| Summary: | This study investigated the nitrogen (N) acquisition from soil and insect capture during the growth of three species of pitcher plants, Nepenthes mirabilis, Cephalotus follicularis dan Darlingtonia californica. 15N/14/N natural abundance ratios (d15N) of plants and pitchers of different age, non-carnivolrous reference plants, and insect prey were used to estimate proportional contributions of insects to the N content of leaves and whole plants. Young Nepenthes leaves (phyllodes) carrying closed pitchers comprised major sinks for N and developed mainly from insect N captured elsewhere on the plant. Their d15N values of up to 7.2‰ were higher than the average d15N value of captured insects (mean d15N value=5.3‰). In leaves carrying old pitchers that are acting as a N source, the d15N decreased to 3.0‰ indicating either an increasing contribution of soil N to those plant parts which in fact captured the insects or N gain from N2 fixation by microorganisms which may exist in old pitchers. The d15N value of N in water collected from old pitchers was 1.2‰ and contained free amino acids. The fraction of insect N in young and old pitchers and their associated leaves decreased from 1.0 to 0.3 mg g-1. This fraction of insect decreased further with the size of the investigated tiller. Nepenthes contained on average 61.5 + 7.6% (mean + SD, range 50-71%) insect N based on the N content of a whole tiller. In the absence of suitable non-carnivorous reference plants for Cephalotus, d15N values were assessed across a developmental sequence from young plants lacking pitchers to large adults with up to 38 pitchers. The data indicated dependence on soil N until 4 pitchers had opened. Beyond that stage plant size increased with the number of catching pitchers but the fraction of soil N remained high. Large Cephalotus plants were estimated to derive 26 + 5.9% (mean + SD of the three largest plants; range: 19-30%) of the N form insects. In Cephalotus we observed an increased d15N value in sink versus source pitchers of about 1.2‰ on average. Source and sink pitchers of Darlingtonia had a similar d15N value, but plant N in this species showed d15N signals closer to that of insect contributed 76.4 + 8.4% (range 57-90%) to total pitcher N content. The data suggest complex patterns of partitioning of insect and soil-derived N between source and sink regions in pitcher plants and possibly higher dependence on insect N than recorded elsewhere for Drosera species
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