Constraints on establishment in an endangered terrestrial orchid: a comparative study of in vitro and in situ seed germinability and seedling development in Nervilia nipponica

摘要

Orchid seed capsules typically contain several thousand seeds, but obstacles to germination and development greatly constrain the establishment of new plants. This study compares the germination response of orchid seeds from the same capsule in situ and in vitro to assess the potential of soil conditions to influence recruitment. Mature capsules of the endangered Nervilia nipponica, a terrestrial orchid endemic to Japan, were harvested from two widely separated populations. On average, over 96% of seeds from all capsules contained an embryo. 2,3,5-Triphenyl tetrazolium chloride stainability was high and, when sown in vitro and cultured for 16 weeks asymbiotically, 76-99% of seeds from all but two capsules germinated and developed rapidly. However, when buried in seed packets for 7 months at the site of the source populations, germination was spatially variable and ranged from 0% to 12% per capsule. Embryo loss in buried seed was extensive, and germination was not significantly greater after a further 14 months of incubation. These results indicate that the window for successful germination under field conditions is short, and that recruitment is limited by microsite. Conservation efforts should focus on habitat management to promote conditions conducive to seed germination, coupled with ex situ propagation.