Asymbiotic germination of mature seeds and protocorm development of Pseudorchis albida (Orchidaceae) are inhibited by nitrates even at extremely low concentrations

Orchidaceae is one of the most endangered plant families, and in vitro propagation techniques play an irreplaceable role in its conservation. While some orchids germinate readily in asymbiotic conditions in vitro, many are reluctant for no obvious reason. The latter group includes Pseudorchis albida, a green terrestrial orchid whose populations are in considerable decline throughout its distribution range. We tested a wide spectrum of cultivation media designed for other terrestrial orchids to identify key components for P. albida mature seed germination and protocorm development. Specifically, we focused on nitrate concentrations and the presence or absence of trehalose, sucrose, and kinetin. Nitrates strongly inhibited both germination and protocorm development, even at surprisingly low concentrations (2 mg NO3-.dm(-3)). Trehalose inhibited seed germination but slightly promoted protocorm development. The highest germination rates were obtained on media containing both kinetin and sucrose and no nitrates, while the fastest protocorm development occurred on media lacking kinetin. Our results imply that even very low nitrogen depositions in natural populations of P. albida may inhibit its recruitment and might therefore represent one of the major causes of the decline of P. albida.