Attenuation of the photobaric-photoacoustic signal in leaves by oxygen-consuming processes

Charland et al. (Biochim. Biophys. Acta 1992, 1098, 261-265) obtained photo acoustic data from sugar maple tree leaves, in which the photobaric part of the photoacoustic signal declined in time following a transition from high light to low light level, which they interpreted as indicating stromal and plasmal oxygen-consuming processes. Here, a simple mathematical model of o xygen diffusion, which includes a continuous distribution of oxygen-consumi ng sinks in the diffusion path from the photosynthetic membrane to the inne r air phase, is presented. The model explains the main features of the depe ndence of the steady-state signal on the modulation frequency and the light intensity, although the numerical agreement between the data and the resul ts of the model is only semi-quantitative, which is discussed. It turns out that at sufficiently high light intensity, or at a short time after a prev ious exposure to a high light level, the effect of oxygen consumption tends to zero because a high stromal oxygen concentration is built up which is s aturating for the uptake process. Within this limitation, the merit of the photoacoustic signal as an indicator for photosynthetic oxygen evolution is preserved, answering recent doubts.