Diel Dynamics of Field Responses to UV and Photosynthetic Radiation in the Aquatic Liverwort Jungermannia eucordifolia and Their Potential Use in UV Biomonitoring

We investigated diel physiological responses of the leafy aquatic liverwort Jungermannia eucordifolia under natural field conditions during two consecutive days. Photosynthetically active radiation (PAR), UV-A and UV-B radiation, together with water temperature, were monitored throughout the study. Physiological responses included: (1) chlorophyll fluorescence assessed through steady-state fluorescence parameters and fast chlorophyll fluorescence induction kinetics (OJIP approach); (2) UV-absorbing phenolic compounds in the methanol-soluble and methanol-insoluble fractions, mainly representing vacuolar and cell wall-bound compounds, respectively, thereby covering different functional modalities of UV protection; both the bulk UV absorption capacity of UV-absorbing compounds and seven individual phenolic compounds were analyzed; and (3) DNA damage. Photosynthetic performance parameters, including the effective photochemical quantum yield of photosystem II (ΦPSII), the maximum photochemical quantum yield of photosystem II (Fv/Fm), and the Performance Index (PI), together with the photoprotection parameter non-photochemical quenching (NPQ), showed pronounced diel fluctuations closely associated with changes in ambient irradiance. ΦPSII, Fv/Fm, and PI were negatively related to irradiance, whereas NPQ exhibited the opposite trend. These responses were consistent with those previously observed in this liverwort under controlled laboratory conditions, suggesting dynamic photoinhibition accompanied by efficient photoprotection of photosystem II (PSII) against excess radiation, probably involving the xanthophyll cycle. Nevertheless, the strong covariation among PAR, UV-A, and UV-B wavebands prevented a clear distinction of their individual contributions. In contrast to chlorophyll fluorescence parameters, most variables associated with UV photoprotection through UV-absorbing compounds did not display consistent diel patterns under field conditions, although some individual compounds showed significant temporal fluctuations. This finding contrasted with previous laboratory experiments, where several UV-absorbing compounds increased under enhanced UV-B. DNA damage was not detected in any sample, consistent with previous field studies conducted under ambient UV-B levels but contrasting with experiments using enhanced UV-B exposure under either field or laboratory conditions. These results contribute to a better understanding of short-term physiological dynamics in aquatic bryophytes and may help improve UV biomonitoring protocols based on these organisms.