Monitoring with lichens – Conductivity methods assess salt and heavy metal damage more efficiently than chlorophyll fluorescence

In the lab, we exposed three foliose lichen species, Lobaria pulmonaria, Parmelia sulcata and Xanthoria aureola, to 0, 0.01, 0.2, and 0.6 M NaCl in combinations with copper and zinc (0, 10, 100, 500 μM). High salt concentrations adversely affected the lichen membrane integrity as measured by conductivity methods, whereas the potential photosystem II efficiency (Fv/Fm) was tolerant. High light was necessary to reduce Fv/Fm in thalli exposed to salt, whereas high light did not aggravate the conductivity. The seashore species X. aureola was much more resistant to salt than the old forest species L. pulmonaria. With respect to Cu and Zn, used concentrations had no (P. sulcata, X. aureola) or small (L. pulmonaria) effects on Fv/Fm. However, both heavy metals substantially increased conductivity in all species, consistent with membrane damage. Thus, the conductivity method detected high salt, high copper and high zinc stress much more efficiently than did the chlorophyll fluorescence method. This suggests that membrane integrity of the mycobiont is more sensitive to salt and heavy metal stress than potential photosystem II efficiency of its autotrophic partners.