Foliar salt spray exclusion and tissue tolerance underlie local adaptation to oceanic salt spray

Premise Surviving under oceanic salt spray is critical for plants in coastal ecosystems, yet the mechanisms of coastal plant resilience to salt spray are not well understood. We investigated mechanisms of salt spray adaptation by comparing five latitudinal pairs of yellow monkeyflower accessions locally adapted to coastal and inland habitats. Methods We measured sodium levels in coastal and inland leaves exposed to experimental salt spray in the lab, and compared leaf surface traits that may contribute to differences in sodium uptake between ecotypes. We assayed tissue tolerance by allowing sodium to enter leaves through wounding and recording time to necrosis for coastal and inland accessions. Key Results Coastal monkeyflowers take up less sodium through the leaf surface under experimental salt spray, which may contribute to local adaptation under oceanic salt spray in coastal habitats. Higher water content per unit leaf area and less water loss under salt spray further reduce sodium concentrations in salt-sprayed coastal leaves compared to inland counterparts. The coastal ecotype also shows greater tissue tolerance to sodium than the inland ecotype. Conclusions Coastal monkeyflowers employ salt spray exclusion and tissue tolerance mechanisms for salt spray resilience. Our results suggest that plant adaptation to coastal habitats may often involve the evolution of multiple mechanisms to survive stress imposed by oceanic salt spray.