The Jeweled Armor of Tillandsia—Multifaceted or Elongated Trichomes Provide Photoprotection

Foliar trichomes of gray-leaved Tillandsioideae (Bromeliaceae) are highly reflective, suggesting a role in protecting the leaf against direct sunlight in exposed niches. The performance of photosystem II, as denoted by the chlorophyll fluorescence characteristic Fv /Fm , was determined for seven Tillandsia species and Vriesea barclayana that were exposed to excessive light, with trichomes either present or removed. Additionally, trichome structure and interaction with light was recorded using extended depth-of-field photomicrography, and reflectance quantified using a novel photographic technique. Trichomes of mesomorphic Type IV life forms (T. cryptantha, T. cyanea) and of the intermediate life form V. barclayana conferred reflectance of between 1 and 11%, which did not significantly influence Fv /Fm when exposed to a high light intensity of 1500 !mol m"2 s"1 (photosynthetically active radiation) for one hour. However, the ornate trichomes of atmospheric species increased the reflectivity of the leaf blade by as much as 18–40%, with a positive correlation apparent between reflectance and photoprotection. Type V Tillandsia andrieuxii, T. caput-medusae, and T. mitlaensis have attenuated trichome wings extending perpendicular to the leaf surface and catching the light (with leaf surfaces appearing gray and fuzzy). This open configuration was observed to facilitate leaf ventilation and the condensation of water vapor on the cooler underlying cuticle, with liquid water subsequently enveloping the trichomes, suggesting a trade-off between water acquisition and light reflectance for air plants from xeric habitats. However, Type IV-V T. albida and T. concolor impound water in leaf bases and the flattened, circular, and overlapping trichome wings did not facilitate dew formation on the cuticle. For these plants with white, smooth leaf surfaces, trichomes are multifaceted and provide more effective photoprotection by scattering light in the manner of cut gemstones.