Ten quirky facts about plant fluorescence
If you had to describe a plant, the first thing you might start with is its colour – and the first colour that usually comes to mind, is green. And indeed, by and large, plants appear green to our eyes because they contain chlorophyll, which absorbs blue and red light much more strongly than green light. Behind this green façade though, plants are secretly glowing. This phenomenon is called solar induced fluorescence, and it gives scientists an unusual way to spy on photosynthesis – all the way from space.
As ESA prepares to launch its Fluorescence Explorer (FLEX) mission on 15 September 2026, here are 10 surprising facts about this faint glow that could transform how we assess the health of Earth's vegetation.
When chlorophyll absorbs sunlight, most of that energy goes into photosynthesis. But a tiny fraction is re-emitted as fluorescence at longer red and far-red wavelengths. It's far too faint for human vision, but sensitive instruments can detect it.
Think of fluorescence as the plant's way of letting some excess excitation energy escape. Under favourable conditions, plants direct most absorbed light towards photosynthetic processes, while the remainder is dissipated as heat or fluorescence.
Chlorophyll fluorescence isn't just one colour. Signals associated with Photosystem II peak around 685 nm, while Photosystem I contributes strongly in the far-red, with a peak around 740 nm. This fluorescence light show emits hundreds of different ‘colours' in this tiny region of the electromagnetic spectrum.
Drought, the availability of nutrients and pests affect photosynthesis and the machinery controlling absorbed light. Consequently, fluorescence can change as stress develops – revealing trouble before leaves visibly wilt. That's one reason fluorescence is so exciting for plant monitoring.
Fluorescence doesn't simply go down whenever a plant is stressed. Suffering from drought or herbicide damage for example, a plant's fluorescence signal can rise or fall, depending on whether it's in the early or latter stages of being stressed. This is why scientists will look at fluorescence together with many other vegetation characteristics to understand what is really happening.
Leaves don't glow in the dark. Solar-induced fluorescence requires sunlight: chlorophyll absorbs incoming light and re-emits a small portion of that energy. It is fundamentally different from organisms such as fireflies that actively produce light through bioluminescence.
Here's the truly space-age part: the signal is faint, but satellites can distinguish it from reflected sunlight. ESA's FLEX mission is specifically designed to measure this vegetation fluorescence from space, providing a new window into how terrestrial ecosystems are functioning.
A conventional spaceborne optical instrument can tell us a lot about vegetation cover and greenness. FLEX aims to go deeper: fluorescence provides information connected to the actual functioning of photosynthesis. Because the amount of fluorescence emitted varies with plant health and environmental conditions, the measurements that FLEX collects will provide scientists with new information about photosynthetic activity and vegetation stress on a global scale.
FLEX won't work alone. It will orbit in tandem with a Copernicus Sentinel-3 satellite. Sentinel-3 can provide information about atmospheric conditions – including clouds, aerosols and water vapour – as well as land-surface characteristics and temperature. This integrated package of quasi-concurrent measurements promises to enable an unprecedented view of global vegetation function and status.
Why go to all this trouble? Because photosynthesis links carbon exchange and the water cycle, and food production. FLEX is designed to produce global vegetation-fluorescence maps at about 300 × 300 metres resolution, helping researchers understand how plants respond to drought, heat and other environmental pressures.
From the faintest glow of a single leaf to the productivity of forests, grasslands and crops across continents, fluorescence offers remarkable insight into how vegetation is functioning. Orbiting 800 km above Earth, FLEX will sense this hidden signal, helping scientists track how photosynthetic activity responds to a changing environment, and how photosynthesis affects the carbon and water cycles.
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