Applications of Chlorophyll Fluorescene in Photosynthesis Research, Stress Physiology, Hydrobiology and Remote Sensing: An introduction to the various fields of applications of the in vivo chlorophyll fluorescence also including the proceedings of the first International Chlorophyll Fluorescence Symposium held in the Physikzentrum, Bad Honnef, F.R.G., 6–8 June 1998