A Tale of Two Extraordinary Adaptations

Desert Rhubarb (Rheum Palaestinum)

EXTREME ADAPTATIONS OF RHUBARB

The Desert Rhubarb (Rheum Palaestinum) in the
Negev Desert possesses unusually large rosette-shaped leaves for a desert plant. Most interestingly, its leaf morphology resembles the slopes it inhabits.  Piqued by this observation, University of Haifa researchers determined that is the only self-irrigating plant in that world known to have this capacity - collecting up to 16 times more water per year than other desert plants in the region.  The study has revealed the plant harvests 4.2 liters per year, akin to the rainfall of a mediterranean climate - in a desert region that receives  only 75 mm of rainfall a year.  Deep depressions in its leaves channel water towards its roots - creating its own mini leaf-oasis.  Yev-Ladun (2009) 



Meanwhile, high in the Himalayas above the treeline at 4000 meters where conditions are so extreme most plants cannot survive, Rheum nobile - the extraordinary Noble Rhubarb stands up to 2 meters tall, a hollow column of overlapping pale-yellow leaves rising from a base of green leaves.  Discovered in the 1840s by Joseph Hooker, Rheum Nobile stands tall on the harsh Himalayan scree-clad slopes withstanding both perishing cold and biting wind as well as an invisible-yet-deadly barrage of ultraviolet light. Amazingly, the translucent column functions as a protective greenhouse, enabling it to grow amazingly large at such an altitude.
A few botanical gardens and amateur enthusiasts grow Rhem nobile but it rarely flowers out of its natural habitat.  The flowers are extraordinary: the hollow columns are actually flower spikes.  Pale yellow leaves "bracts" grow from each spike that surround and hide the flowers inside.  Kew Gardens herbarium head David Simpson says it displays great botanical novelty.  Joseph Hooker noted that in winter, after the fruit and seeds have formed, the dead seed bearing stems are in dismal keeping with the surrounding winter desolation. Nicholls (2013)

In 1964 Sasuke Naoao wrote "the flowers open in a self-made warm room" that boosts pollination, he suggested by providing favorable conditions for insects.

 

Rheum Nobile

                                                   

So even though this blog takes a slight detour from the glacial refugia topic, it is pertinent because it does make one question why if one angiosperm could manage such extreme adaptations, why were there not more? 

Weeds usually are noted for their phenotypic plasticity, but this example of the adaptation of Rhubarb (Rheum ssp) of the polygoniaceae family to survive in both extreme aridity and extreme cold is truly amazing.