Biomechanics and hydraulics of Opuntia ficus-indica roots exposed to extreme drought

Succulent plants possess traits that allow them to complete physiological functions under extreme environments and root are at the frontline of the stress: the drying soil. Previous works in succulent plants have reported the extraordinary reversible mechanism of root shrinkage that disconnects plants from drying soils, reestablishing the hydraulic connection when water availability is restored. Yet, this rectifier-like mechanism would require complex biomechanical and hydraulic control at organ, tissue, and cell level. In here we evaluated the changes in hydraulic and mechanical behavior of Opuntia fine roots under extreme drought stress. Using a combination of techniques, we found that fine roots get more elastic as drought stress gets more extreme, allowing cells to modify their shape while preventing permanent damage. Furthermore, we found abrupt decreases in Lpr, that coincided with increased root shrinkage, suberin deposition and structural damage inside the endodermis via lacunae formation and possibly cell wall folding. Our data suggest that, in drought stressed succulent plants, the biomechanics of organs, tissues, and possibly cell walls are deeply coupled with belowground hydraulics, highlighting the need to continue working on deciphering the physiological mechanism that governs the interplay between mechanics and hydraulics at cell level in fine roots during drought.