Diurnal courses of photosystem II (PS II) activity of Xanthoria elegans were continuously monitored using a novel technique of chlorophyll fluorescence imaging in the field (maritime Antarctica) over a period of 7 days. The lichen specimens grew on rock surfaces in a north-facing coastal area of Galindez Island (Argentine Islands). A portable fluorometer FluorCam equipped with a CCD camera and image analysis software was placed over a thallus so that false colour images of FV/FM and quantum yield of photochemical processes in PS II (ΦII) distribution over the thallus could be taken over the course of a day. Simultaneously, microclimatic parameters of the habitat were recorded: air and thallus temperatures, relative air humidity, PPFR (photosynthetic photon fluence rate) and global radiation. Photosynthetic processes in X. elegans were activated by rainfall or water from melting snow. After thallus hydration, FV/FM and ΦII gradually decreased from their maximum values due to evaporation and progressive loss of water from the thallus. Chlorophyll fluorescence imaging showed that the thallus dehydration started from the margins and moved towards the central thicker thallus zones. These zones exhibited higher activity of PS II than the margins throughout the day. The rate of inhibition of photosynthetic processes in PS II was faster at the margins than in the centre of the thallus. Dependence of ΦII on thallus water potential (WP) was investigated during gradual dehydration under laboratory conditions. After a decrease in WP from zero to −7 MPa, X. elegans exhibited only a 13·8% decrease in ΦII from its maximal value. Within the range of WP of from −8 to −20 MPa, the ΦII decrease was more rapid reaching the critical point (ΦII=0) at WP of approximately −25 MPa. Chlorophyll fluorescence imaging and WP measurements showed that X. elegans is capable of maintaining detectable photosynthetic activity even at extremely low WP. Based on our microclimatological data, X. elegans may be photosynthetically active under field conditions for several hours after being wetted by rain or snowfall. It is suggested that chlorophyll fluorescence imaging is a powerful technique, transferable to field conditions and capable of visualizing heterogeneity of photosyntetic processes over a lichen thallus subjected to periodic dehydration.

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Nine temperate grass species were screened for their potential to grow in the low-light conditions typical of gaps in dense vegetation. To this end, photosynthetic photon flux densities (PFD) were simulated in a growth chamber (PFD 100, 50 or 25 lmol photons m -2 s -1 ). Relative and absolute growth rates (RGR and AGR, respectively) of the species were regressed on ten different ecophysiological and morphogenetic plant attributes. No significant relationships were found between plant attributes and relative growth rate, while six attributes explained a significant proportion of the interspecific variance in absolute biomass growth: net photosynthetic rate at growth PFD (P net ) (75.5%), leaf apparent quantum yield of CO 2 fixation (62.5%), leaf dark respiration rate (65.2%), leaf compensation PFD (71.0%), root: shoot ratio (66.4%) and plant nitrogen content on a mass basis (42.0%). Only species with extremely low allocation to roots and very high (relatively speaking) net photosynthetic rates were able to grow fast in low light. Specific leaf area (SLA), instantaneous photosynthetic nitrogen use efficiency (PNUE) and leaf nitrogen content on a mass basis as well as on an area basis were not significantly related to growth. The absence of effects of plant traits on RGR, unlike for AGR, could arise from a relationship that we observed between AGR and a fitted start value of the biomasstime course (i.e. seed mass or germination time). This suggests that interspecific differences in the very early growth stages of the plants were responsible for differences in successful development under low light, rather than differences in RGR. Based on its high explanatory power, its relative constancy with plant age and the lack of effect of growth PFD, P net would be the best candidate for characterizing potentially shadetolerant species that are likely to establish in dense vegetation in the field.