Photosynthesis Research 40: 45-53, 1994. © 1994 Kluwer Academic Publishers. Printed in the Netherlands. Regular paper The use of photothermal radiometry in assessing leaf photosynthesis: II. Correlation of energy storage to Photosystem II fluorescence parameters Alexandra R.J. Driesenaar ~, Ulrich Schreiber 2 and Shmuel Malkin 1 ~Biochemistry Department, The Weizmann Institute of Science, Rehovot 76100, Israel; 2Lehrstuhl Botanik I, Universitiit Wiirzburg, Mittlerer Dallenbergweg 64, D-8700 Wiirzburg, Germany Received 1 June 1993; accepted in revised form 29 November 1993 Key words: infra-red, quantum yield of PS II, temperature Abstract Following the first part of this work (Malkin et al. (1991) Photosynth Res 29: 87-96), where modulated photothermal radiometry (PTR) was used to measure energy storage (ES) in intact leaves as a function of PT00 redox state, we report here on simultaneous ES and fluorescence measurements, which characterize the state of PS II. PTR monitors the conversion of modulated light into heat by measuring the modulated infra-red radiation emitted from the sample. The ratio [PTR+-PTR_]/PTR+, where PTR indicates the PTR signal and the subscripts +,_ indicate the presence or absence of saturating background light, is used to quantitate ES. We searched carefully for the right conditions where the background light does not introduce a significant rise in the leaf temperature, which influences the PTR signal as such, otherwise the above ratio deviates from the true ES. Under such conditions, ES and the fluorescence parameters,. F (momentary fluorescence level) Fm' (fluorescence of fully reduced PS II reaction centers) were measured during the induction phase of photosynthesis and in the steady state. ES and the parameter 3' = (Fm'-F)/Fm', considered by Genty et al. ((1989) Biochim Biophys Acta 990: 87-92) to reflect the yield of PS II, had similar kinetics during the induction phase. Both reached a final maximum plateau after about 4-5 min. of illumination. In different experiments, where the measuring light intensities varied, 3' was approximately linearly related to ES. This linear relationship was found in the same way also in steady-state measurements, where these parameters varied by using different background light intensities. Extrapolation to an ES value of zero indicates a finite non-zero value of 3'. A possible explanation for this may be found in the existence an electron transport cycle around PS II which does not store energy in the range corresponding to the modulation frequency used (ca. 3.6 Hz). Abbreviations: ES- energy storage; PTR- photothermal radiometry; PTR_, PTR+- PTR signal in absence or presence of background light, respectively Introduction The modulated heating caused by the absorption of modulated light can be measured by photo- thermal radiometry (PTR), which involves the detection of the modulated part of the emitted infra-red irradiation (Nordal and Kanstad 1981). PTR is therefore a suitable technique to measure photochemical energy storage. In the first part of this work (Malkin et al. 1991), PTR was used to determine photosynthetic energy storage by measurements in intact leaves. A direct correla- tion between energy storage and the redox state of PT00 was demonstrated when the modulated