The paradox of higher light tolerance during desiccation in rare old forest cyanolichens than in more widespread co-occurring chloro- and cephalolichens

Abstract

Desiccation tolerance was quantified in four cyanolichens (Lobaria hallii, Lobaria retigera, Lobaria scrobiculata, Pseudocyphellaria anomala), one cephalolichen (Lobaria pulmonaria) and one chlorolichen (Platismatia glauca) from xeric and mesic, open and closed North American boreal forests. These sympatric epiphytes were exposed to 0%, 33%, 55% and 75% relative humidity with or without medium light (200 μmol m⁻² s⁻¹) for 7 d. Permanent and temporary photoinhibitory damage was recorded as viability measures. All species tolerated well the drying in darkness, but L. hallii and L. retigera, associated with a very humid climate, showed minor damage at the hardest drying (silica gel). Simultaneous exposure to medium light severely aggravated the drying damage at all relative humidity levels. Combined drying-light exposure was particularly devastating for the widespread chloro- and cephalolichens, whereas cyanolichens, including rare old forest species, were fairly resistant. The ability to recover after combined drying-light stress (this study) correlated positively with increasing species-specific water holding capacities (from the literature). Cyanolichens, depending on liquid water and large internal water storage, probably require strong drying-light resistance to handle long periods between hydration events, whereas chlorolichens can regularly maintain their photosynthetic apparatus during frequent and rapid activation by humid air on clear mornings.

Fig. 1

Maximum quantum yield of photosystem II (F_v/F_m; measured in hydrated thalli of Norwegian Lobaria pulmonaria after 48 h of recovery subsequent to drying treatment) as a function of the percentage water content in thalli experiencing 7 d of drying in darkness (closed circles) or at 200 μmol m⁻² s⁻¹ (open circles). Drying treatments were given in boxes in which thalli were in equilibrium with 0% (the lowest water content), 35%, 55% and 75% relative humidity. Error bars show standard errors.

Fig. 2

Level of photoinhibition after combined drying (0%, 35%, 55% and 75% relative humidity) and light exposure (200 μmol m⁻² s⁻¹) in Lobaria pulmonaria and Platismatia glauca from different habitats (black circles, xeric open; gray circles, mesic open; white circles, mesic closed) as specified in Table 3. Each symbol represents the mean ± 1SE. See Table 4 for ANOVA results.

Fig. 3

Kinetics during recovery (log-scale) for hydrated thalli of the studied species at low light after 7 d of drying treatment in darkness (closed symbols and solid lines) and in medium light (200 μmol m⁻² s⁻¹; open symbols and dotted lines). Each symbol represents the mean ± 1SE. Relative humidity during drying: triangles apex down, 0%; triangles apex up, 35%; squares, 55%; circles, 75%.

Fig. 4

The relationship between thallus water holding capacity (WHC; from Gauslaa & Coxson, 2011) and permanent depression in F_v/F_m measured in this study as a percentage of the start values after a 7-d combined drying and light exposure ( = 0.469; P = 0.005). Open symbols, the chlorolichens Platismatia glauca (triangles apex up) and Lobaria pulmonaria (circles). Closed symbols, Pseudocyphellaria anomala (squares), Lobaria scrobiculata (triangles apex down), L. hallii (triangles apex up) and L. retigera (circles). Each error bar (vertical as well as horizontal) represents ± 1SE (n = 39–40 for WHC and n = 19–20 for F_v/F_m) from each type of habitat (see Table 3).