How do plant biostimulants work? Claims and mechanisms of plant biostimulation

Plant biostimulants have been accepted as a new regulatory category of fertilising products in different parts of the world, but their properties and use still need to be clarified. The main reason for this is that their definition is based on claims of agricultural effects – which are increased nutrients use efficiency, improved tolerance to abiotic stress, enhanced product qualities and increased availability of soil nutrients, according to the new regulation (EU) 2019/1009 – but how these claims should be substantiated in practice is still unclear, generating skepticism among growers. The EU approach consists of defining harmonized standards, relating to agreed definitions, quality criteria and protocols for generating data on product efficacy. Compliance of the data package provided by the companies with the adopted standards will be regarded as sufficient to validate product claims, as indicated on the labels of CE-marked products.

Notwithstanding the regulatory developments, achieving confidence in the products will depend on the scientific knowledge of the mechanisms of biostimulation. Although many scientific articles on biostimulants have been published in peer-reviewed journals over the last decade, several bottlenecks can be identified in the understanding of biostimulants’ action. For example, many articles use single substances (which can be multicomponent), whilst most marketed products combine several biostimulant substances, sometimes added to micro-nutrients. This makes it difficult to assign the observed effects to individual constituents or to specific interactions between constituents. Secondly, biostimulants aim to interact with the life processes of plants, which respond to many environmental factors and also depend on the genetic makeup of the variety. Thirdly, a plant is now regarded as a ‘superorganism’, associated with microbes, and many biostimulants will interact with the plant-associated microbiota. Other difficulties stem from the blurred borders between biostimulation and biocontrol, and also between nutrients and biostimulants, with some nutrients acting on physiological processes in ways that are similar to those of biostimulants.

A roadmap to tackle some of the above issues was discussed.