The hydration of selected lichens (Cladonia mitis, Cladonia
bellidiflora, Cetraria islandica, Parmelia saxatilis,
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/p0=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/m0) and the relative (scaled
water) proton densities of
solid matrix of lichen (β) were evaluated for all lichens investigated.