Biological soil crusts are communities composed of cryptogamic organisms such as lichens, mosses, cyanobacteria and green algae that form a skin on soils in areas where vascular plants are excluded or limited by water availability or temperature. The lichen Psora decipiens (Hedw.) Hoffm. is a characteristic key organism in these communities in many different biomes. The species has a generalistic ecology and high morphological variation, which contributes to the ability of the species to withstand environmental changes. We investigated whether different populations, based on site and associated morpho-anatomical differences, incorporate functional water relations and how/whether this was driven by changes in abiotic factors. Samples were collected from two climatically distinct sites, one ‘dry’ site in southern Spain and one ‘wet’ site in the Austrian Alps. Our results showed that samples from the dry site had a significantly thicker epinecral layer, higher specific thallus area, a faster water uptake and contained more water per dry mass, all of which contributed to a much slower drying rate. Both populations showed a highly adjusted water gain that incorporates functional water relations and diffusion properties as a result of local water availability. We show eco-physiological and morphological mechanisms that underlie the high variability in P. decipiens and suggest how these might provide ecological benefits for this generalist lichen species.

The drying rate of whole seeds of Ekebergia capensis (Meliaceae) was shown to influence the response to desiccation, with rapidly dried seeds surviving to lower water contents. Short-term rapid drying (to water contents higher than those leading to viability loss) actually increased the rate of germination. The form of the time course of decline of axis water content varied with drying rate; slow drying could be described by an exponential function, whereas with rapid drying initial water loss was faster than predicted by an exponential function. These observations suggest that slow drying brought about homogeneous dehydration and that the rapid drying was uneven across the tissue. This raised the possibility that the different responses to dehydration were a function of different distributions of water in the axis tissue under the two drying regimes. However, ultrastructural observations indicated that different deleterious processes may be occurring under the different drying treatments. It was tentatively concluded that a major cause of viability loss in slowly dried material was likely to be a consequence of aqueous-based processes leading to considerable membrane degradation. Uneven distribution of tissue water could not be rejected as a contributory cause of the survival of rapidly dried seeds to low bulk water contents. The differential response to dehydration at different drying rates implies that it is not possible to determine a ‘critical water content’ for viability loss by recalcitrant seeds.

The viability of Norway maple seeds harvested at mass maturity (57.3% moisture content) was reduced from 100% to only 38% if dried rapidly (at 15% RH and 15°C for 3 days) to 4.7% moisture content. In contrast, slow drying for 32 days (the optimum duration of several investigated) to 29.9% moisture content enabled 93% of the seeds to survive subsequent rapid desiccation to 3.5% moisture content. This is similar to the 95% viability shown by seeds harvested 40 days after mass maturity and then dried rapidly to 4.4% moisture content. However, fruits or seeds harvested at mass maturity and then held moist for 21 days also showed 94 and 91% viability after subsequent rapid desiccation to 3.8 and 3.3% moisture content, respectively. Thus a post-ovule-abscission programme is required before Norway maple seeds are able to tolerate rapid enforced desiccation to low moisture contents, but loss in moisture during this period is not essential to the development of desiccation tolerance.