Ecophysiology of leaf trichomes.

This review examines how leaf trichomes influence leaf physiological responses to abiotic environmental drivers. Leaf trichomes are known to modulate leaf traits, particularly radiation absorptance, but studies in recent decades have demonstrated that trichomes have a more expansive role in the plant-environment interaction. Although best known as light reflectors, dense trichome canopies modulate leaf heat balance and photon interception, and consequently affect gas exchange traits. Analysis of published studies shows that dense pubescence generally increases reflectance of visible light and near-infrared and infrared radiation. Reflective trichomes are also protective, reducing photoinhibition and UV-B related damage to leaf photochemistry. Little support exists for a strong trichome effect on leaf boundary layer resistance and transpiration, but recent studies indicate they may play a substantive role in leaf water relations affecting leaf wettability, droplet retention and leaf water uptake. Different lines of evidence indicate that adaxial and abaxial trichomes may function quite differently, even within the same leaf. Overall, this review synthesises and re-examines the diverse array of relevant studies from the past 40 years, illustrating our current understanding of how trichomes influence the energy, carbon and water balance of plants, and highlighting promising areas for future research.