Photooxidative stress can therefore be an important limiting factor for poikilohydrous auto-
trophs. Bryophytes and algae that are restricted to shady habitats were shown to have limited
photoprotective capacities (O
¨
quist and Fork 1982b). Negative effects of strong light were
observed in hydrated lichens in the tropical, temperate (Coxson 1987a,b), and Mediterranean
region (Manrique et al. 1993, Valladares et al. 1995). Nevertheless, tolerance to strong light
can be enhanced by acclimation. Cyanobacterial mats taken from exposed habitats proved to
be highly tolerant to high irradiance, whereas cyanobacteria from shaded sites were very
sensitive (Lu
¨
ttge et al. 1995). Field studies have revealed, for instance, that the cyanobacterial
lichen Peltigera rufescens was at least photoinhibited under certain conditions in winter
(Leisner et al. 1996). On the other hand, cryptogam species in Antarctica such as Umbilicaria
aprina, Leptogium puberulum, Xanthoria mawsonii,andHennediella heimii were very resistant
to the combination of low temperatures and high irradiance while the thallus was photosyn-
thetically active (Schlensog et al. 1997, Kappen et al. 1998a, Pannewitz et al. 2003, 2006).
In hydrated autotrophs, photosynthetic productivity is maintained because only that part
of the light energy that is in excess to that used for energy conservation is thermally dissipated
by a mechanism that requires zeaxanthin, a carotenoid of the xanthophyll cycle, and the
protonation of a special thylacoid protein (Niyogi 1999, Heber et al. 2006). Thermal energy
dissipation should be in equilibrium in hydrated autotrophs with ongoing photosynthesis.
This means that energy dissipation is in equilibrium with energy conservation based on charge
separation, the production of a strong oxidant and a reductant in the reaction centers of PS II.
If energy dissipation caused is speeded up (photostress), it would inhibit photosynthesis
(Wiltens et al. 1978, O
¨
quist and Fork 1982a, Demmig-Adams et al. 1990b). Downregulation
of photosynthetic processes and the so-called dynamic or recoverable photoinhibition (i.e.,
inhibition of photosynthesis by light, but no damage) has been observed in a number of
poikilohydrous plants, bryophytes, and lichens (e.g., Seel et al. 1992, Leisner et al. 1996,
Ekmekci et al. 2005), and as a result, avoidance of photooxidation (Eickmeier et al. 1993,
Valladares et al. 1995, Calatayud et al. 1997, Heber et al. 2000, 2001, Bukhov et al. 2001).
A prevention of photooxidative damage by drying may be apparent from the fact that
isolated Trebouxia as well as green-algal lichens resisted photostress in the field by quick
desiccation under high irradiances (O
¨
quist and Fork 1982b, Leisner et al. 1996). This would
resemble in effect the strategy of poikilochlorophyllous plants that radically destruct the
photosynthetic apparatus during desiccation (Smirnoff 1993). It was hypothesized that
the photosynthetic apparatus of homoiochlorophyllous autotrophs cannot be affected by
strong irradiance because it undergoes a functional dissociation between light harvesting
complexes and photosystem II during desiccation (Bilger et al. 1989, Smirnoff 1993). However,
water content has been proved to influence both dynamic and chronic photoinhibition of
lichens (Valladares et al. 1995, Calatayud et al. 1997). Some air-dried lichens typical of shady
habitats exhibited even damage after exposure to high light (Valladares et al. 1995, Gauslaa and
Solhaug 1996, 1999, 2000, Gauslaa et al. 2001). In addition, stenopoikilohydrous mosses were
more damaged by drying at high irradiance than at low irradiance (Seel et al. 1992a).
According to recent findings since Shuvalov and Heber (2003), it has become apparent
that reaction centers are capable of charge separation even in the absence of water (Heber
et al. 2006a). Thus, functional reaction centers would cause damaging oxidative reactions. A
revised and more comprehensive approach to understanding photoprotection in desiccated
autotrophs has recently come from Heber and coauthors (Heber et al. 2000, 2001, Heber and
Shuvalov 2005, Kopecky et al. 2005, Heber et al. 2006a,b) who have demonstrated that more
than one photoprotective mechanism of energy dissipation is active in lichens and bryophytes.
Available evidence suggests that zeaxanthin-dependent energy dissipation remains active
upon desiccation (Eickmeier et al. 1993, Kopecky et al. 2005, Georgieva et al. 2005), but it
is not clear whether the zeaxanthin-dependent energy dissipation is fast enough to prevent
charge separation in functional reaction centers particularly in lichens and xeric bryophytes.
Francisco Pugnaire/Functional Plant Ecology 7488_C002 Final Proof page 30 30.4.2007 7:54pm Compositor Name: DeShanthi
30 Functional Plant Ecology