Correlation between chlorophyll fluorescence and photoacoustic signal transients in spinach leaves

Chlorophyll fluorescence and photoacoustic transients from dark adapted spinach leaves were measured and analyzed using the saturating pulse technique. Except for the first 30 s of photosynthetic induction, a good correlation was found between photoacoustically detected oxygen evolution at 35 Hz modulation frequency and electron flow calculated from the fluorescence quenching coefficients qP and qN. The induction kinetics of the photothermal signal, i.e., the photoacoustic signal at 370 Hz, reveal a fast (t r <10 ms) and a slow (t r ≈1 s) rise component. The fast component is suggested to be composed of the minimal thermal losses in photosynthesis and thermal losses from non-photosynthetic processes. The slow phase is attributed to variable thermal losses in photosynthesis. The variable thermal losses were normalized by measuring the minimal photothermal signal (H0) in the dark-adapted state and the maximal photothermal signal (Hm) during a saturating light pulse. The kinetics of the normalized photochemical loss (H-H0)/(Hm-H0) obtained from high-frequency PA measurements were found to correlate with the kinetics of oxygen evolution measured at low frequency.