Introduction:
Vegetation health monitoring by fluorescence imaging is a recent development in remote sensing and initial experiments have revealed the potential of imaging technique in detecting the quality of agro-foods. It is known that agro-food of high quality can only be produced by physiologically active plants which grow without any biotic, abiotic or anthropogenic stress. Later sustainance of its quality depends on the stage of fruit development or ripening at the time of harvesting and packaging.
The laser-induced fluorescence (LIF) spectra of plant leaves consists of the blue (440 nm) and green (520 nm) fuorescence bands as well as the red (690 nm) and far-red (740 nm) chlorophyll emission bands shown in the figure.
The LIF signatures such as the fluorescence intensity ratios (blue/red, blue/far-red, red/far-red, etc.) and associated spectral features are presently used in remote detection of plant stress. It has been proved that the LIF characteristics permit an early detection of stress and strain conditions in leaves and associated damage to the photosynthetic apparatus.
Recently, a new high resolution fluorescence imaging Nd:YAG laser system that allows fast and large scale screening of fluorescence gradients and fluorescence changes over the whole leaf surface was developed, jointly by physicists of Strasbourg-GOA and plant physiologists of Karlsruhe-Botanisches II, for detectión of vegetation stress. This system was used to remotely detect stresses in different kinds of plants, both under laboratory and field conditions. Further studies have shown that this technique has the ability to measure the fluorescence images of leaves , fruits, and agro-food at the four LIF emission bands for arriving at parameters useful in stress detection.
Activities at the Botanical Institute II of Karlsruhe University, Germany:
A compact fluorescence imaging system(FIS) is being established in Karlsruhe(TH) as part of a joint research program "Quality Assessment of Agro-food by Fluorescence Imaging (QAAFFI)" of German, French and Hugarian physicists and plant physiologists. This system consists of a xenon flashlamp with interference filters centered at 440, 520, 690 and 740 nm, a beam expander, a CCD camera and spectral analysis software. Measurement and analysis of high resolution fluorescence images of agro-foods such as apple, grapes, peach, strawberries, lemons, bell peppers(paprika) and leafy vegetables are being carried out using the FIS system. Ripening and senescence effects can be seen in the fluorescence image ratios (F440/F690, F440/F740, F440/F520, and F690/F740) before they become visually detectable. The study would be of use in conservation of agro-foods and its quality monitoring during different stages harvest, ripening and storage.
Because of the obvious advantages of whole area sampling over point sampling, this technique can collect much more information from the whole leaf surface or several leaves or fruits as has not been possible with any other plant or agro-food screening method. Also since the fluorescence imaging is non-destructive measurements can be repeatedly performed with the same leaf, vegetable, fruit or crop plant, thereby guaranteeing a high accuracy in the assessing physiology and quality of agro-food. Thus, unlike the laser-induced fluorescence imaging system (LIFIS), the FIS system would be cost effective with possiblitites for easier adaptation to non-destructive evaluation of agro-food quality during growth, harvest and storage in agriculture/industry.