Photoacoustic detection of photosynthetic oxygen evolution from leaves. Quantitative analysis by phase and amplitude measurements

The photoacoustic signal from an intact leaf was analyzed as a vectorial summation of photothermal and photosynthetic oxygen-evolution contributions. A method is outlined to estimate each contribution separately. The amplitude of the oxygen-evolution component relative to that of the photothermal singnal decreases as the modulation frequency increases due to two processes which specifically damp the oxygen-evolution modulation: (1) diffusion of oxygen from the chloroplasts to the cell boundary, and (2) electron-transfer reactions occurring between the photochemical act and oxygen evolution. The effects of the two processes are well separated and are observed over different ranges of modulation frequency. Analysis of the data leads to a consistent estimation of the oxygen diffusion coefficient and also to a preliminary idea on the limiting time constant on the donor side of Photosystem II. The dependence of the photoacoustic oxygen-evolution signal on the intensity of added nonmodulated background light is used to construct the light saturation curve of (gross) Photsynthesis, with an estimation of the ratio maximal rate / maximal quantum yield. The photoacoustic method is distinguished by its sensitivity and rapidity (a single measurement takes approx. 1 s), far better than any other method to measure gross photosynthesis. The only disadvantage is in the fact that the quantum yield of oxygen evolution is determined in a relative basis only. Attempts to calibrate the photoacoustic measurements in an absolute sense are underway.