The Ecological and Physiological Significance of Leaf Surface Wetness

One of my current interests is the functional and morphological response of leaves to surface wetness. Plant responses to leaf wetness are interesting because CO2 diffuses about 10,000 times more slowly through a film of water than through air. Slowed diffusion has physiological consequences. We have documented reduction of up to 46% in CO2 uptake for wet versus dry leaves. Predictable precipitation (rain, fog, dewfall) that causes leaf surface wetness for some part of the day is common in many habitats. Because of the functional importance to CO2 uptake, we hypothesized that plants would have adaptations to avoid or prevent the formation of water films on leaf surfaces if they occurred in habitats where wetting events were common.

Cindy Fornstrom, Bill Simons, and I designed a study to evaluate adaptations for avoiding the formation of water films on leaf surfaces to document the frequency and duration of natural leaf wetness in different habitats, including montane and subalpine meadows, forests, and ponds. We estimated leaf wettability by determining the tendency for moisture to accumulate in spherical droplets versus films. Leaves that repel water into spherical balls are not wettable because droplets tend to roll off the leaf surface. Cindy and Bill also examined leaves microscopically to determine density of stomatal pores and trichomes (leaf hairs).

Plants in meadow habitats intercepted moisture on nearly 90% of nights during the summer and were frequently wet in the morning when conditions for temperature, light, and water status were optimal for photosynthesis. Surprisingly, floating water lily leaves were rarely wet at dawn. Floating leaves stayed warm at night because of close contact with the warm pond. Leaves of understory forest plants were rarely wet because they were protected from radiational heat exchange with a cold sky by the forest canopy.

Cindy, Bill, and I also discovered a variety of leaf characteristics that influenced the extent and duration of surface wetness. Many species had adaptations for minimizing the amount of leaf surface area in contact with water. Trichomes and properties of leaf cuticles (waxes) had particularly important influences on leaf wettability. Furthermore, the surface with the most stomata was the least wettable!

Our results suggest that leaf wetness may be a selective pressure shaping the evolution of leaf surface characteristics in plants, including the type and density of trichomes, the location of stomatal pores, and properties of the cuticle. Plants in tropical cloud forests stay wet for days at a time, compared to only hours at a time in the montane habitats we studied. Some tropical plants have drip tips for removing surface water. Who knows what other adaptations to leaf wetness remain to be discovered! With new undergraduate research collaborators, I am beginning studies in tropical habitats to more fully understand plant adaptation and evolution in response to leaf surface wetness.