Desert mosses withstand intense sunlight while desiccated and
metabolically inactive. We used in situ field experiments to uncover the
effects of natural and reduced levels of UV radiation on maximum
Photosystem II (PSII) quantum efficiency (Fv/Fm) and on the relative
abundance of photosynthetic pigments and antioxidants in Syntrichia
caninervis. We tested the hypothesis that if UV is a stressor, reduction
of natural UV levels will result in increased photosynthetic efficiency,
but that such reduction will de-harden plants and increase vulnerability
to PSII damage with UV exposure. We also measured photosynthetic
efficiency over a simulated winter recovery period to assess sustained
non-photochemical quenching (NPQ) and its subsequent relaxation.
Finally, we measured the effect of UV reduction on photosynthetic
pigment and antioxidant abundance. All field-collected plants had low
Fv/Fm at collection but recovered over eight days in winter conditions.
Plants in the low-UV treatment had lower Fv/Fm during recovery than
those exposed to natural UV levels and had higher zeaxanthin, lutein,
tocopherols, and a higher ratio of chlorophyll a to chlorophyll b.
Natural S. caninervis undergoes sustained NPQ that takes days to relax
and for efficient photosynthesis to resume. Reduction of UV radiation
from sunlight has adverse effects on recovery of Fv/Fm.