Farmers around the globe wish to monitor their crops regularly. At present, researchers from a university in China have created a wide-angle imaging system that uses chlorophyll’s fluorescence to observe the health of agricultural crops (Appl. Opt., doi:10.1364/AO.56.009762).
More than just photosynthesis
Chlorophyll is the important pigment that takes in solar energy and initiates photosynthesis in plants. Although a major portion of the absorbed energy is used up for photosynthesis, a small portion is discharged as fluorescence, and researchers can use evaluations of the fluorescence to observe plant stress.
For their imaging system, scientists headed by Xu Liu from Zhejiang University in Hangzhou used a high-power 460-nm LED as the light source because chlorophyll fluoresces in the red end of the visible spectrum. A relay lens and light pipe were used to alter the light spot’s shape to match the rectangular shape and aspect ratio of the cooled-type charge-coupled device (CCD) sensor of the system.
Imaging plant stress
The researchers developed the electronics of the imaging system in order to emit light pulses—good enough to initiate fluorescence, but not adequate to activate photosynthesis—and to time the CCD array to obtain images only if the target foliage fluoresced. The following system had the ability to image over an area of nearly 45 cm x 34 cm, a significantly larger area of view than commercially accessible systems. As the researchers stated, wider imaging area is indispensable because spot inspections of narrow areas of view might arbitrarily miss not-so-healthy plants from among the healthy ones.
To investigate the system, Liu and his colleagues imaged the foliage of cucumber seedlings inside a greenhouse. The researchers stressed certain seedlings by depriving them of adequate water or nitrogen-based supplements. The team used the series of images obtained from the seedlings to compute the way two important photochemical parameters altered in the time course of several days.
The system developed at Zhejiang has the potential to monitor seven or eight cucumber seedlings in just one image. Statistical analysis of the parameters from the series of images can offer a snapshot of the overall health of a crop field.