Biostimulant potential of dark septate endophytes and Schizophyllum commune on petunia growing in peat-reduced and peat-free substrates

Introduction Peat has been widely used in horticultural substrates in Europe for decades due to its low cost, availability, and favorable physicochemical properties. However, its extraction from bogs contributes significantly to greenhouse gas emissions. Reducing peat use is therefore crucial to mitigate the environmental impact of horticultural production. This study aims to investigate the potential of fungal inoculation belonging to dark septate endophytes (DSEs): Rhexocercosporidium sp., Sordaria sp., and the basidiomycetous fungus Schizophyllum commune , and the impact of substrate sterilization by steaming to enhance petunia plant growth, nutrient availability, and substrate stability in peat-reduced and peat-free horticultural substrates. Methods To achieve the aim of the study, two greenhouse experiments using Petunia×hybrida were conducted: The first one assessed the DSEs Rhexocercosporidium sp. and a newly isolated Sordaria sp. as plant roots inoculants in peat-free substrates (50% wood fibers, 35% green compost, 10% perlite, 5% clay). The second experiment tested Schizophyllum commune as a substrate stabilizer in peat-reduced substrates (50% peat, 25% green compost, 25% digestate); all treatments were tested under steamed and non-steamed conditions. Fungal colonization of petunia roots by DSEs was confirmed microscopically, and plant biomass, nutrient content, and chlorophyll levels were measured after six weeks Results Rhexocercosporidium sp. significantly enhanced the biomass of plants grown in steamed peat-free substrates compared to the non-steamed substrates. In contrast, Sordaria sp. successfully colonized roots but did not promote significant growth. However, chlorophyll content improvement was only observed in plants grown in steamed peat-free substrates. Both the live and mock (autoclaved) inocula of Schizophyllum commune supported plant growth in both steamed and non-steamed substrates, suggesting indirect improvements to substrate conditions, although direct structural changes were not assessed in this study. Discussion These findings highlight the potential of the DSE strain Rhexocercosporidium sp. and Schizophyllum commune as functional bio-inoculants that support plant biomass production and improve nutrient dynamics in peat-reduced and peat-free substrates, thereby reducing reliance on peat and contributing to more sustainable horticultural practices.