A faint glow emitted from plants during photosynthesis could be key to measuring the health of large areas of forests and croplands in real time. The glow can be detected by spectrometers aboard orbiting satellites. Currently, the standard technique for measuring photosynthesis, called eddy covariance, relies on ground-based, tower-mounted detectors that monitor only smaller tracts of vegetation.

Through a ground-based study, a research team has now helped to confirm that this subtle glow, known as solar-induced chlorophyll fluorescence (SIF), can serve as a strong proxy for photosynthetic activity in a deciduous forest. The glow is invisible to the naked eye.

The team was led by geoscientists from Brown University in Providence, R.I., and from the Marine Biological Laboratory in Woods Hole, Mass. Its findings have been published in the journal Geophysical Research Letters
“We found that solar-induced chlorophyll fluorescence is highly correlated with canopy photosynthesis at diurnal and seasonal scales,” said Xi Yang, a postdoctoral researcher at Brown and the study’s lead author.

“This is the first time anyone has linked fluorescence to photosynthesis over a long time scale in a deciduous forest and validated orbital measurements of fluorescence with ground-based measurements.”

“Photosynthesis plays an important role in the global carbon cycle and climate change studies, but we cannot estimate it accurately at global scale.” Yang indicated that he was inspired by works in Geophysical Research Letters  and in Biogeosciences, each of which independently estimated global, solar-induced chlorophyll fluorescence using satellite data.

“However, no ground validations of these satellite estimates of SIF have been done, and the relationship between SIF and photosynthesis at different time scales [is] not entirely clear. This is an important research question, and I like to design field instruments, so we started this work.”

In the photosynthetic process, chlorophyll molecules in the leaves of plants absorb photons from sunlight. The plant converts this light energy into sugar and other carbohydrates using carbon dioxide absorbed from the atmosphere. The faint glow is the result of a small percentage of photons not absorbed by the chlorophyll. They are re-emitted as lower-energy photons.

During the summer of 2013 in Massachusetts’ Harvard Forest, the researchers compared photosynthesis measurements from the forest’s eddy covariance tower with fluorescence data taken with their tower-mounted spectrometer system.

Yang said he designed the system, FluoSpec, to measure chlorophyll fluorescence. “We used one spectrometer to measure both solar and vegetation spectra, with the help of a mechanical switch – it switches between the incoming lights from sun and vegetation, so that we can automatically measure both spectra in a short time period.” Two long fiber optics – one pointed at the sky and the other at the vegetation – were connected to the switch.

The readings from the ground-based spectrometer were compared to readings from a spectrometer aboard the European Space Agency’s Global Ozone Monitoring Experiment-2 (GOME-2) satellite. The researchers found that fluorescence measurements from the ground-based spectrometer and the satellite were tightly correlated to photosynthesis as measured by eddy covariance.

The study also revealed that the fluorescence measurements appeared to capture day-to-day fluctuations in photosynthesis, as well as fluctuations over time. Fluorescence could be a much better way of getting real-time data compared with other remote-sensing methods now in use, Yang said.

Yang said that there are proposals to launch an orbital system to track photosynthesis but none have yet been funded. He is optimistic, though. “Photosynthesis is an important process and has enormous applications in both basic research and people’s daily life. As technology progresses, we will see satellites for this purpose.”