The photoacoustic technique is used to monitor thermal dissipation processes as well as oxygen evolution and uptake in intact leaves. It was shown to be capable of providing significant information on the properties of alternative Photosystem I-dependent electron transport routes. The potential of this method becomes particularly high if photoacoustic signals are detected simultaneously with other parameters enabling independent characterization of the redox states of photosystems II and I. The effects of elevated temperatures on the function of photosystem I-driven electron transport routes were examined using photoacoustic signals simultaneously measured with either modulated chlorophyll fluorescence for the characterization of photosystem II or absorbance changes at 830 nm for the assay of photosystem I. The maximum turnover time of photosystem I in methyl viologen-treated leaves exposed for 5 min to 45°C was estimated from the light-response of variable thermal emission to be 105 ms. Maximum size of the pool of reductants located in the chloroplast stroma and available for photosystem I was calculated as 140 eq:P700. That pool was rapidly exhausted in methyl viologen-treated leaves, and that process accelerated following an increase in irradiance. The recovery of the pool occurred in the dark with half-time of about 1 min. The «far-red drop» in photosystem I activity measured as variable heat emission was absent when the photosystem receives electrons from stromal reductants instead of photosystem II. The approach developed and described here can be easily spread over other aspects of stress plant physiology.