Simultaneous detection of photosynthetic energy storage and oxygen evolution in leaves by photothermal radiometry and photoacoustics

We have measured simultaneously the photothermal radiometry and the photoacoustic signals from intact leaves. We have confirmed that while the former senses that part of the modulated absorbed radiation not used in photosynthesis, but converted into heat, the latter, at low modulation frequencies, senses not only this heat but also the modulated oxygen evolution resulting from photosynthesis in the leaf. When photosynthetic activity is saturated upon additional excitation with strong non-modulated light, the photothermal radiometry signal increases (virtually all absorbed modulated light being converted into heat), while at the same time the photoacoustic signal decreases, because virtually no modulated oxygen evolution occurs any more. At higher modulation frequencies the behaviour of the photoacoustic signal closely follows that of the photothermal radiometry signal. We have used combined photothermal radiometry / photoacoustic measurements to estimate directly the yield of chemical energy storage in various plant species which applies for different times after excitation. Measurement of light saturation curves for wheat and Siberian pea bush leaves and of action spectra for the latter confirm the similarity between photothermal radiometry and high-frequency photoacoustic signals, and their difference from the low-frequency photoacoustic signal. Combined use of photothermal radiometry (or high-frequency photoacoustics) and low-frequency photoacoustics can thus provide more information than any one method alone. Experiments on intact chloroplasts and on a blue-green alga demonstrate that photothermal radiometry and photoacoustic methodologies can also be used for these tissues.