A first-of-a-kind global map of land plant fluorescence shows stronger photosynthetic activity in the Northern Hemisphere in July when light and temperature conditions were most conducive to plant growth, and the reverse in December. The maps are based on data from a spectrometer aboard the Japanese satellite GOSAT. Image: NASA’s Earth Observatory |
Scientists
from NASA’s Goddard
Space Flight
Center have produced groundbreaking global maps
of land plant fluorescence, a difficult-to-detect reddish glow that leaves emit
as a byproduct of photosynthesis. While researchers have previously mapped how
ocean-dwelling phytoplankton fluoresce, the new maps are the first to focus on
land vegetation and to cover the entire globe.
To date,
most satellite-derived information related to the health of vegetation has come
from “greenness” indicators based on reflected rather than fluorescent light.
Greenness typically decreases in the wake of droughts, frosts, or other events
that limit photosynthesis and cause green leaves to die and change color.
However,
there is a lag between what happens on the ground and what satellites can
detect. It can take days—even weeks—before changes in greenness are apparent to
satellites.
Chlorophyll
fluorescence offers a more direct window into the inner workings of the
photosynthetic machinery of plants from space. “With chlorophyll
fluorescence, we should be able to tell immediately if plants are under
environmental stress—before outward signs of browning or yellowing of leaves
become visible,” said Elizabeth Middleton, a NASA Goddard-based biologist
and a member of the team that created the maps.
The new
maps, based on data collected in 2009 from a spectrometer aboard a Japanese
satellite called the Greenhouse Gases Observing Satellite (GOSAT), show sharp
contrasts in plant fluorescence between seasons. In the Northern Hemisphere,
for example, fluorescence production peaked during July, while in the Southern
Hemisphere it did in December.
The new
findings help confirm previous lab and field experiments that suggest
chlorophyll fluorescence should taper off in the fall as the abundance of green
foliage declines and stress increases as a result of lower temperatures and less
favorable light conditions.
While
additional research is required to sort out the subtleties of the fluorescence
signal, the new maps are significant as they demonstrate the feasibility of
measuring fluorescence from space.
In the
future, the Goddard team expects that fluorescence measurements will complement
existing measures of “greenness” in a variety of ways. They could help farmers
respond to extreme weather or make it easier for aid workers to detect and
respond to famines. Fluorescence could also lead to breakthroughs in
scientists’ understanding of how carbon cycles through ecosystems—one of the
key areas of uncertainty in climate science.
In the future, fluorescence measurements could help farmers detect disease, droughts and other problems before they take a heavy toll on crops. Image: John Deere Corp. |
“What’s
exciting about this is that we’ve proven the concept,” said Joanna Joiner,
the deputy project scientist for NASA’s Aura mission and the leader of the
Goddard team that created the maps. “The specific applications will come
later.”
Glowing plants?
The same mechanism that makes plants fluoresce causes a range of everyday
objects to glow intensely under black light.
However,
plants fluoresce in specific parts of the blue, green, red, and far-red
spectrum. Chlorophyll fluorescence from green foliage, for example, is produced
at the red and far-red wavelengths.
“In
plants, fluorescence is not something that you can see with your naked eye
because background light overwhelms it,” explained Joiner, the lead author
of the paper. When sunlight strikes a leaf, disc-like green structures called
chloroplasts absorb most of the light and convert it into carbohydrates through
photosynthesis.
Chloroplasts
re-emit about two percent of incoming light at longer, redder wavelengths. This
re-emitted light—fluorescent light—is what the Goddard scientists measured to
create their map. Fluorescence is different than bioluminescence, the
chemically-driven mechanism lightning bugs and many marine species use to glow
without exposure to light.
For decades,
scientists have measured fluorescence in plants by exposing leaves to laser
beams that, like black light, make fluorescence more apparent. Such experiments
have revealed much about how certain types of plants fluoresce, but researchers
have not been able to use lasers to measure the phenomenon across broad swaths
of the Earth’s surface.
To create
their global fluorescence map, Joiner and her colleagues used a different
technique. They analyzed an unusually dark section of the infrared portion of
the solar spectrum embedded within a feature called a “Fraunhofer line.” There
is little background light at the line they focused on—at about 770 nanometers—which
made it possible to distinguish the faint fluorescence signal.
The future
of fluorescence
The new findings have implications for both current and upcoming satellite
missions. In the near term, awareness of the fluorescence signal should help
atmospheric scientists refine measurements of carbon dioxide and methane from
the GOSAT mission.
From space, farmland in northwest Minnesota looks like a patchwork quilt. Fields change hue with the season and with the alternating plots of organic wheat, soybeans, corn, alfalfa, flax or hay. Image: NASA’s Earth Observatory |
The creation
of the maps also bolsters the argument that an experimental mission being
developed by the European Space Agency (ESA)—the Fluorescence Explorer (FLEX)
mission—would make significant breakthroughs. The ESA is currently in the midst
of feasibility studies and has not yet set a launch date for FLEX.
The findings
also suggest that NASA’s Orbiting Carbon Observatory-2 (OCO-2), a mission that
is designed to measure carbon dioxide levels much like GOSAT, should be able to
make useful fluorescence measurements on a global scale. OCO 2 will launch no
earlier than February of 2013 from Vandenberg Air Force Base in California.
The maps, published
online in Biogeosciences, represent
just a first attempt to detect terrestrial fluorescence on a broad scale and
will be enhanced and expanded over time, the scientists involved in the project
emphasized.
More work
needs to be done, for example, to understand how plant fluorescence varies
depending on light conditions. In strong afternoon light, the conditions that
GOSAT made its observations, unstressed plants produce a stronger fluorescence
signal than stressed plants. However, complicating matters, the reverse is true
in the morning or evening when light is less intense.
To
disentangle the two opposing effects, the Goddard-based group plans to continue
refining the mathematical methods they used to calculate fluorescence.
Meanwhile, groups of scientists at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.—as
well as Japanese and European research groups—are in the process of honing
similar fluorescence-monitoring methods.
