Blue, green, and red fluorescence emission signatures of green, etiolated, and white leaves

Laser-induced fluorescence emission of green terrestrial vegetation has become of high interest in remote LIDAR techniques in recent years. The remote sensing of blue and red fluorescence signatures appears to be a suitable future tool to determine the state of health of plants. In this contribution we report about major factors which determine the intensities of the UV-laser (337 nm) excited blue fluorescence near 450 nm and the red chlorophyll fluorescence between 650 nm and 800 nm as well as the ratio of the blue to the red fluorescence F450/F690. In contrast to green plants grown in a phytochamber or at low light intensities, plants grown in the field at a high photon flux density (PFD) showed high values for the ratio of the blue to the red fluorescence F450/F690, which was due to a relatively low intensity of the red chlorophyll fluorescence. The strongly increased ratio F450/F690 was primarily caused by a much reduced penetration depth of the exciting UV light into the leaf, which seemed to be due to substances absorbing in the epidermal layers. Etiolated wheat leaves exhibited a stronger blue fluorescence intensity than green leaves and showed also a small maximum around 530 nm (green fluorescence). White leaves of wheat treated with the bleaching herbicide norfluorazone (10⁻¹ M) and leaves from which the photosynthetic pigments had been extracted by acetone showed a fourfold and tenfold increase of the blue fluorescence, respectively. Isolated chloroplasts and thylakoids did not exhibit a blue-green fluorescence. It is concluded that the blue fluorescence is mainly caused by phenolic plant substances located in the cell wall and / or vacuoles of leaves. Partial reabsorption of the emitted blue and red fluorescence by the photosynthetic pigments modifies the shape of the fluorescence spectra. (author)