Testing biostimulants to validate the claims through multi-scale assays and a meta-analysis

Biostimulants are made available on the market according to their intended effects on crop plants. The European law has set a legal definition for “plant biostimulants” based on their effects on crop plants and/or soil, referred to as “claims” (Regulation (EU) 2019/1009). Consequently, standardized procedures will be required to validate the claims of biostimulants. Which protocols will prove sufficient to demonstrate their efficacy still needs to be determined. Although many data are available, from the laboratory to the field, it seems important to generate parallel data sets by using the same biostimulants at the laboratory, greenhouse and field scales. By combining the data sets, the relevance of the different tests and the efficacy of the field and contained use approaches can be best evaluated and compared. In order to set up an evaluation platform for biostimulants in the Walloon Region of Belgium, the project BioStimTest has been launched by a gathering of public and private partners. Organic and mineral (as well as microbial) commercial biostimulants are being tested on winter wheat, maize and potato with field trials. On the other hand, greenhouse and laboratory bioassays are performed using monocotyledonous and dicotyledonous plant models and include seed germination tests and plant cultivation under more or less artificial conditions (in vitro assays, hydroponic culture and pot plants in greenhouse). Improvement of nitrogen use efficiency and of drought tolerance are the two biostimulant claims considered by the project. Accordingly, different fertilization regimes and rain supplies limitation by controlled crop coverage are used in the field experiments, under the temperate climate of Belgium, whereas contrasted nitrogen or water supply or polyethylene glycol use are implemented in laboratory and greenhouse bioassays. Data produced by these bioassays and plant performance in field trials are compared with a meta-analysis in order to develop claims-validating protocols at the laboratory and/or greenhouse scale which are most predictive of efficacy under field conditions.