Introduction

In times of global change, with increasing frequency of high-intensity rainfall events (Allan, 2011) aggravating the loss of arable soil through erosion while a growing world population demands ever more food (Pimentel et al., 1995), the interaction of vegetation with atmospheric water has never been a hotter topic in plant science. Plants, and their leaves in particular, intercept rainfall and act as condensation surfaces for fog and dew, and thereby completely change how this precipitation reaches and impacts the ground below (Dunkerley, 2020). The retention of surface water on leaves not only alters the hydrological cycle by increasing evaporation, it also has diverse impacts on the plant itself (summarized in Dawson and Goldsmith, 2018).

The effects of surface water retention on plants are, however, strongly context dependent. On the one hand, persistent wetness on leaves can impede transpiration and photosynthesis (Aparecido et al., 2016, 2017; Berry and Goldsmith, 2020); however, on the other hand, foliar water uptake (Berry et al., 2019; Schreel et al., 2020) can boost photosynthesis and growth (Eller et al., 2013; Carmichael et al., 2020). Depending on the relative solute content of the interstitial fluid and the surface water, nutrients can leach from the leaf (Tukey, 1970) or be taken up (Templer et al., 2015). In epiphytic bromeliads, wettable leaves have gained an important function for water and nutrient uptake (Zambrano et al., 2019), and many species store rain water in tightly sealed leaf ‘tanks’ (Freschi et al., 2010; Ladino et al., 2019). Leaf surface wetness has been shown to promote epiphyll and pathogen growth (Huber and Gillespie, 1992), but mutualistic fungi benefit too (Arnold et al., 2003).

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In general, it appears that temporary water cover on leaf surfaces can have benefits, but long-term wetness tends to be disadvantageous. Therefore, plants have ubiquitously evolved adaptations to promote water shedding from their leaves. These can be simplified into two general mechanisms: (i) increased water repellency of the leaf surface and (ii) steeper leaf inclination angle. In the following, we will explore both strategies in detail and discuss their interaction with each other as well as trade-offs with other leaf functions, and implications for leaf ecology and evolution. We will first consider the (simpler) case of a droplet or water layer on a static leaf, as might occur after rain or as a result of condensation, before exploring the more complex effects of drop impacts during rain. Finally, we will take a look at some specialized adaptations such as the ‘drip tips’ on the leaves of a diversity of tropical species, and anisotropic surface structures promoting directional water transport.

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