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Root morpho-physiology changes during the habitat transition from soil to canopy of the aroid vine Rhodospatha oblongata.

BACKGROUND AND AIMS
The aroid vine Rhodospatha oblongata is characterized by a habitat change from terrestrial to canopy, relying on aerial roots at maturity to obtain water and nutrients from the forest soil. We hypothesize that morpho-physiological acclimation occurs in roots as they grow under atmospheric conditions. These changes would guarantee the whole plant survival of aroid vines in the new and potentially stressful habitat of the canopy.


METHODS
Terrestrial and aerial roots were compared on a morpho-physiological basis. Root anatomy, water balance, water absorption capacity via fluorescent tracer, and photochemical activity via chlorophyll fluorescence were measured.


KEY RESULTS
While thin fasciculate roots occur on terrestrial crawling individuals, two clearly distinct aerial roots (anchor and feeder) are produced on canopy individuals, which are both adhered to the host trunk. The color of both aerial roots change during development from red and brownish to striped and green at maturity. Color changes are induced by the replacement of epidermis, exodermis, and outer cortex by an inner layer of lignified cork on the root region exposed to the atmosphere. In the root region that is in contact with the host, covering substitutions do not occur and both exodermis and lignified cork, along with several epidermal hairs, appear. Water retention capacity was higher in green roots than in other root types. Rehydration capacity via water absorption by hairs of aerial roots was confirmed by fluorescence. Chlorophyll fluorescence data indicated low levels of photosynthetic capacity in both root types.


CONCLUSIONS
Plants should evolve strategies to survive stress situations. The transition from soil to canopy imposes abiotic changes and potentially stressful situations to R. oblongata. We conclude that the morpho-physiological changes observed represent an important strategy that permits the maintenance of aroid roots and the survival of R. oblongata in the canopy.