A Speed Gun for Photosynthesis

On a recent sunny afternoon, David Allen was standing by a third-floor window in a research building at the National Institute of Standards and Technology (NIST), holding in his hands a device that looked like a cross between a video camera and a telescope. The NIST campus is in suburban Gaithersburg, Maryland, but looking out the window, Allen could see 24 hectares (60 acres) of tulip tree, oak, hickory and red maple—a remnant of the northeastern hardwood forest that once dominated this landscape.

Allen mounted the device on a tripod and pointed it out the window at the patch of forest below. The device wasn’t a camera, but a type of optical sensor that, if the science bears out, will be able to estimate the rate of photosynthesis—the chemical reaction that enables plants to convert water, carbon dioxide (CO₂) and sunlight into food and fiber—from a distance.

That measurement is possible because when plants are photosynthesizing, their leaves emit a very faint glow of infrared light. That glow is called Solar Induced Fluorescence, or SIF, and in recent years, optical sensors for measuring it have advanced dramatically. The sensor that Allen had just mounted on a tripod was one of them.

“If SIF sensors end up working well,” Allen said, “I can imagine an instrument that stares at crops or a forest and has a digital readout on it that says how fast the plant is growing in real time.” 

Such a device would revolutionize agriculture, forestry and the study of Earth’s climate and ecosystems.

Allen is a NIST chemist whose research involves remote sensing—the technology that’s used to observe Earth from outer space. Remote sensing allows scientists to track hurricanes, map terrain, monitor population growth and produce daily weather reports. The technology is so deeply embedded in our everyday lives that it’s easy to take for granted. But each type of remote sensing had to be developed from the ground up, and the SIF project at NIST shows how that’s done. 

Some satellites are already collecting SIF data, but standards are needed to ensure that those measurements can be properly interpreted. NIST has a long history of developing standards for satellite-based measurements, and Allen’s research is aimed at developing standards for measuring SIF. Doing that requires a better understanding of the biological processes that underlie SIF, and for that, Allen teamed up with outside scientists.

At the same time that Allen was aiming a SIF sensor through that third-floor window, a team of biologists from Boston University and Bowdoin College was in the NIST forest measuring photosynthesis up close. A pair of them spent the day climbing into the canopy on an aluminum orchard ladder. Once there, they would use a portable gas exchange analyzer to measure photosynthesis directly based on how much CO₂ the leaf pulled out of the air. They also measured SIF at close range.

Other scientists checked on specially designed sap flow sensors they had installed on the trunks of trees to measure the movement of water toward the leaves for photosynthesis.

“We’re measuring the vital signs of the trees,” said Lucy Hutyra, the Boston University ecologist who led the team of scientists on the ground. The idea was to use those ground measurements to make sense of the SIF data collected from a distance.

“If we measure an increase in photosynthesis at the leaf, we should see a corresponding change in the optical signal,” Hutyra said. 

The research was also taking place at still a higher level. That afternoon, Bruce Cook and Larry Corp, scientists with NASA’s G-LiHT project, flew over the NIST forest in a twin-turboprop plane that carried multiple sensors, including a SIF sensor and Light Detection and Ranging (LiDAR) sensors that mapped the internal structure of the forest canopy. The aircraft made six parallel passes over the forest at about 340 meters (1,100 feet, slightly above the minimum safe altitude allowed by FAA regulations), the instruments peering out from a port cut into the belly of the aircraft.

That gave the scientists three simultaneous measurements to work with: from the ground, from the window above the forest and from the air. They’ll spend months correlating the data.

“It’s tricky, because when you go from the leaf level to the forest level, you often get different results,” Allen said. For instance, at the forest level, the SIF signal is affected by the variations in the canopy, including its contours and density. “We’re still studying those effects.” 

Currently, there is no reliable way to measure photosynthesis in real time over a wide area. Instead, scientists measure how green an area is to gauge how much chlorophyll is present—that’s the molecule that supports photosynthesis and gives leaves their color. But if a plant lacks water or nutrients, it may be green even if the photosynthetic machinery is switched off.

SIF may be a much better indicator of active photosynthesis. When plants are photosynthesizing, most of the light energy absorbed by the chlorophyll molecule goes into growing the plant, but about two to five percent of that energy leaks away as SIF. The amount of leakage is not always proportional to photosynthesis, however. Environmental variables also come into play.

The NIST forest is a test bed for understanding how all those variables interrelate. In addition to SIF data and the vital signs of trees, the scientists are collecting environmental data such as temperature, relative humidity and solar irradiance. They’re also figuring out the best ways to configure and calibrate the SIF instruments.

“We’d like to see robust, repeatable results that make sense,” Allen said. “That will allow us to scale up from the leaf level, to the forest level, to the ecosystem level, and to estimate photosynthesis from measurements made at any of those scales.”

Making SIF scalable is a key part of the measurement standard that Allen is working to create, and it will go from the ground level to measurements made from outer space.

Using SIF to measure photosynthesis in real time would allow farmers to use only as much irrigation and fertilizer as their crops need, and only when they need it. Forest managers would be able to know how fast their timber is growing without having to tromp through the woods with a tape measure. Environmental managers would be able to monitor the recovery of damaged or deforested habitats after a drought or forest fire.

And scientists would have a powerful new tool for studying how plants help regulate the amount of CO₂ in the atmosphere.

Humans add CO₂ to the atmosphere when they burn fossil fuels, and land-based plants remove roughly a quarter of that CO₂ through photosynthesis. But the environmental factors that affect that process are not well understood, mainly because scientists haven’t had a good way to measure the uptake of CO₂ at the ecosystem level. SIF measurements, and the standards for interpreting them accurately, might help solve that problem.

“CO₂ exchange by plants is one of the most important biological processes on the planet,” Allen said, “and SIF will give us a new way to see that process in action.”

Related Content

• New Method for Identifying Carbon Compounds Derived from Fossil Fuels
• NIST’s Greenhouse Gas Measurements Program