Study of a Method of Measuring the Transverse Distribution of Illuminating Laser Light and Laser-Induced Chlorophyll Fluorescence in Plant Leaves Using A Micro-Fluorescence Imaging (MFI) System.

Abstract

The chlorophyll fluorescence spectra of green plant leaves induced by a visible laser have strong emission peaks near 685 and 740nm. In order to examine the transverse distributions of the illuminating laser light and laser-induced chlorophyll fluorescence (687nm : F687 and 741nm : F741) within rice leaves, a micro-fluorescence imaging (MFI) system was devised using a microscope, a CCD camera with an image intensifier (I.I.), Ar+ lasers oscillating at 488 and 514.5nm, a He-Ne laser, an image processor, and a microcomputer. Transverse distributions of illuminated laser light and chlorophyll fluorescence in leaves, projected on the surface of rice leaves with different chlorophyll contents, were measured for laser lights with various wavelengths (488, 514.5, and 632.8nm). The distributions of light projected at 541.5 and 632.8nm were observed at a greater depth from the leaf surface than that projected at 488nm. The attenuation curve of the incident laser light in the mesophyll tissue was well approximated by an exponential function. As a result, the attenuation coefficient α for incident light of various wavelengths and of leaves of various chlorophyll contents could be determined. In order to examine the self-absorption from distributions of the chlorophyll fluorescence in leaves with different chlorophyll contents, the profiles of the F741/F687 ratio within the mesophyll tissue were measured at various wavelengths. The F741/F687 ratio increased in the case of light with a high chlorophyll absorption coefficient. We conclude that the MFI system is a useful method of study to select the spectral quality of light for plant irradiation.