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Plant-Monitoring Fluorometer Has Its Day in the Sun

Photonics Spectra
Jun 2000
Daniel C. McCarthy

Through the ages, plants -- and particularly crops -- have evolved to survive adverse conditions, not to be highly productive. Agricultural research, however, strives to improve both the stamina and the yield of crops by understanding how they perform under extreme exposure to water, light, disease and other conditions. Photosynthetic performance is one measure of health, and one measure of photosynthetic performance is chlorophyll fluorescence.

Chlorophyll returns about 1 to 3 percent of sunlight as fluorescence, explained C. John Whitmarsh, a plant physiologist for the US Department of Agriculture's Agricultural Research Service. "When you damage plants, less light comes off," he added. "Some of that damage is reversible, and the plant compensates quickly. We're interested in longer-term damage that requires protein synthesis for repairs."

For years, researchers like Whitmarsh used kinetic fluorometers integrating high-frequency modulated light sources to excite fluorescence. Although these instruments allowed in vivo measurement of the rate, efficiency and regulation of photosynthesis, they were unable to provide image analysis of whole leaves or plants or under full sunlight.

Agricultural researchers used a CCD camera and two arrays of modulated orange LEDs and continuous blue light to image the fluorescent emission of chlorophyll from a 1-month-old periwinkle leaf (left) and a 1-year-old leaf (right). The strength of the fluorescence indicates the photosynthetic activity of the leaf. Photosynthesis is weakest where the leaf appears blue, medium where it appears green and strongest where it appears red. The fluorometer used can measure fluorescence dynamics for plants in the field under direct sunlight.

Recently, Whitmarsh selected the FluorCam, a fluorometer from Photon Systems Instruments that incorporates digital cameras and two arrays of modulated orange LEDs. It uses two arrays, each integrating 350 LEDs linked by a microchip to control color, intensity and pulse rate on the leaf. The LEDs uniformly illuminate the leaf with high-intensity 10-µs pulses to measure its fluorescence.

"The [fluorescence] technology has been around about 14 years, but what's new is the use of charge-coupled digital cameras and arrays of rapidly modulated LEDs," explained Whitmarsh. The camera is aimed between the arrays and captures a full image of the fluorescence signal every 40 ms -- although more expensive cameras can work faster, he noted.

"It's remarkable, because it can measure fluorescence without interference from sunlight, enabling us to identify the pattern of damage on a leaf," Whitmarsh said.

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