Light Reflects Off Leaves, Unveils Details of Plant Evolution

Scientists can identify plants by the light they reflect.
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Light filters through the forest canopy, with colorful leaves along the forest floor.
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Joshua Mayer via Flickr 

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Tess Joosse, Contributor

(Inside Science) -- Every fall, the trees outside our windows shine bright with shades of red, orange and yellow. But light can reveal much more about leaves and the plants that made them. New research shows that the light a plant reflects explains important details about how it evolved and where it sits on the tree of life. These details could be used to identify plant species based on the collection of different wavelengths of light the leaves reflect, known as their spectra.

"As evolutionary history unfolds, different traits and attributes of plants change over time," said Dudu Meireles, a plant evolution researcher at the University of Maine, who led the study. Those changes accumulate, and branching groups on the tree of life are identified by their different evolved traits.

Meireles and his colleagues suspected a correlation between spectra and evolutionary history. Since differences in plant traits such as chemistry and shape produce different interactions with light, the researchers thought related plants that share traits might reflect similar light patterns. "But we didn’t have a model of the mechanism," he said.

In the new study, published recently in the journal New Phytologist, the team described how their model merges the way leaves reflect light with the way traits have evolved over time. 

"This work links the physics of light to biology and evolution," said Woody Turner, a conservation scientist with NASA who was not involved in the study.

While related species reflect similar light profiles, the researchers showed not all groups of related plants reflect the same type of light. Pigments including chlorophyll and carotenoids reflect light and give plants distinct colors. Other plant traits interact with light in ways we can’t see, but still indicate their identity. Gymnosperms such as conifers are most associated with the shortwave infrared region. But grasses and other monocots correlate with the visible and near-infrared wavelengths.

This means scientists have to measure the entire light spectrum, rather than a few predefined bands, to properly identify a plant by its reflectance. By measuring the full spectrum reflecting off an unknown leaf, scientists can use the model to gauge what group of plants the leaf belongs to.

Meireles and his colleagues measured the light reflected by the leaves of 544 diverse plant species to confirm the model is broadly applicable. As with any model, it’s just a framework. "It’s an oversimplification of how evolution actually works," Meireles said. With that in mind, the researchers see this model, and spectra in general, as a tool to help measure plant identity and biodiversity on a wider scale.

Biodiversity loss is accelerating globally, and determining which species are present in an ecosystem is essential for assessing how the ecosystem is changing. Monitoring this is typically a laborious process. It isn’t always possible to count every species by hand, Meireles said.

By collecting spectral data remotely from airplanes or satellites and applying this new model, scientists could estimate which plants live within a landscape without even picking up a leaf.

To measure individual plants, Meireles and his colleagues beamed artificial light at single leaves and used a hand-held instrument to measure their spectra. But at atmospheric levels, the Sun provides the light. Specialized sensors can remotely detect hundreds of light measurements reflecting from Earth at a resolution of meters. The researchers think this could allow them to map out what general plant types are within a certain area.

NASA is working to put observation satellites into orbit with instruments capable of collecting these types of data. While Meireles and his colleagues used their model to evaluate individual leaves in the lab, a satellite-based system could do the same for forests across the globe.

“For a global problem, you need a global solution,” Turner said.

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Tess Joosse is a journalist and student in the science communication graduate program at UC Santa Cruz. She grew up in Chicago, studied biology at Oberlin College, and now lives in California. You can find her on Twitter @tessjoosse, where she tweets about writing, baking, and her dog Leo.