Growth, Nitrogen Uptake, and Resource Allocation in the Two Tripartite Lichens Nephroma arcticum and Peltigera aphthosa during Nitrogen Stress

• 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 15 N 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 15 N 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.