Morpho-anatomical responses of leafless Vanilla spp. roots to drought and habitat degradation

Human activities and climate crises have resulted in severe disturbances affecting tropical forests. Here, we address the responses of leafless Vanilla spp. to open habitats, drought, and scarcity of large trees as phorophytes, with a focus on the root system. First, we compared the abundance of vanilla plants and root morphology in conserved and degraded forests. Next, we analysed the variations in root anatomical traits along a rainfall gradient. Our results showed contrasting responses depending on the ecoregion. In the northern humid forest, both the density of phorophytes and the number of vanillas per phorophyte were higher in the degraded forest than in the conserved forest. No such effects were observed in the southern dry forest. Furthermore, vanilla vines in the degraded humid forest developed more filiform roots for anchorage to smaller supports. Vanilla species occurring in semiarid climates, such as V. bosseri and V. decaryana, have larger underground roots with a thicker cortex to compensate for water rarity, as well as more vascular bundles to promote hydraulic conductivity, and more aerenchyma to decrease radial transport. Xylem vessel diameter is a plastic trait that increases with drought severity, as observed in one V. madagascariensis ecotype. These findings underline the high plasticity of the root system in leafless Vanilla spp. when facing habitat degradation and drought, and provide insights into their possible use in conservation and crop breeding.