The effect of nitrogen on growth and key thallus components in the two tripartite lichens, Nephroma arcticum and Peltigera aphthosa

Relationships between growth, nitrogen and concentration of unique biont co mponents were investigated for the tripartite lichens Nephroma arcticum (L. ) Torss. and Peltigera aphthosa (L.) Willd. Nitrogen availability was manip ulated during 4 summer months by removing cephalodia and their associated N -2 fixation activity, or by weekly irrigation with NH4NO3, Chlorophyll and ribulose 1.5-biphosphate carboxylase/oxygenase (Rubisco), and chitin and er gosterol were used as photobiont and mycobiont markers, respectively. Nitro gen concentrations were similar in older and newer parts of the same thallu s, varying between 2 and 5 g m(-2), with P. aphthosa having higher concentr ations than N. arcticum. Both chlorophyll (Chl a) and chitin were linearly correlated, with thallus N. but N. arcticum invested more in fungal biomass and had lower Chi a concentrations in comparison with P. aphthosa at equal thallus N, During the 4 months, control and N-fertilized thalli of N, arct icum increased in area by 0.2 m(2) m(-2) and P. aphthosa by 0.4 m(2) m(-2). Thallus expansion was significantly inhibited in samples without cephalodi a, but there was no effect on lichen weight gain. Mean relative growth rate (RGR; mg g(-1) d(-1)) was 3.8 for N. arcticum and 8.4 for P. aphthosa, whe n time (d) reflected the lichen wet periods. RGR was 2-3 times lower when b ased on the whole time, i.e. when including dry periods. The efficiency (e) of converting incident irradiance into lichen biomass was positively and l inearly correlated with thallus Chi a concentration to the same extent in b oth species. The slower growth rates of N. arcticum, in com; parison with P . aphthosa, could then be explained by their lower nitrogen and Chi a conce ntrations and a subsequently lower light energy conversion efficiency. Func tional and dynamic aspects of resource allocation patterns of the two liche ns are discussed in relation to the above findings.