Nitrate is a well-known regulator of several processes in plants, including seed germination. It generally induces germination, but its effect is species-specific, and germination of some species is even inhibited. It is not clear what drives these differences. Nevertheless, it is striking that the germination of initially mycoheterotrophic orchids is generally inhibited by nitrate, indicating that this may be an adaptive trait allowing the seeds to detect sites with low nutrient availability, favourable for mycoheterotrophic plants. Whether this unusually negative response to nitrate is a general feature of mycoheterotrophic plant germination remains unknown. We focused on the family Ericaceae, which contains lineages that are either fully or initially mycoheterotrophic together with autotrophic ones, and we tested the germination response of selected species to nitrate. We found high variability in the germination responses even within members of this family. Fully and initially mycoheterotrophic species reacted more negatively to nitrate compared to autotrophic species. However, the species responses to nitrate also strongly correlated with seed dimensions. As a control, we also tested responses to another common nitrogen form, ammonium ion, but these showed little or no correlations with the tested predictors indicating that the observed germination responses are specific to nitrate. Two factors may be responsible for the negative response of mycoheterotrophic species of both Ericaceae and orchids to nitrate: seed size and the mode of carbon acquisition (mycoheterotrophy), nevertheless, both factors are closely linked. Thus, mycoheterotrophic plants in general, which nearly all produce tiny dust seeds, are likely to be negatively affected by nitrate. 

Seeds were cultivated axenically in vitro on media with various amount of nitrate or ammonium ion. We also measured dimensions of dry ripe seeds.