Evidence for wavelength-dependent light screening of cyanobionts and phycobionts in Lobaria during dehydration

Lichens have little control over their hydration status, but rapid physiolo gical and optical responses allow them to thrive through repeated dehydrati on cycles. Chlorophyll fluorescence and thallus optics were monitored durin g progressive dehydration of two ecologically similar old forest epiphytic lichens of the genus Lobaria harbouring different photobionts. Thalli of Lo baria scrobiculata (cyano-lichen) and L. pulmonaria (phyco-lichen) were har vested from a red maple (Acer rubrum) forest in southeastern New Brunswick, Canada. Simultaneous excitation during dehydration-rehydration cycles with amber light (590 nm) preferentially absorbed by phycobilisomes and blue li ght (450 nm) preferentially absorbed by chlorophyll was used to measure wav elength-selective changes in excitation of photosystem II activity of thall us samples. The ratio of PSII steady-state fluorescence yield from these tw o excitation sources (phi 590/phi 450) declined in the cyano-lichen during dehydration, but rose in the phyco-lichen. These dual-wavelength excitation experiments did not however support the hypothesis of specific decoupling of the light harvesting phycobilisomes in the cyano-lichen during dehydrati on. instead, the opposing patterns of phi 590/phi 450 in the cpano- and phy colichens suggest a symbiotic interaction in which the mycobiont cortex dev elops wavelength-dependent screening of light transmission during dehydrati on to generate photoprotection specific to the absorbance spectra of the Pa rticular photobionts' light harvesting antennae. Changes in reflection from and transmission of light by the thallus support the hypothesis that optic al changes in the thallus cause this screening.