Natural UV exposure alters photosynthetic biology and improves recovery from desiccation in a desert moss.

Plants in dryland ecosystems experience extreme daily and seasonal fluctuations in light, temperature, and water availability. We used an in-situ field experiment to uncover the effects of natural and reduced levels of UV on maximum PSII quantum efficiency (Fv/Fm), relative abundance of photosynthetic pigments and antioxidants, and the transcriptome in the desiccation-tolerant desert moss Syntrichia caninervis. We tested the hypotheses that: (1) S. caninervis plants undergo sustained thermal quenching of light (non-photochemical quenching; NPQ) while desiccated and after rehydration, (2) a reduction of UV will result in improved recovery of Fv/Fm, but (3) one year of UV removal will de-harden plants and increase vulnerability UV damage to photosynthetic efficiency. All field-collected plants had extremely low Fv/Fm after initial rehydration but recovered over eight days in lab-simulated winter conditions. UV-filtered plants had lower Fv/Fm during recovery, higher concentrations of photoprotective pigments and antioxidants such as zeaxanthin and tocopherols, and lower concentrations neoxanthin and chlorophyll b than plants exposed to near natural UV levels. Field-grown S. caninervis underwent sustained NPQ that took days to relax and for efficient photosynthesis to resume. Reduction of solar UV radiation adversely affected recovery of Fv/Fm following rehydration.