Does long-term salt stress of environmentally relevant concentrations affect the physiology of inland lichens? : the importance of rainfall to restore thallus vitality

During winter seasons, sodium chloride is the most widely used de-icing agent for road maintenance. The aim was to determine the effect of long-term spraying with salt solutions of environmentally relevant concentrations on the physiology of two epigeic lichens i.e., Peltigera didactyla and Cladonia rei. We also aimed at studying lichen responses to rainfall simulations between salt treatments to verify whether the liquid water supply would improve the physiological condition of both symbiotic partners. Long-term salt stress led to a gradual loss of cell membrane integrity, but the damage was lower in the case of an experiment in which thalli were treated with salt solutions and water. Salt stress significantly affected the peroxidation of membrane lipids in both studied species, but their responses differed. Thiobarbituric acid reactive substances (TBARS) level in P. didactyla increased very late in comparison to C. rei, in which the highest TBARS concentrations were observed at the beginning of the experiment, and then decreased over time. This may be associated with the activation of defence mechanisms protecting against reactive oxygen species. The cyanobiont in P. didactyla was more sensitive to salt stress than the green algal photobiont in C. rei. A low possibility of photosynthesis reactivation of P. didactyla at high humidity may explain the earlier decrease in $F_{V}/F_{M}$ compared to C. rei; whereas the stronger reaction of this species may result from irreversible changes associated with ionic effects. Moreover, significantly higher $F_{V}/F_{M}$ values were recorded in the salt-water experiment in both species. Therefore, we conclude that rainfall could cause at least a partial recovery of the water potential by photobionts and trigger the process of photosynthesis. This is of key importance for lichens, which are constantly exposed to salt stress in the winter season, and rainfall may enable them to continue their physiological processes and survival