Trichome mediated external water transport may compensate for reduced vascular efficiency in atmospheric epiphytic Bromeliaceae.

Abstract

Complex trichomes in the leaves of epiphytic Bromeliaceae absorb water and nutrients, while also facilitating long distance water transport along the leaf surface, a phenomenon previously characterized for two Tillandsia species. This study aimed to determine trichome traits that govern external water transport speed, and its relation to life form, xylem transport capacity and environmental conditions. Using near-infrared optical techniques, we characterized trichome-mediated transport in 19 species and analyzed its association with trichome and vascular traits, functional group, and habitat parameters. External leaf water transport was observed in 10 species, all of which were atmospheric life forms (nebulophytes and pseudobulbs). Transport speed positively correlated with trichome area, wing length, and degree of overlap. Species with higher trichome overlap had lower xylem capacity (Kx ) and tracheid diameter and numbers, suggesting that the atmospheric life form is related to secure, inefficient vascular systems, which may be partly compensated with external transport. External transport was more common in species from habitats with high maximum vapor pressure deficits and low aridity indices, suggesting it enhances water uptake by rapidly redistributing water across available trichomes before evaporation can occur.