Biological complexity in rapid biostimulant screening across multiple seasons

Microalgae are a diverse group of photosynthetic microorganisms and offer a sustainable source of bioactive compounds for next generation plant biostimulants. In this study we screened the biostimulant effect of 15 aqueous microalgal extracts, obtained by maceration and centrifugation, using Arabidopsis thaliana root growth assays. Due to significant variations in root growth in treated and untreated (control) seedlings, even under controlled conditions, an integrative statistical strategy, incorporating dimensional reduction approaches, was employed to address this complexity. This analysis revealed significant seasonal variations in root growth, likely sensed by endogenous plants mechanisms, despite controlled growth conditions. By employing statistical methods and accounting for seasonal effects, Porphyridium purpureum and Chlorococcum sp. were identified as potent strains, consistently stimulating root growth by 15–45%. To the best of authors' knowledge, no prior studies have explicitly addressed this issue in biostimulant assays. The observed circannual rhythm suggests that results from in vitro assays may vary significantly depending on the season in which they are conducted, and that future studies should carefully consider this factor. These findings provide valuable insights for optimizing rapid root/germination screening assays and guiding extract selection for evaluating biostimulant potential.