Evaluating leaf water potential at turgor loss to assess drought tolerance in temperate and Mediterranean shrub species for dry urban environments

Shrubs represent a substantial proportion of the plant material used in urban plantings, owing to their compact size, variety of forms, and diverse habitat requirements. These elements serve as effective design components within urban green spaces, especially in sites where space for large trees is severely limited. Urban sites are characterised by stress for plants, due to limited space and challenging, often arid, growing conditions. Water stress is a significant limiting factor for plant growth in urban areas. Consequently, practitioners require reliable, quantitative data on shrub drought tolerance to guide their decisions. In this study, leaf water potential at turgor loss ( Ψ P 0 ) has been selected as a screening trait. The Ψ P 0 It is a highly instructive plant trait and a quantifiable metric of physiological drought tolerance. The objective of this study is to ascertain the Ψ P 0 of 20 temperate and 20 Mediterranean shrub species cultivated within the Royal Botanic Gardens, Kew, in London. Moreover, this study aims to determine whether Ψ P 0 is a reliable screening trait for evaluating drought tolerance across distinct shrub species from different biomes. The findings indicate that the values ranged from -3.80 MPa ( Bupleurum fruticosum ) to -1.61 MPa ( Phlomis fruticosa ). This study demonstrates that there is no significant difference between Mediterranean and temperate species overall. Still, there is considerable variation in drought-coping mechanisms among species in both biomes, with species adopting strategies tailored to their needs and ecological niches. However, it is hypothesised that many Mediterranean species, such as Teucrium ackermanii , Phlomis grandiflora , Cistus albidus , and Halimium halimifolium , might have evolved drought-avoidance strategies and, consequently, did not exhibit the anticipated negative Ψ P 0 . Therefore, it can be concluded that Ψ P 0 cannot be regarded as an exhaustive screening trait, and it is not always a reliable indicator of drought tolerance across distinct shrub species from different biomes. Further research is needed to evaluate the resilience of available, common, and novel shrub species to current and future urban climate scenarios. This assessment should be customised to the species' provenance and specific ecological niches, and it is imperative to identify which leaf, stem, and root traits are instructive in characterising a species' response to drought. • This study demonstrates that there is no significant difference between Mediterranean and temperate species as a whole, but considerable variation in drought-coping mechanisms at the species level in both biomes, where species adopt strategies tailored to their needs and specific ecological niches. • Most Mediterranean shrub species did not demonstrate the anticipated negative turgor loss point Ψ P 0 response. • Mediterranean plants have evolved traits that facilitate drought avoidance, including deep root systems and reduced growth of leaves and stems. • The mean Ψ P 0 value for all 40 shrub species and cultivars in this study was -2.54 MPa. • The overall species Ψ P 0 value ranged from -3.80 MPa ( Bupleurum fruticosum ) to -1.61 MPa ( Phlomis fruticosa ).