The aquatic liverwort Jungermannia exsertifolia subsp. cordifolia was cultivated for 15 d under controlled conditions to study the single and combined effects of cadmium and enhanced ultraviolet (UV) radiation. Both cadmium and UV radiation caused chlorophyll degradation and a decrease in the maximum quantum yield of photosystem II (PSII), together with an increase in the mechanisms of non-photochemical dissipation of energy (increase in the xanthophyll index). Cadmium was more stressing than UV radiation, since the metal also influenced photosynthesis globally and caused a decrease in net photosynthetic rates, in the effective quantum yield of photosynthetic energy conversion of PSII, and in the maximal apparent electron transport rate through PSII. Ultraviolet radiation increased the level of trans-p-coumaroylmalic acid and cadmium increased trans-phaselic and feruloylmalic acids. The increase in these compounds was probably related to both a more efficient absorption of harmful UV radiation and an enhanced protection against oxidative stress. DNA damage was specifically caused by UV-B radiation, but was intensified under the presence of cadmium, probably because the metal impairs the DNA enzymatic repair mechanisms. Ultraviolet radiation and cadmium seemed to operate additively on some physiological processes, while other responses were probably due to either factor alone.

Temporal physiological variations in relation to environmental factors, in particular to ultraviolet (UV) radiation, have been studied in bryophytes from circumpolar latitudes, but not in mid-latitudes with longer growing seasons. In addition, seasonal and interannual changes in individual UV-absorbing compounds (UVAC) have not been previously studied in bryophytes. To fill these gaps, samples of the aquatic liverwort Jungermannia exsertifolia subsp. cordifolia were collected on a monthly basis during 3 years from a mountain stream in northern Spain. Sclerophylly index, chlorophyll fluorescence, DNA damage, the bulk UV absorbance of methanolic extracts and the concentration of five UVAC (hydroxycinnamic acid derivatives) were measured. Interannual changes were little marked, probably because the 3 years studied were environmentally similar. In summer-autumn, with respect to seasonal variations, newly grown tender young shoots with high F(v)/F(m) accumulated higher amounts of several hydroxycinnamic acid derivatives than in winter-spring. DNA damage was not detected in any of the samples analyzed. p-Coumaroylmalic acid was the compound best associated with radiation changes, and the best model explaining UV-B took into consideration the concentration of this compound and the ozone level. The specific effects of UV radiation could not be separated from the effects caused by other environmental factors, such as global radiation or temperature, because all these variables were correlated. However, indirect evidence strongly suggests that seasonal changes in bulk UV absorbance and p-coumaroylmalic acid are mainly driven by UV radiation. This compound may be a promising physiological variable to be used for UV bioindication.

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We analyzed the form of the UV-absorption spectra and calculated the bulk UV-absorption capacity of the methanolic extracts (BUVACME) in 15 bryophytes (five liverworts and 10 mosses) from unshaded aquatic habitats of Tierra del Fuego (Argentina). BUVACME was obtained as the area under the absorbance curve (AUC) in several ranges of the UV band to allow comparison with the literature data. Both the spectra form and the AUC values depended on the species considered. The spectra showed either no, one or two defined peaks. The peaks were probably due to phenolic derivatives, which could act as both screening compounds and antioxidants. The different AUCs calculated were highly and significantly correlated, and thus it may be unimportant which of these is used. The AUC values of most liverworts were higher than those of most mosses and in particular, the liverworts Noteroclada confluens and Triandrophyllum subtrifidum showed much higher BUVACMEs than those analyzed in any other bryophyte. Thus, the accumulation of UV-absorbing compounds might often increase protection against UV radiation in liverworts, but rarely in mosses. This is in line with the evolutionary differences between these two groups of bryophytes. Except for the two abovementioned liverwort species, BUVACME of aquatic bryophytes from Tierra del Fuego is not particularly different to that found in bryophytes from other zones of the planet.