Multifunctionality Is Key to Understanding Diversity of Leaf Trichomes: A Case Study Using an Evolutionary Model Tree Species Metrosideros polymorpha

Various plant species have leaf trichomes, and their amounts and forms can vary across environmental conditions. Several functions have been considered as adaptive roles of leaf trichomes; however, most of the previous studies focused on a single role in each environment for a given species. In the present review, we introduce the multifunctionality of leaf trichomes using an evolutionary model tree species Metrosideros polymorpha Gaud. (Myrtaceae), which exhibits the remarkable intraspecific variation in leaf trichomes (especially on the lower surface) across diverse environments in the Hawaii Islands. We examined three functions in relation to carbon gain and water conservation: diffusion resistance (Function I), enhancement of foliar water absorption (Function II), and defense against specialist gall makers (Function III). In dry alpine areas, the benefits of Functions I and II can contribute to increasing carbon gain, whereas the effect of Function III may be limited due to less insect abundance, which explains the high abundance of the heavily pubescent individuals. In dry, warm areas, Function I results in increasing water loss and decreasing carbon gain through overheating while Function III becomes significant to impede gall formation that increases drought stress, which explains the high abundance of thinly pubescent individuals. In wet areas, all three functions are not advantageous, which may explain the dominance of glabrous individuals. Overall, we suggest that leaf trichomes of M. polymorpha serve multiple functions depending on environmental conditions, and that the concept of multifunctionality is key to understanding the variations and ecological significance of leaf trichomes.