Sclerified Cork Outperforms the Exodermis: Root Water Permeability Decreases in the Soil-To-Canopy Transition of the Aroid Vine Epipremnum aureum.

The aroid vine Epipremnum aureum undergoes changes in habitat and growth axis direction from terrestrial (plagiotropic) to canopy (orthotropic) conditions. Since aerial roots connected to the forest soil are vital for water and nutrient uptake in these vines, we hypothesize that morphophysiological acclimation occurs, enabling root survival under atmospheric conditions. Root morpho-anatomy, water balance, water absorption assayed via fluorescent tracer, and photochemical activity measured by chlorophyll fluorescence were analyzed. Gentle mechanical abrasion was applied to remove root coverings and compare water retention capacity in intact versus abraded roots. During the soil-to-canopy transitions, the root surface shifted from smooth, light brown in terrestrial roots to rough, dark brown surface in aerial roots, a change resulting from exodermis replacement by a sclerified cork in aerial regions. Small host-contacting regions of aerial roots retained exodermis and epidermal hairs instead of developing a cork, establishing spatial-dimorphism. Intact aerial roots with sclerified cork presented lower water-loss permeance than terrestrial roots with exodermis; this difference disappeared after abrasion, confirming the functional role of tissue replacement. Optical and fluorescence microscopy identified a ligno-suberized nature for the covering tissues, differing in structure and size between host-contact versus atmospheric regions of the aerial roots. While water absorption through root hairs was confirmed in host-contact, chlorophyll fluorescence suggests minimal photosynthetic capacity in atmospheric region. We propose that exodermis replacement by a sclerified cork is induced by steeper water potential gradients under atmospheric conditions. This anatomical novelty mitigates water loss in aerial roots, enabling E. aureum to thrive in the canopy habitat.