Growth, nitrogen uptake, and resource allocation in the two tripartite lichens Nephroma arcticum and Peltigera aphthosa during nitrogen stress

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Summary: Lichen responses towards nitrogen stress, both increased exposure and deprivation of N, were investigated by measuring N uptake, growth, ergosterol, chitin and Chla in two tripartite nitrogen‐fixing species, Nephroma arctium and Peltigera aphthosa. The lichens were irrigated with different N forms, enriched in 15N to assess N uptake, during 3 months in the field, with a total N dosage of 500 mg m−2. Nitrogen deprivation was induced by removing the nitrogen‐fixing cephalodia. The lichens took up 11–134 mg N m−2 of the added N, corresponding to 1–4% of their total thallus N. Uptake was 4 times higher for NH4+ than for NO3−, and the highest 15N concentrations were found in newly synthesized tissue. Both forms of N stress affected thallus expansion rates in both species. It is concluded that the two lichens were able to maintain a balanced tissue N concentration despite large variations in N supply, and that assimilated N might be transported to growing apices. Alternatively, N assimilation from external sources might be greater in the margins than in the mature thallus. Thallus expansion was sensitive to N stress, apparently being tightly coupled to N assimilation.

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