Apparent electron transport rate - a non-invasive proxy of photosynthetic CO 2 uptake in lichens.

Main Conclusion: During desiccation, both apparent electron transport rate (ETR _app ) and photosynthetic CO ₂ uptake peak when external water has evaporated. External water, causing suprasaturation, weakens the strong correlation between ETR _app and CO ₂ 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 CO ₂ uptake. Hydration level, however, is a complicating factor, particularly during suprasaturation that strongly reduces CO ₂ diffusion. Here, the cephalolichen Lobaria pulmonaria and two chlorolichens Parmelia sulcata and Xanthoria aureola were excessively hydrated before photosynthetic CO ₂ uptake and Φ _PSII using imaging fluorescence tools were simultaneously measured while drying at 200 µmol photons m ^-2  s ^-1 . CO ₂ 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. CO ₂ 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 CO ₂ 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 CO ₂ uptake. For new lichen species, calibration of ETR _app against CO ₂ 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 CO ₂ uptake in thalli without external water but overestimates photosynthesis during suprasaturation.