The conventional interpretation of the Permanent Wilting Point (PWP) as a fixed soil suction value (1500 kPa) is not only agronomically limited but also a significant barrier to sustainable urban water management. This approach overlooks the dynamic soil-plant-atmosphere continuum, resulting in inefficient irrigation practices in green infrastructure that exacerbate water demands and strain urban metabolism. This study introduces a sustainability-focused framework that redefines wilting thresholds based on plant physiological responses to soil-water availability. Through systematic experiments on a common urban grass (Axonopus compressus) in contrasting soils (clayey loam vs. silty sand), we demonstrate that PWP is highly soil-specific, occurring at 1600 kPa for the clayey loam but only at 300 kPa for the silty sand. By correlating soil suction with stomatal conductance and photosynthetic efficiency, we reveal that fine root architectures in coarser soils dramatically enhance water-use efficiency, doubling plant drought resilience. These findings challenge the one-size-fits-all model of irrigation and pave the way for a sustainability transition in urban planning. We emphasise that adopting soil- and species-specific PWP frameworks is crucial for selecting climate-resilient vegetation, optimising water resources, and enhancing the circular metabolism of future cities.

Sustainability, Metabolism, Wilting point, Unsaturated soil, Drought