Aroid aerial roots answer new pressures with new morphology

Plant roots display a variety of morphological and physiological adaptations to challenging environments such as arid soils, aquatic media, and an epiphytic lifestyle. Little is known, however, about how different roots on the same individual can adapt to a habitat transition presenting contrasting environments across the whole of the plant.

Aroid vines begin life growing over the forest floor. Once a vine locates a suitable tree, it grows upward and into the canopy. This stark change in habitat presents new stresses to the aerial portion of the plant, which produces both anchor and feeder roots. The former are short, branching, and affix the vine to the host; the latter are long, unbranching, and connect with the forest soil. Both types maintain contact with the tree.

In a new paper published in Annals of Botany, lead author Arinawa Liz Filartiga and colleagues compared the terrestrial and aerial roots of the aroid vine Rhodospatha oblongata using traditional and fluorescence microscopy to study root anatomy, water balance, water absorption capacity, and photochemical activity.

The authors found changes in the morphology and physiology of R. oblongata roots as they transitioned from a terrestrial to and aerial habitat. These changes improved the roots’ light and water use in canopy conditions. The principal modification in the aerial roots is a loss of exterior tissue composed of the epidermis, exodermis, and outer cortex. These tissues are replaced by a layer of lignified cork – a rare occurrence in monocot roots – that is accompanied by a colour change from red/brown to green.

The green aerial roots had an increased water retention capacity and at least a weak ability to photosynthesize. The area of the root in contact with the host retains both exodermis and lignified cork, as well as epidermal hairs, and is specialized for water absorption, while the outer surface is specialized more for water retention, creating a functional dimorphism.

“The transition from soil to canopy imposes abiotic changes and potentially stressful (e.g., drought, photodamage, herbivory) situations to R. oblongata,” write the authors. “We conclude that the morpho-physiological changes in the roots of R. oblongata observed in this study represent an important strategy that allows its growth and survival in the canopy.”