Hydration-driven metabolic activity enhances the resilience of Xanthoria parietina (L.) Th. Fr. to nitrate stress

Lichens can utilise nitrogen in molecular, reduced, and oxidized forms, but they preferentially use ammonium due to low energetic cost of its assimilation. The effects of ammonium on lichen physiology have been extensively studied, whereas much less is known about lichen responses to nitrate. Lichens depend on external water sources to hydrate the thallus, which triggers metabolic activity essential for dealing with environmental stress. We aimed to determine the effect of exposure to nitrate under different humidity conditions on the physiological response, as well as ammonium and nitrate accumulation in Xanthoria parietina . Lichen samples were exposed to nitrate for 30 days under two humidity conditions, with physiological traits measured before and after the experiment. Our study demonstrated that air humidity and the related time of thallus hydration strongly influenced the physiological response of X. parietina . Lichens exposed to high air humidity maintained hydration for longer, which extended the period of potential metabolic activity. This contributed to low oxidative stress and high water-soluble protein concentrations. Conversely, nitrate exposure under low-humidity conditions triggered oxidative stress due to the accumulation of toxic ammonia in cells. Lichens were able to take up nitrate ions; however, their ability to assimilate these ions remains unclear. The revealed pattern suggests that the duration of metabolic activity may be a critical factor in determining lichen resilience to nitrate stress. Our results have important implications in the context of climate change, in which water availability is becoming increasingly unpredictable, suggesting that metabolic activity sustained under favourable moisture conditions may be critical for maintaining lichen physiological performance. • High humidity prolongs lichen hydration and extends the period of metabolic activity • Nitrate exposure under low humidity induces oxidative stress via ammonia accumulation • Prolonged metabolic activity triggers lichen tolerance to nitrogen stress