**3. Photoacoustics and photosynthesis**

In principle, photosynthesis generates three phenomena which may be detected by photoacoustics, with adequate setups. The thermal expansion of tissue, liquids and gases due to light energy converted to heath is termed as the **photothermal** signal. That is generated always, when photosynthetic tissue, or cell is exposed to a light pulse, since never is all of the light absorbed by plant tissue stored as products of the process. The unused fraction of the absorbed light energy is converted to heat, resulting in measurable pressure transient. When a leaf is illuminated by a pulse of light, the resulting photosynthetic photolysis of water causes the evolution of a burst of gaseous oxygen. That process leads to an increase in pressure, a change which is readily detected by a microphone as the **photobaric** signal. For detailed definitions and description see review by Malkin (1996). In addition to these two, absorption of light by components of the photosynthetic apparatus, such as PSI, or PSII, is accompanied by change in its spatial conformation and volume change, or **electrostriction.**
