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

Abstract

Summary Lichen responses towards nitrogen stress, both increased exposure and deprivation of N, were investigated by measuring N uptake, growth, ergosterol, chitin and Chl a 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 NH 4 + than for NO 3 − , 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.