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

Abstract

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 morphophysiological 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. Terrestrial and aerial roots were compared on a morphophysiological basis. Root anatomy, water balance, water absorption capacity via fluorescent tracer, and photochemical activity via chlorophyll fluorescence were measured. While thin fasciculate roots occur on terrestrial crawling individuals, two clearly distinct aerial roots (anchor and feeder) are produced on canopy individuals, which both adhere to the host trunk. The colour of both aerial roots changes during development from red and brownish to striped and green at maturity. Colour 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 aerial roots. Plants should evolve strategies to survive stress situations. The transition from soil to canopy imposes abiotic changes and potentially stressful situations on R. oblongata. We conclude that the morphophysiological changes observed represent an important strategy that permits the maintenance of aroid roots and the survival of R. oblongata in the canopy.