Boundary-line trade-off in bryophytes between UV photoprotection and photosynthetic capacity, but not desiccation tolerance.

<h4>Background and aims</h4>Coping with both UV-B radiation and desiccation was essential for early land plants. In bryophytes, UV-B screening and desiccation tolerance partially depend on the physico-chemical features of cell walls, which may also constrain photosynthesis. This study tested the relationship between the accumulation of UV absorbing compounds (UVACs) and photosynthetic capacity and desiccation tolerance.<h4>Methods</h4>Gas exchange, chlorophyll fluorescence, desiccation tolerance and UVACs were analyzed for Colombian, Antarctic, Spanish and Australian species collected from locations of contrasting UV exposure. Apparent photosynthesis, quantum efficiency and heat dissipation were measured using PAM-coupled systems. Desiccation tolerance was evaluated by recovery of Fv/Fm after long-term dry storage (14 days). Cell wall-bound and soluble UVACs were quantified through spectrophotometry after methanolic and alkali extraction. Trait relationships were assessed by correlation tests and quantile regression.<h4>Key results</h4>A boundary line trade-off was found between apparent photosynthesis and cell wall-bound UVACs: no species simultaneously exhibited both high photosynthetic rates and large amounts of UVACs in its cell walls. The three phyla of bryophytes were segregated across this gradient, with mosses showing the highest photosynthetic capacity, but lowest UVACs and liverworts and hornworts presenting the highest UVACs together with low photosynthetic rates. No trend was observed with chlorophyll concentration or quantum efficiency of photosystem II. Long-term desiccation tolerance was not correlated with either cell wall-bound nor soluble UVACs. On the contrary, it was associated with non-photochemical quenching, which is possibly linked to a minimum ROS scavenging capacity indispensable for recovering functionality after a long-term desiccation/rehydration cycle.<h4>Conclusions</h4>Bryophytes exhibit a boundary-line trade-off between UV protection and photosynthetic capacity, possibly driven by limitations to CO2 diffusion associated with the accumulation of cell wall-bound UVAC. In contrast, desiccation tolerance appears to rely on different physiological mechanisms, related to heat dissipation.<p></p>