Fluorescence Imaging for Better Crop Monitoring

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The new crop imager showed changes in photosynthesis as the nitrogen deficiency deteriorated for cucumber seedlings over time. ΦPSII and Fv/Fm are two photosynthetic parameters calculated from chlorophyll fluorescence. The white rectangular box shows an area of 10 cm by 10 cm, the imaging area for some commercial chlorophyll fluorescence imaging systems. [Image: Haifeng Li, Zhejiang University] [Enlarge image]

All farmers want to keep an eye on their crops. Now scientists at a Chinese university have developed a wide-view imaging system that uses the fluorescence of chlorophyll to monitor the health of agricultural crops (Appl. Opt., doi:10.1364/AO.56.009762).

More than just photosynthesis

Chlorophyll is the vital pigment that absorbs solar energy and drives photosynthesis in plants. While a good portion of that absorbed energy goes into photosynthesis, a fraction of it is emitted as fluorescence, and scientists can use measurements of that fluorescence to detect plant stress.

Researchers led by Xu Liu at Zhejiang University in Hangzhou chose a high-power 460-nm LED as the light source for their imaging system, since chlorophyll fluoresces in the red end of the visible spectrum. A light pipe and relay lens changed the shape of the light spot to match the rectangular shape and aspect ratio of the system's cooled-type CCD sensor.

Imaging plant stress

The team designed the imaging system's electronics to send out pulses of light—just enough to drive fluorescence, but not enough to stimulate photosynthesis—and to time the CCD array to acquire images only when the target foliage fluoresces. The resulting system images over an area of up to 45 cm x 34 cm, a considerably larger field of view than commercially available systems. A wide imaging area is important, according to the researchers, because spot checks of narrow fields of view could randomly miss struggling plants in a bed of healthy greenery.

Liu's team tested the system by imaging the foliage of cucumber seedlings in a greenhouse. The scientists stressed some of the seedlings by depriving them of sufficient levels of either water or nitrogen-based nutrients. From the sequence of images of the seedlings, the group could calculate how two key photochemical parameters changed over the course of several days.

The Zhejiang system can capture seven or eight cucumber seedlings in a single image, and statistical analysis of the parameters from a sequence of images can provide a snapshot of a crop field's overall health.

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