Mapping out a leaf's mountains and valleys.

Cartographers have created topographical maps of much of the world, but they have overlooked at least one important area: the surfaces of leaves. Because a leaf's surface can make it more or less hospitable to microbes, insects, and insecticides, such maps would prove useful to a broad range of scientists, from biologists to agronomists.

Now, Wendy L. Mechaber of the University of Arizona in Tucson and her colleagues have remedied that oversight. Her team's maps "are an interesting development, and some will find [them] a very useful tool," says Sanford D. Eigenbrode of the University of Idaho in Moscow.

Surface features can determine how well water, agricultural sprays, or even beneficial microbes stay on a leaf, Mechaber notes. Moreover, the surface has a big effect on some of a leaf's residents, studies show. For example, to infect a plant, the bean rust fungus must distinguish minute variations in the topography of its leaves. When an insect sits on a leaf, the amount of its pheromones that escape into the air may depend on the thickness of the layer of fatty lipid molecules that coat most leaves and can absorb volatile compounds.

Researchers have relied on scanning electron microscopes to create pictures of leaf surfaces. However, the pictures are distorted and provide inaccurate data on important features such as the height of the lipid layer, explains Mechaber.

Using atomic force microscopy, an imaging technique normally employed by physicists and materials scientists to measure much smaller objects, Mechaber and her colleagues magnified leaf sections from a perennial cranberry vine (Vaccinium macrocarpon) 1,600 times and determined their three-dimensional coordinates. When measuring height, they incorporated data only on the lipid layer and not, for example, data on surface hairs.

They then used a standard geologic mapping program to produce the topographical maps, they report in the May 14 Proceedings of the National Academy of Sciences.

The scientists compared five leaves that grew during the current growing season with four older ones. The young leaves have more lipids, which formed "a regular pattern of broad expanses or plateaus," the team reports. The older leaves' surfaces lack both the broad, elevated areas and a clear pattern. The wear and tear of exposure to organisms and the elements change leaves' topography over time, the group suspects, "much as geologic erosion preserves some land features while strongly changing others."