Do laboratory measurements misrepresent leaf water repellency? Impacts of sampling season and measurement environment

Leaf water repellency (LWR) plays a crucial role in the interfacial interactions between plant leaf surfaces and water. Nevertheless, whether measurement environments affect the characterization of LWR remains poorly understood. Meanwhile, uncertainties regarding the relationship between droplet static morphology and critical dynamics of water droplets on leaf surfaces hinder the elucidation of the ecological functions of LWR. Thirty evergreen plant species were selected in a humid urban environment, and adaxial contact angle (CA) and retention angle (RA) were measured under field and laboratory conditions during summer and winter. Mixed-effects models were used to access the effects of measurement environment and season. Measurement environment significantly affected RA, with field-measured values being 11.7° higher than laboratory measurements, whereas CA showed no significant difference between the two environments. Relative to summer, CA and RA decreased significantly by 14.1° and 21.4° respectively, in winter. CA and RA were positively correlated in our observations, with a stronger correlation in winter than in summer. By integrating our observations with previous studies, we found a concave-down parabolic relationship between CA and RA. These findings that the characterization of LWR depends on the measurement metric and environmental conditions. Standardized sample preparation and measurement conditions are therefore important for reliable assessment of LWR and for understanding vegetation-mediated water interactions.