The Effect of Trichomes on Leaf Boundary Layer Conductance to Heat Transfer

Leaves of many plant species possess epidermal hairs, or trichomes, whose functions remain largely speculative. I used dynamic infrared (IR) thermography to test the hypothesis that leaf hairs impede the transfer of heat between leaves and the adjacent air, manifesting as a reduction in leaf boundary layer conductance to heat (gbh). For five broadleaved species, I transiently heated leaves with a radiative light source, and I recorded the subsequent decline in leaf temperature in leaf patches with and without hairs with an IR camera at 5 Hz. I used a model of leaf energy balance to infer gbh from the observed time constant for leaf cooling (tau). Wind was provided by an array of computer fans, through a wind tunnel, and wind speed and air temperature were measured continuously with a hot-wire anemometer and fine-wire thermocouple placed below the leaf. The infrared camera was calibrated against a blackbody prior to each measurement. In over 95% of experiments, cooling was slower (tau was greater) in the hairy patch than in the bald patch. Similarly, inferred gbh was greater in the bald patch, by an amount ranging from 7% to 87% across species. Contrary to some theoretical predictions, enhancement of gbh by leaf hairs was not consistently related to the depth of the hair layer across species. These data confirm the long-held speculation that leaf hairs reduce boundary layer conductance and may, therefore, significantly impact both leaf energy balance and gas exchange rates. Moreover, they further demonstrate the value of dynamic IR thermography for studying leaf energy exchange