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Photosystem activity status of the green algal (Pseudocyphellaria lividofusca) and cyanobacterial (P. knightii) components of a photosymbiodeme were continuously monitored in the field over a period of 35 days. The photosymbiodeme grew on a Nothofagus menziesii tree at Lake Waikaremoana, Urewera National Park, North Island, New Zealand. Two Mini-PAM fluorometers were placed so that the chlorophyll a fluorescence, temperature and PPFD (photosynthetically active photon flux density) could be recorded every 30 min for green algal and cyanobacterial parts of the thallus. Microclimate conditions were also recorded with a datalogger. The study confirmed the already known ability of green algal lichens to reactivate from high humidity alone whilst cyanobacterial species need liquid water, here obtained from rainfall. The photosystems of P. lividofusca were activated on every day and positive ETR (relative electron transport rate) occurred on all but 3 days. Activation level depended on the overnight relative humidity. P. knightii was activated and had positive ETR on only 13 days when rainfall had occurred. Both species were mostly inactive above 12°C but differed at low temperatures. P. knightii showed no activation at very low temperatures, –2 to 0°C, since these only occurred on clear, rain-free nights. PPFD was always very low, mostly below 80 µmol m–2 s–1, and both species were inactive at higher PPFD. The three-dimensional structure of the thallus seemed to contribute to the hydration. The cyanobacterial sectors were more appressed to the trunk and needed substantial rainfall to rewet whereas the green algal lobes were more distant from the trunk and this probably caused more rapid desiccation as well as lower temperatures. It is suggested that the longer active periods for photosynthesis by P. lividofusca are balanced by several factors: first, depressed net photosynthesis at high thallus water contents after rainfall, a feature not shown by P. knightii; second, possible lower maximal net photosynthetic rates; and third, the possibility of greater respiratory rates when thalli have been hydrated by high relative humidity. There is little evidence for high PPFD differently affecting the photosymbiodeme components since sustained, high PPFD did not occur. It has been reported that the photosystems of cyanobacterial species from photosymbiodemes can reactivate at high relative humidity but the results obtained here suggest that it is not ecologically significant. [ABSTRACT FROM AUTHOR]

A lichen growing in a continental Antarctic region with low temperatures and strong irradiance in summer was investigated for evidence of photoinhibition. Field experiments with Umbilicaria aprina from a sheltered site with heavy snowpack showed no effects of photoinhibition when the lichen was exposed to strong sun irradiance for nearly 11 h a day. This was evident from CO2 exchange and simultaneous chlorophyll a fluorescence measurements. CO2 exchange was also not affected if quartz glass allowing greater UV penetration, was used as a lid for the cuvette. The dependency of net photosynthesis on photosynthetic photon flux density suggests that the lichen is photophilous. [ABSTRACT FROM AUTHOR]

Many lichens show seriously depressed net photosynthesis (NP) at high thallus water contents due to increased carbon dioxide diffusion resistance through blockage of diffusion pathways by water. The soil lichen Diploschistes muscorum, however, shows no depression and NP is close to maximal even at the highest thallus water content. We investigated whether lichen substances (lecanoric and diploschistesic acids) in the cortex and medulla contributed to this ability to maintain high NP. Dry thalli were extracted with water-free acetone and, after this treatment, were found to be fully viable to the extent of continued growth after replanting in the field. No differences were found in the response of NP to thallus water content between the normal and extracted thalli, in fact the response curves were often nearly identical. Thus, in this species it seems that lichen substances did not maintain the water-free diffusion pathways and some other explanation, possibly structural, needs to be sought. [ABSTRACT FROM AUTHOR]

CO 2 exchange rate in relation to thallus water content (WC, % of dry weight) was determined for 22 species of lichens, mainly members of the genera Pseudocyphellaria and Sticta, from a temperate rainforest, Urewere National Park, New Zealand. All data were obtained in the field, either using a standard technique in which the lichens were initially wetted (soaked or sprayed, then shaken) and allowed to slowly dry, or from periodic measurements on samples that were continuously exposed in their natural habitat. A wide range of WC was found, with species varying from 357 to 3360% for maximal WC in the field, and from 86 to 1300% for optimal WC for photosynthesis. Maximal WC for lichens, wetted by the standard technique, were almost always much less than the field maxima, due to the presence of water on the thalli. The relationships between CO 2 exchange rate and WC could be divided into four response types based on the presence, and degree, of depression of photosynthesis at high WC. Type A lichens showed no depression, and Type B only a little at maximal WC. Type C had a very large depression and, at the highest WC, CO 2 release could occur even in the light. Photosynthetic depression commenced soon after optimal WC was reached. Type D lichens showed a similar depression but the response curve had an inflection so that net photosynthesis was low but almost constant, and never negative, at higher WC. There was little apparent relationship between lichen genus or photobiont type and the response type. It was shown that high WC does limit photosynthetic CO 2 uptake under natural conditions. Lichens, taken directly from the field and allowed to dry under controlled conditions, had net photosynthesis rates that were initially strongly inhibited but rose to an optimum, before declining at low WC. The limiting effects of high WC were clearly shown when, under similar light conditions, severe photosynthetic depression followed a brief, midday, rain storm. Over the whole measuring period the lichens were rarely at their optimal WC for photosynthesis, being mostly too wet or, occasionally, too dry. Photosynthetic performance by the lichens exposed in the field was similar to that expected from the relationship between the photosynthetic rate and WC established by the standard procedure.