A small plant from the Chinese mountains feeds on insects to compensate for the poverty of its environment. This ability has just been demonstrated by a series of experiments. The plant captures its prey, digests them, then recovers their nitrogen for reproduction.
The species, named Saxifraga candelabrum, grows between 2500 and 3300 m above sea level. It occupies rocky crevices of the Qinghai-Tibet Plateau and the Hengduan Mountains, in southwestern China. The soil there provides few nutrients, which favors unusual strategies to survive.
Saxifraga candelabrum.
a) Flowering plants thriving in stone crevices.
b) Plants covered with insects.
c) Erect scape; close-up of the rachis.
d) Leaf with a captured insect.
e) Calyx and pedicel.
These photos (especially c to e) show the reddish, sticky glandular hairs as well as the insects captured on different parts of the plant.
Its stems, leaves and flower-bearing branches are covered with small reddish glandular hairs. These produce a viscous substance capable of immobilizing tiny insects. The researchers observed an average of 71 prey on the adult plants studied, mostly non-biting midges.
However, the mere presence of stuck insects was not enough to classify this flower among carnivorous plants. To demonstrate this, the team had to verify four steps: attract prey, capture it, digest it and absorb its nutrients. Experiments conducted in the field and in the laboratory confirmed each of them.
The scientists notably detected a phosphatase in the glandular hairs. This enzyme helps release certain nutrients contained in animal tissues. They then placed fruit flies enriched with nitrogen-15 on the plant, a stable form of nitrogen that is easy to track in tissues.
Two weeks later, this nitrogen was present in several parts of the plant. The flowers contained more of it than the leaves, indicating a transfer to the reproductive organs. The non-carnivorous species used as controls did not show this enrichment, ruling out simple external contamination.
The plant also seems to avoid capturing its own pollinators. Its prey are mostly very small insects, while larger flies and bees can free themselves. This selection allows the flower to gain nutrients without compromising the pollination necessary for seed production.
Charles Darwin had proposed as early as 1875 that some saxifrages with sticky hairs could be carnivorous. He had not succeeded in demonstrating it with the means available to him. This study confirms his intuition about this plant group and now opens up research into other inconspicuous species, perhaps carnivorous without having yet been recognized as such.