Sensitivity of the relative F level of chlorophyll fluorescence induction in leaves to the heat stress

The (F-F)/F value of the fluorescence induction curve is shown to be a more suitable parameter to detect a wider range of heat stress damage to thylakoid membranes as compared to quantities t (time of fluorescence rise from F to (F+F)/2 level) and {Mathematical expression} (the fluorescence induction time defined as the area above the induction curve normalized to F=1). A method for exact and automatic F determination is presented. A break point in the quality and behaviour of the fluorescence induction curve of barley leaves incubated at 49°C was reached at the moment (about 240 s) when the transformation of PS II active (Q-reducing) to PS II inactive (Q-non-reducing) centres was completed. The meaning of the standard F and F/F parameter was then changed. The method of F determination described here may help to increase the analytical value of the standard chlorophyll fluorometers. © 1994 Kluwer Academic Publishers.