Wetting and drying cycles have been studied in much detail for lichens in arid habitats. Photosynthesis of lichens is related in a complex and delicate way to the transient water conditions of the thallus (Fig. 11.22). At very low water content, i.e. below 20% of dry weight, the lichens are metabolically dormant showing neither photosynthesis nor respiration. Between 20% and 50% water content, photo-synthetic net CO2-uptake increases sharply and then reaches a plateau as optimal water content is attained. However, when the water content increases further and thalli are fully saturated with water, CO2-assimilation is depressed. This is due to increased limitation of photosynthesis by CO2 diffusion when the capillary system of the lichen thallus is infiltrated. Thus, upon drying the assimilation rates may first increase again and then decline as the thalli desiccate (Lange 1988).
Both protection and repair mechanisms are essential during desiccation as well as rehydration. Reactive oxygen species can be formed during desiccation and rehydration, especially when the use of exitation energy of the photosynthetic apparatus by photochemical work is reduced. Protection mechanisms such as by the redox state of glutathione are effective in both lichens and mosses (Kranner 2002; Mayaba et al. 2002). However, we must assume that in the desiccated state water structures required for enzymatic reactions including photochemical work of CO2-reduction and photorespiration as well as epoxidases and de-epoxidases of the xanthophyll cycle (Sect. 4.1.4, Box 4.4) are not intact, and therefore, these excitation-energy using processes are negligible in dry homoiochlorophyllous material.
Thus, by the excitation of chlorophyll in homoiochlorophyllous plants in the dry state high irradiance causes photo-oxidative stress. An interesting feature of
homoiochlorophyllous desiccation tolerant lichens and bryophytes is the loss of ground fluorescence of chlorophyll a, F (see Sect. 4.1.7) in the desiccated state in great contrast to desiccation tolerant vascular plants. While F decreases during desiccation and increases again during rewetting in mosses and lichens, the opposite dynamics are found in vascular plants (Lange et al. 1989; Calatayud et al. 1997; Eickmeier et al. 1993; Heber et al. 2000, 2001).
High irradiance causes photodamage in dried vascular plant leaves but not in dried mosses (Heber et al. 2000). This shows that the reduction of F to very low levels in the latter can be considered as a protection mechanism against photodamage under full sunlight on exposed rock surfaces of inselbergs in the dehydrated stage. Effective mechanisms of chlorophyll fluorescence quenching mediate the conversion of excitation energy into heat when metabolism can no longer control it. Two
processes are involved in this, namely zeaxanthin-dependent energy dissipation in the antenna of photosystem II (see Sect. 4.1.4) and desiccation induced thermal energy dissipation in the reaction centres (RCs) of photosystem II (Deltoro et al. 1998; Heber et al. 2000,2006a,b; Bukhov et al. 2001). The latter is essential in desiccated lichens and mosses because zeaxanthin does not protect RCs directly from photo-oxidation and in the absence of photosynthetic electron transport dynamics in the dry state dissipation must be extremely fast given the half-lives of first and second singlet excited states of chlorophyll of 10-11 to 10-9 and 10-15 to 10-13, respectively (Box 4.3).
Homoiochlorophylly is an important prerequisite of rapid recovery upon rewa-tering (Tuba et al. 1996b; Csintalan et al. 1999). Kinetics of recovery can vary considerably among mosses (Csintalan et al. 1999; Proctor 2000; Proctor and Smirnoff 2000). In the example of Fig. 11.23 the recovery of F has a very fast initial phase with a drastic increase within less than 1 min followed by a more gradual increase lasting much longer. Protein synthesis is not required during the fast initial phase (Proctor and Smirnoff 2000), but the gradual increase following the first rapid phase suggests that in addition to an immediate reactivation slower repair mechanisms involving protein synthesis, such as that of the D1 protein may be involved (Proctor and Smirnoff 2000).
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