Seeing the fluorescence for the trees

Caren B. Les, caren.les@photonics.com

The subtle fluorescence that plants emit during photosynthesis – invisible to the naked eye – could provide a rapid picture of how healthy the world’s land vegetation really is.

Using satellites – instead of the ground-based or airborne instruments commonly used to measure the fluorescence – scientists from NASA Goddard Space Flight Center in Greenbelt, Md., have developed global maps of land plant fluorescence that could help identify plant life that is under environmental stress. Sensing this fluorescence remotely – from space – could perhaps help farmers and aid workers to more quickly address critical issues such as crop failures and famine.

Satellites have up until now used “greenness” indicators based on reflected light rather than fluorescence to help monitor land plants. The problem with this method is that there is a time lag of days or weeks before a diminishing level of plant greenness – caused by frost, drought or the changing seasons, for example – can be detected by satellite.

Chlorophyll fluorescence enables scientists to determine immediately whether plants are under stress before the leaves show outward signs of decline, said Elizabeth Middleton, a biologist at NASA and part of the team that created the maps.

The chlorophyll fluorescence from green foliage is produced at red and far-red wavelengths. Background light overwhelms it. When sunlight strikes a leaf, the leaf’s chloroplast structures absorb a good portion of the light and convert it into carbohydrates through photosynthesis. About 2 percent of this light is re-emitted at the longer, redder wavelengths.

A newly developed global map of land plant chlorophyll fluorescence shows stronger photosynthetic activity in the Northern Hemisphere in July (above) and the reverse in December (below). The data was collected in 2009 by a spectrometer aboard the Japanese satellite GOSAT. Images courtesy of NASA’s Earth Observatory.

Scientists now can use lasers to measure chlorophyll fluorescence across the Earth’s surface, whereas, previously, they could measure it only on a much smaller scale.

High spectral resolution data was gathered from the Thermal And Near-Infrared Sensor for carbon Observation – Fourier Transform Spectrometer on the Japanese Greenhouse gases Observing SATellite (GOSAT). The researchers compared their information with “greenness” data provided by the satellite-based Moderate Resolution Imaging Spectroradiometer Enhanced Vegetation Index.

To make their maps, they analyzed a dark section of the infrared portion of the solar spectrum embedded with a “Fraunhofer line.” There is little background light at the line on which they focused – at about 770 nm – making it possible to distinguish the faint fluorescent signal.

The researchers hope that their maps will help scientists understand carbon cycles through ecosystems.