Sundberg B, Nasholm T, Palmqvist K
The effect of nitrogen on growth and key thallus components in the two tripartite lichens, Nephroma arcticum and Peltigera aphthosa
Plant Cell and Environment: 2001 24:517-527
Relationships between growth, nitrogen and concentration of unique biont components were investigated for the tripartite lichens Nephroma arcticum (L.) Torss. and Peltigera aphthosa (L.) Willd. Nitrogen availability was manipulated 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 ergosterol were used as photobiont and mycobiont markers, respectively. Nitrogen concentrations were similar in older and newer parts of the same thallus, varying between 2 and 5 g m(-2), with P. aphthosa having higher concentrations 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, arcticum 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 cephalodia, 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, when time (d) reflected the lichen wet periods. RGR was 2-3 times lower when based on the whole time, i.e. when including dry periods. The efficiency (e) of converting incident irradiance into lichen biomass was positively and linearly correlated with thallus Chi a concentration to the same extent in both 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 concentrations and a subsequently lower light energy conversion efficiency. Functional and dynamic aspects of resource allocation patterns of the two lichens are discussed in relation to the above findings.
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