Laser-induced fluorescence imaging of plants using a liquid crystal tunable filter and charge coupled device imaging camera

Abstract

We developed a laser-induced fluorescence imaging system for plant monitoring use, with which it was possible to make an image at any wavelength between 430nm and 750 nm. The excitation source for the fluorescence was a cw ultraviolet laser diode with 398nm, and the detector was an image-intensified charge coupled device. A liquid crystal tunable filter was used as the fluorescence wavelength selection device. All of the system performance including the wavelength tuning was electrically controlled, so that it could be operated with no mechanical vibration noise.The fluorescence images of a coffee tree leaf obtained at 440 nm, 530 nm, 685 nm, and 740 nm clearly showed a distribution pattern of the fluorescence intensity over the leaf. The pattern reflected different 1 physiological statuses of the plant. Advantages of the imaging system were experimentally discussed on a point of detection of inhomogeneous physiological activities over a plant leaf. 2In agriculture and ecological science, there are strong demands to monitor plant growth process based on physiological activities, such as photosynthesis, light use efficiency, response to growth environmental stress, retrieval process from disease and so on. Plant fluorescence can be available for these requirements. As the fluorescence is released to outside after light use occurred inside of every part of plant, it can inherently contain physiological information. Using a laser as an excitation source for laser-induced fluorescence (LIF) has some advantage, in particular, an ultraviolet laser diode (LD) is an ideal excitation source because of compactness, solid-state construction, and a low power consumption. Combination of a laser with an imaging device offers another possibility to the measurement, which is LIF imaging. This is especially effective to understand the spatial heterogeneity of plant activities.Expansion to a LIF imaging lidar (light detection and ranging) makes possible of in vivo remote monitoring of vegetation. [1][2][3] In this short technical report we describe the LIF imaging system that was developed for monitoring of plant's physiological activities.There were three basic concepts we required the system. First, the system should possess an ability of multispectral image monitoring. Several distinctive wavelengths in the plant LIF such as 430, 440, 450, 460, 475, 525, 530, 550, 600, 680,3 685, and 740nm were reported. The wavelength and the intensity varied in accordance with PH, 4 temperature, 5 stress, 6 plant species, 7 seasons 8 , and excitation source. 9If reflectance information (wavelength) on a plant such as physiological reflectance index 10 can be also monitored together with the fluorescence information, the system will be more practical. Thus multi spectral monitoring is anticipated, but it is unfavorable to prepare many filters. Second, the system does not involve as few mechanically driven devices as possible, so that precise positioning in the detection is achievable. Positioning directly influences results of image a...

Scientific and technical challenges in remote sensing of plant canopy reflectance and fluorescence

“…The laser pulses of actinic light, which can be discriminated from static and panchromatic background light, are applied to elicit fluorescent transients when measuring fluorescence from a distance (Cecchi et al, 1994;Corp et al, 2006). The footprint of such a light detection and ranging (LIDAR) laser beam can be expanded from several centimetres up to metres to cover larger observation areas or to decrease the power of the excitation source (Saito et al, 2005). The first field laser-induced vegetation fluorescence was observed by Measures et al (1973).…”

A portable fibre-probe ultraviolet light emitting diode (LED)-induced fluorescence detection system

“…The wavelength region around 440 nm called the blue fluorescence and around 494-533 nm called the green fluorescence is derived from the products of the plant's secondary metabolism such as ferulic acid derivatives, phenyl propanoids and partially mesophyll origin and tends to be a proper index of a plant's health and stress. The spectra at the peak of 740 nm (far red fluorescence) and that of 690 nm (red fluorescence) are also good indicators to evaluate chlorophyll concentration [20,21]. Any increase in the blue or green fluorescence shows stress on the leaf of the plant or on the fruit.…”