Initial stages of lichen hydration observed by proton magnetic relaxation.

The hydration of selected lichens (Cladonia mitis, Cladonia bellidiflora, Cetraria islandica, Parmelia saxatilis, and Xanthoria parietina) was investigated using gravimetry and proton magnetic free induction decays (FIDs). The hydration from gaseous phase and dehydration to gaseous phase showed first-order kinetics. The amount of water which was non-removable in the air-dry state (relative humidity p/p =9%) did not depend significantly on the lichen species and was found to be 5·6±1·0% of the d. wt. The proton FID Gaussian component from the solid matrix of thallus structure, and two (or, depending on lichen species, one averaged) liquid signals coming from water tightly bound on the surface of thallus solid matrix and from loosely bound or free water, were recorded. The bound-water component was distinguished by its motional properties and by its proximity to endogenous paramagnetic centres present in solid matrix (presumably PS II reaction centres of the photobiont). Mild dehydration (from gaseous phase) could completely remove the loosely bound water fraction, leaving the system below the water percolation threshold and below the water clustering point, emphasizing the passivity of lichen response to desiccation shock. In the species in which the one average liquid component was recorded, bound water behaved similarly. The hydration at which free water pool vanishes (ΔM/m ) and the relative (scaled to water) proton densities of solid matrix of lichen (β) were evaluated for all lichens investigated.