Abstract

Carnivorous and parasitic plants have captured attention not only for public but also for researchers for centuries. Instead of absorbing inorganic nitrogen from soils, they can obtain some or most of their nutrients from heterotrophic organisms. With such special strategies of ‘carnivory’ and ‘parasitism,’ they indeed successfully survive in oligotrophic conditions. However, the detailed mechanisms have remained unclear, in short, by which these unique plants may have multiple channels in the nitrogen acquisition from soils, insects, and/or mycorrhizal fungi. Here we use the stable isotope ratios of nitrogen within amino acids to characterize the trophic position (TP) and the nitrogen acquisition of these unique plants. The results show that carnivorous plants register at the TP of 2.1 ± 0.2, being consistent with the TP of organisms that feed on animal diets, while parasitic-mycoheterotrophic plants register at the TP of 1.0 ± 0.1, being identical to the TP of typical photoautotrophs such as modern plants. These trophic tendencies provide strong evidence for the presence of two types of trophic adaptation to oligotrophic conditions: Carnivorous plants directly absorb amino acids derived from insect preys through their carnivorous leaves, whereas parasitic-mycoheterotrophic plants absorb ammonium ion derived from mycorrhizal fungi through their roots. Our findings thus illuminate that there are multiple strategies for nitrogen acquisition among plants under oligotrophic conditions, which will help to clarify the classification of carnivorous and parasitic plants and to advance the knowledge of plant evolution in the earth history.