Apparent electron transport rate - a non-invasive proxy of photosynthetic CO₂ uptake in lichens

During desiccation, both apparent electron transport rate (ETR_app) and photosynthetic CO₂ uptake peak when external water has evaporated. External water, causing suprasaturation, weakens the strong correlation between ETR_app and CO₂ uptake. Lichens are poikilohydric organisms passively regulated by ambient conditions. In theory, apparent electron transport rate (ETR_app), estimated by photosystem II yield measured in light (Φ_PSII), is a proxy of photosynthetic CO₂ uptake. Hydration level, however, is a complicating factor, particularly during suprasaturation that strongly reduces CO₂ diffusion. Here, the cephalolichen Lobaria pulmonaria and two chlorolichens Parmelia sulcata and Xanthoria aureola were excessively hydrated before photosynthetic CO₂ uptake and Φ_PSII using imaging fluorescence tools were simultaneously measured while drying at 200 µmol photons m^-2 s^-1. CO₂ uptake peaked when hydration had declined to a level equivalent to their respective internal water holding capacity (WHC_internal) i.e., the water per thallus area after blotting external water. CO₂ uptake and ETR_app in all species were highly correlated at hydration levels below WHC_internal, but weaker at higher hydration (chlorolichens) or absent (cephalolichen). Yet, at a specimen level for the two chlorolichens, the correlation was strong during suprasaturation. The CO₂ uptake-ETR_app relationship did not differ between measured species, but may vary between other lichens because the slope depends on cortical transmittance and fraction of electrons not used for CO₂ uptake. For new lichen species, calibration of ETR_app against CO₂ uptake is therefore necessary. At intrathalline scales, Φ_PSII during drying initially increased along thallus margins before reaching maximum values in central portions when hydration approached WHC_internal. WHC_internal represents the optimal hydration level for lichen photosynthesis. In conclusion, ETR_app is an easily measured and reliable proxy of CO₂ uptake in thalli without external water but overestimates photosynthesis during suprasaturation.