Phenotypic Cuticle Plasticity at High Elevation: Is Microstructure and Microchemistry Related to Water Permeability?

Kalmia procumbens (K. procumbens), a ubiquitous alpine dwarf shrub, thrives at high elevations, particularly on wind-exposed sites. Plants on contrasting north- and southeast-facing slopes at ~2237 m elevation exhibit differences in leaf colour and growth, suggesting acclimative strategies. Leaves from the southeast-facing slope, exposed to higher leaf temperatures (54°C), stronger winds (21 m ∙ s^-1), and increased solar irradiation (2319 µMol photons m^-2 ∙ s^-1), developed thicker cuticles (16 µm) than leaves from north-facing slope (11 µm). Raman imaging revealed that cutin and triterpenoids built the foundation in all cuticles on the adaxial and abaxial leaf sides sampled from the two contrasting slopes, while flavonoids accumulated mostly in the outer adaxial cuticle layer and reached the highest values at the N-site. The thicker cuticle of the S-site was mainly composed of cutin and triterpenoids, while the flavonoids were restricted to a thinner outer layer. Minimum diffusive conductance (g_min) was lower in the S-site leaves, which may be associated to their thicker cuticle. The water permeability (g_min) increased exponentially with temperature in leaves from both slopes. Under heat, above 38°C, north-facing leaves with higher flavonoid content lost increasingly more water. While the flavonoids will defend bacterial and fungal pathogens and have a vital role in enhancing plant resilience, they seem to promote higher water permeability of the cuticle of K. procumbens. By combining physiological, structural and chemical insights, our findings suggest that micro-environmental factors play a significant role in driving acclimative responses in K. procumbens. Cuticle structure, composition and function are finely tuned to alpine microhabitats and illustrate a distinct potential for phenotypic adjustment to environmental and biotic constraints.