Far-red light enhances photosynthesis in the shade-adapted old forest lichen Lobaria pulmonaria

Far-red (FR) light (700-750 nm) is strongly enriched beneath forest canopies, but its impact on photosynthesis in shade-adapted lichens is not well understood. We investigated the role of FR light in photosynthesis of Lobaria pulmonaria, an oldforest cephalolichen. Spectral measurements showed that FR photons contributed up to 32% of the extended photosynthetic photon flux density (ePPFD; 400-750 nm) under the canopy, compared to only 11% in open areas. Gas exchange data showed that FR light alone supported moderate CO₂ uptake, but photosynthetic rates increased markedly when 50-100% FR was combined with background red light (R), demonstrating the Emerson enhancement effect. When comparing absorbed photons, FR light was even more efficient than R. Chlorophyll fluorescence data indicated that conventional electron transport rate (ETR) calculations often overestimate photosynthetic activity under high FR, underscoring the need for improved methods. These results suggest that FR light is crucial for lichen growth and survival in shaded forests. We recommend using ePPFD sensors in ecological studies to more accurately quantify relevant light availability for photosynthetic organisms.