Leaf trichomes reduce boundary layer conductance

Leaf trichomes (hairs) have multiple hypothesized functions, of which several require empirical evidence. An important, yet controversial, proposed function of trichomes is to influence the leaf boundary layer, which would affect leaf temperature, transpiration and photosynthesis, and may confer differential benefits depending on climate. We used dynamic infrared thermography to test whether trichomes reduce the boundary layer conductance to heat ( g), impeding heat transfer between leaves and air. For five species, with trichome lengths of 135-780 μm, we transiently heated leaves with a radiative light source, measured the time constant for subsequent leaf cooling simultaneously in two adjacent leaf regions (with and without trichomes) with an IR camera, and inferred g using an energy balance model. Cooling was slower in hairy leaf regions relative to bald regions, corresponding to a lower g in hairy regions, by 2.4% to 39% across species. Contrary to prior theory, the resistance added by trichomes was unrelated to the depth of the hair layer (i.e., trichome height) across species. Simulations predicted that the reduction in g by trichomes would influence energy balance and gas exchange rates by up to a few percent, with the direction and magnitude of such effects depending sensitively on environmental conditions.