Abstract
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 bh),
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 bhusing an energy balance model. Cooling was slower in hairy leaf regions
relative to bald regions, corresponding to a lowerg bh 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 ing bh 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.