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

The (F_pl-F_o)/F_v 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 _1/2 (time of fluorescence rise from F_o to (F_o+F_m)/2 level) and % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaacq% aHepaDaaaaaa!39D5!\\overline \tau \] (the fluorescence induction time defined as the area above the induction curve normalized to F_v=1). A method for exact and automatic F_pl 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_B-reducing) to PS II inactive (Q_B-non-reducing) centres was completed. The meaning of the standard F_v and F_v/F_m parameter was then changed.The method of F_pl determination described here may help to increase the analytical value of the standard chlorophyll fluorometers.Abbreviations F_o initial fluorescence - F_m maximal fluorescence - F_pl fluorescence at first inflection point (plateau) - F_v variable fluorescence (F_v=F_m–F_o) - PSM plant stress meter - SD standard deviation