Functions of Gelatinous Fibers in the Roots and Shoots of Fouquieria splendens

Fouquieria splendens (ocotillo) is a large, basally branching, shallow-rooted, drought deciduous shrub native to the Chihuahuan and Sonoran Deserts. Ocotillos, like all Fouquieriaceae, have gelatinous fibers (g-fibers), which typically occur in tension wood (TW) of eudicotyledons and can reorient stems and roots. The goal of the research was to determine whether the presence of g-fibers in TW in branches was a response to mechanical stress and whether g-fibers in roots functioned in pulling ocotillos towards the soil. It was hypothesized that TW in the branches aided in resisting gravitational stresses and bending from high winds and that TW in the roots would pull the shoots downward to prevent them from falling over, or provide tension to prevent uprooting. To address the hypotheses, the anatomy of shoots that were displaced or fixed in place was compared to those in their native state in the field. To study the g-fiber function in roots, young ocotillos were planted at different depths and with their caudices (stem/root axis) at different angles from vertical. Shoots had far greater TW coverage in cross-section in sides of the branches experiencing tension than in sides experiencing compression for all treatments. There was also greater TW coverage in the basal regions of branches than in more distal regions, suggesting that they were resisting bending due to static loads as well as possibly dynamic loads (wind). Coverage of TW in taproots differed between sides experiencing tension and compression for plants with half-exposed caudices planted 45 from normal. Further, there was no evidence of contraction in ocotillo roots. However, because g-fibers occurred in lateral roots, all roots may function in resisting tensile stresses. In addition to g-fibers, several other fiber types occurred in ocotillos, indicating an unusual amount of fiber diversity.