This volume contains the papers, presented during a conference, organized jointly by the "Opzoekingsstation van Gorsem" and the "Limburgs Universitair Centrum", Belgium from 22 to 27 August 1982. For this third meeting, the chosen topic was the effect of different stresses on photosynthesis. Most of the research in this field is realized on water stress and temperature stress; this situation is refllected in the conference programme. However, the imp- tance of the other factors such as light, CO , salinity, anaerobiosis, was 2 also emphasized especially during the important discussion sessions. We express our gratitude to Drs. J. Gale, P. Jarvis, G.H. Krause, P.E. Kriedemann and P.S. Nobel for their excellent leadership during the discussion sessions. Particular thanks are also due to Dr. H.~i. Woolhouse who gave us an excellent inaugural address and whose erudition largely contributed to the interest of the discussions. For the first time in our experience of editors, we decided to use camera ready copies in order to publish more rapidly the proceedings and at a lower price. For a lot of reasons (among other things the bad choice of type of letter to be used and the choice of instructions to authors which were not perfectly followed by the authors), the technical presentation of this book will appear as non homogeneous; we accepted this lack of homogeneity with the hope tbat the publication time would be shorter in spite of the fact that, some authors delivered their manuscript with delay.