Leaf trichomes can alleviate water deficit stress and maintain diurnal photosynthesis by increasing leaf wetness and water absorption in Metrosideros polymorpha

Leaves frequently become wet throughout their lifetime, and leaf wetting affects the physiological activities of leaves. Leaf trichomes, which are often observed in plants inhabiting dry areas, may prolong the duration of leaf wetness and increase foliar water uptake (FWU), thereby contributing to greater carbon gain through improved leaf water status. However, the effects of leaf trichomes on leaf wetness and physiological activities have not been examined across a wide range of environmental conditions. We focused on Metrosideros polymorpha, a dominant tree species that exhibits significant variation in the amount of abaxial leaf trichomes across diverse environmental gradients in the Hawaiian Islands. Pubescent individuals are abundant in dry areas, whereas glabrous individuals are common in moderately wet areas. In the field, the presence of a greater amount of hydrophilic leaf trichomes significantly prolonged the duration of leaf wetness, by up to 100 min/day. When nighttime leaf wetting was experimentally prevented, the duration of high shoot-level photosynthetic rates decreased, suggesting that nighttime leaf wetness can increase the duration of high stomatal openness during the daytime. In laboratory experiments, the FWU ability was greater in pubescent individuals inhabiting dry, cold areas than in glabrous individuals inhabiting wet, warm areas. These findings suggest that the leaf trichomes of M. polymorpha improve leaf water status and increase carbon gain by increasing the duration of leaf wetness and FWU, which may contribute to the fitness of this species in dry environments.