Impact of construction-derived substrates on seed germination and early development of Borago officinalis: insight for sustainable management of urban green spaces

Urban vegetation is exposed to intense environmental stress and anthropogenic disturbance, which can strongly constrain plant growth and development. Urban soils differ from natural soils in structure and functionality, being characterized by high compaction and elevated levels of pollutants. They frequently contain anthropogenic materials such as construction and demolition waste (CDW) and asphalt residues (AS), which further alter soil features. Due to their abundance and inert nature, the reuse of these materials as alternative substrates for urban green infrastructure warrants experimental investigation. This study assessed the effects of asphalt and two types of sands frequently used in pavement construction, namely standard sand (SS) and recycled sand (RS), on the germination and early development of Borago officinalis. Seeds were sown in trays filled with the test substrates, while potting soil (SO) served as control treatment. We monitored seed germination success, seedling emergence, mortality, biomass accumulation, and evaluated morpho-anatomical and physiological traits such as leaf area, shoot and root dry weight, stomatal density, stomatal conductance, and photochemical efficiency. Seedling emergence and survival were significantly reduced in AS, where mortality reached 60% and leaf development was strongly delayed. As expected, SO plants exhibited the highest biomass and physiological performance, with greater transpiration and lower stomatal density, indicating favourable water use efficiency. Overall, our findings suggest that while AS is unsuitable for early plant development, recycled CDW materials may serve as alternative, though suboptimal, components in the construction of urban Technosols. Their reuse could ease landfill pressure, limit the environmental costs of natural soil exploitation, and support key ecosystem services, thus aligning with circular economy goals and fostering more sustainable urban greening.