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

Surviving the effects of 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.

We measured sodium levels in coastal and inland leaves exposed to experimental salt spray in the laboratory and compared leaf surface traits that may contribute to differences in sodium uptake between ecotypes. We assayed tissue tolerance by allowing sodium to enter leaf discs through the cut edges and recording time to necrosis for coastal and inland accessions.

The leaf surface of coastal accessions took up less sodium from the experimental salt spray than the inland accessions, which may contribute to local adaptation to oceanic salt spray in coastal habitats. Higher water content per unit leaf area and less water loss under salt spray further reduced sodium concentrations in salt-sprayed coastal leaves compared to inland accessions. The coastal ecotype also had greater tissue tolerance to sodium than the inland ecotype.

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.