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The master of adaption: The Noble Rhubarb

Have you ever heard of Rheum nobile or Sikkim rhubarb? Probably not. To be fair, it is a very rare and not especially beautiful plant – but it is a master of adaption and a little wonder on its own.

The plant is native to the Himalayas and grows at an altitude of 4000-4800 m. This zone is also called the alpine tundra. Not even trees are able grow here, as the climate is way too cold and too windy. The alpine tundra is characterized by dwarf shrubs, growing close to the ground to withstand the harsh winds. And in between these, you can find our Noble Rhubarb towering 1-2 meters tall. It even has small green flowers and bears fruits.

How is that possible?

The Noble Rhubarb is also called the glasshouse plant, and this is the whole trick: The bracts of the plant form a protective shield against wind, cold and moreover harmful UV-B radiation, which is quite strong in this altitude. Impressively, the bracts are translucent (light can pass through) and this creates a small greenhouse effect for the small green flowers and ripening fruits hiding inside the leaves. Once the fruits are ripe, the bracts die off and uncover the fruits.

But how are the flowers pollinated if they are hidden behind their protective shell?

Fruits of the Noble Rhubarb

In fact, scientists struggled for a long time with the answer. Generally, you won’t find many pollinators (insects) in the alpine tundra, thus the small amount of plants growing there only show a limited fruit set. But not our Nobile Rhubarb. About 98% of the flowers are pollinated and form fruits. Just in 2014, Bo Song and colleagues found out that the flowers of the Nobile Rhubarb are regularly visited by a fly species: a seed-parasitic fly fungus gnat (to be a bit more specific: fungus gnats of the genus Bradysia sp.). The adult female flies seek the flowers to lay their eggs and while doing that pollinate the flowers. Their larvae feeds on the seeds and flowers. The research group found out, that a specific volatile compound in the floral scent attracts the flies. And even if the larvae feeds on the flowers, the seed production is still high enough for a huge fruit set.

Have a look at the plant, the fungus gnat and their larvae:

(a) A plant at the anthesis stage. (b) A plant in the middle of the seed development stage. (c) A flower head concealed by bracts. (d) Female and male Bradysia sp. mating outside the bract. (e) A female fly visiting a flower. (f) An ovipositing female fly. (g) A pollinated stigma. (h) A fruit infested by a fly larva. (i) Pupae of fly under litter. Bo Song et al., 2014.

The Noble Rhubarb is indeed a very interesting plant and a master of adapting to its environment.

Source and further reading:

Song, B., Chen, G., Stöcklin, J., Peng, D. L., Niu, Y., Li, Z. M., & Sun, H. (2014). A new pollinating seed‐consuming mutualism between Rheum nobile and a fly fungus gnat, Bradysia sp., involving pollinator attraction by a specific floral compound. New Phytologist, 203(4), 1109-1118.

Picture Source

1] http://flower.onego.ru/other/rheum.html

2] Copyright © 2019 ChineseAlpines.com

3] Bo Song et al., 2014

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