Increasing fungal abundance in the substrate enhances seed germination
in a fully mycoheterotrophic orchid
Abstract
Mycorrhizal associations are essential for orchid germination and
seedling establishment, and thus may constrain the distribution and
abundance of orchids under natural conditions. Previous works have shown
that germination and seedling establishment in several orchids often
decline with increasing distance from adult plants, resulting in
nonrandom spatial patterns of seedling establishment. In contrast,
individuals of the fully mycoheterotrophic orchid Gastrodia confusoides
often tend to have random spatial patterns of distribution within bamboo
forests. Since G. confusoides is parasitic on litter-decaying fungi, its
random spatial patterns of distribution may be due to highly scattered
patterns of litter-decaying fungi within bamboo forests. To test this
hypothesis, we first identified the main mycorrhizal fungi associating
with developing seeds and adult plants using Miseq high-throughput
sequencing. Next, we combined seed germination experiments with
quantitative PCR (qPCR) analyses to investigate to what extent the
abundance of mycorrhizal fungi affected spatial patterns of seed
germination. Our results show that seed germination and subsequent
growth to an adult stage in G. confusoides required a distinct switch in
mycorrhizal partners, in which protocorms associated with a single
Mycena fungus, while adults associated with a fungus from the genus
Gymnopus. A strong, positive relationship was observed between
germination and Mycena abundance in the litter, but not between
germination and Gymnopus abundance. Fungal abundance was not
significantly related to the distance from the adult plants, and
consequently germination was also not significantly related to the
distance from adult plants. Our results provide the first evidence that
the spatial distribution and abundance of litter-decaying fungi are
distributed randomly within the bamboo forest and independently from G.
confusoides adults.