Ontogenetic changes in root traits and root-associated fungal community composition in a heteroblastic epiphytic bromeliad

Heteroblastic variations among leaf traits is a well-known process, especially in bromeliad species that show abrupt changes, but little effort was directed to test whether comparable ontogenetic variation occurs among root traits and their fungal partners. Usually considered for their mechanical role, roots of bromeliads may also play a role in resource use that we expect to differ between ontogenetic stages, namely nutrient pulse-supplied atmospherics (young stage) and continuously-supplied tanks (later stage). On a selection of adventitious roots of the heteroblastic epiphyte Lutheria splendens, we explored 1) ontogenetic variations in morphological and anatomical root traits, namely the number of adventitious roots and tips, total root length, average root diameter, root tissue density, specific root length and root tissue areas, and 2) ontogenetic changes in the composition of the root-associated fungal communities using ITS1 region-based high-throughput sequencing. Plant size, taken as an indicator of gradual ontogenetic variations, and plant stage had significant effects on root traits, unless for root tissue density. The number of adventitious roots and tips, the total root length, average root diameter and all root tissue areas (i.e. velamen, outer cortex, inner cortex, vascular cylinder) increased gradually with ontogeny while the specific root length decreased abruptly. The root-associated fungal communities varied with plant stage. The tank stage showed higher proportion of mycorrhizal fungi and a lower abundance of dark septate endophyte as compared to atmospheric stage. Overall, we demonstrated significant ontogenetic variations of root traits and fungal partners in bromeliad species that suggest important changes in the strategy for resource use as plants mature. We invite future root trait studies to better account for ontogenetic variations.