bio Tech, University of Liège, 2 Passage des Déportés, Gembloux, Belgium

²Horticell, Department of Plants and Crops, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, Ghent, Belgium

1.1 The Biostimulant Concept

The concept of biostimulant has gained an increasing interest in plant science over the last decade. There are essentially two reasons for that. The first one is the adoption of the term by the scientific community, with the aim of describing a range of substances and microorganisms showing characteristic effects on plant growth, development and metabolism (Figure 1.1). The second one is related to the regulation and marketing of agricultural input products and the need to clarify their typology in order to accommodate a range of innovative products, which appear to be distinct from both conventional fertilizers and pesticides. Indeed, biostimulants are not providing nutrients as their primary function, as opposed to fertilizers, and they do not target pests and pathogens, as is the case for pesticides.

The term ‘biostimulant’ was coined to describe plant‐applied substances showing positive effects on plant growth and stress tolerance, which is not related to the plants' nutrient content [1]. Acting in minute amounts, biostimulants are often suggested to harbour hormone‐like activities. However, this view does not align with the mechanisms of action reported for complex substances like seaweed extracts, humic acids (HAs) and protein hydrolysates (PHs), for which there is only limited support for hormonal basis of biostimulation. Since many microbes from the soil and rhizosphere can evoke similar effects as biostimulatory substances on crops and horticultural plants, the term ‘biostimulant’ was later applied to Plant‐Growth‐Promoting Rhizobacteria (PGPR), including other microorganisms referred to as biofertilizers.

Figure 1.1 Number of documents using the word biostimulant over the years in the Scopus database. The words ‘biostimulant’ (queries: ‘biostimula* OR bio‐stimula*’) and ‘plant’ (query: ‘plant*’) were searched in the titles, abstracts and keywords, over the period from 1997 to 2018.

Extensive discussions about the definition of biostimulants have taken place and there is no consolidated definition today in both the academic and regulatory arenas [1–3]. However, distinctive features of biostimulants emerge from current definitions: (i) plant biostimulants (PBs) are substances or microorganisms, or combinations thereof, intended to be applied to cultivated plants or to their root environment; (ii) they are primarily defined by their effects on plants – the ‘claims’ – not by their compositions and origins, which are very diverse; (iii) the claims are expected benefits to whole plants and crops, both in quantitative terms (i.e. yield under stress or non‐stress conditions, nutrient use efficiency (NUE)) and in qualitative terms (i.e. products' quality traits); (iv) these beneficial effects are not due to their nutrient contents or to direct or indirect plant protection effects, but to regulatory effects on plant metabolism and development, which remain largely to be defined.

In line with this, the following definition has been proposed in the scientific literature [1]: ‘a plant biostimulant is any substance or microorganism applied to plants with the aim to enhance nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrients content. By extension, plant biostimulants also designate commercial products containing mixtures of such substances and/or microorganisms’ [1]. The European Biostimulants Industry Council (EBIC) has introduced a similar definition, based on the agricultural functions of biostimulants: ‘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 stress, and crop quality’ (EBIC, http://www.biostimulants.eu/). However, EBIC defines biostimulants as products applied to plants, which are most often formulated mixtures of substances and/or microorganisms, while the previous definition from the scientific literature aims to cover both the ‘ingredients’ and the final products.

Regulatory advances in European Union provide support to the definition of EBIC in the sense that biostimulants are regarded as one of the ‘product function categories’ recognized by a future regulation on CE‐marked fertilizing products, which will repeal the current regulation (EC) No 2003/2003 on EC fertilizers. Indeed, the new regulation will use a double typology: ‘product function categories’ refer to the claimed actions and intended uses of the products (fertilizers, liming products, soil improvers, biostimulants, etc.), while ‘component material categories’ describe the materials used to produce them. Specifications are defined for each category and compliance of the CE‐marked fertilizing products with the corresponding EU‐harmonized standards will be the prerequisite to their placing on the European market. Labelling provisions are also part of the new regulation. Within this regulatory system, biostimulants will be considered as fertilizing products, as supplied to the farmer, hence potentially containing multiple ingredients. The ingredients will be substances and/or microorganisms, referring to one or more of the component material categories. Provisions for ensuring both the safety and the efficacy of the marketed products are being defined.

The complexity of PBs is expected to increase in the future for reasons of very different nature, as listed in the following:

• The so‐called ‘circular economy’ is a major driver of the development of biostimulants, which means that more agricultural, urban and industrial waste materials will be recycled as fertilizing products on agricultural and horticultural crops. Such waste‐derived products are highly complex substances.
• Although substances and microbes have been regarded as two distinct types of biostimulants and have mobilized different scientific and technical expertise so far, new insights have generated the prospect that the combination of both in innovative fertilizing products is promising in terms of product characteristics, providing both complementarity and synergism.
• Commercial – good and not so good – reasons prompt companies to elaborate unique products and manufacturing processes, mixing multiple active ingredients. Combining multiple bioactive compounds maximizes the chance to deliver some positive effects on plants, which seems of interest when marketing biostimulants of poorly understood action mechanisms, and intellectual property on the manufacturing process gives the opportunity to secure the profitability of R&D investments.
• Combining bioactive compounds to unravel new and valuable characteristics in the final product is a driver for the development of complex biostimulants. The combination of bioactive compounds may result from their co‐existence in complex substances ab initio (like in seaweed extracts), but can also be obtained by the intentional blending of compounds and substances.

The speculation that the biological effects of biostimulants are due to novel, emerging properties of complex substances has led to the suggestion that it can be used as a criterion to define biostimulants and allows the differentiation from known nutritional and hormonal effects [3]. A biostimulant is defined by Yakhin et al. as ‘a formulated product of biological origin that improves plant productivity as a consequence of the novel or emergent properties of the complex of ...

Table of contents

• Cover
• Table of Contents
• List of Contributors
• Series Preface
• Preface
• Part I: Introduction
• Part II: Examples of Plant Biostimulants
• Part III: Methods to Screen for New Biostimulants
• Part IV: Biostimulants' Mode of Action
• Part V: Biostimulants – A Practical Guide
• Index
• End User License Agreement