**4. A leaves**

Photoacoustic methods provide unique capabilities for photosynthesis research. The pulsed photoacoustic technique gives a direct measurement of the enthalpy change of photosynthetic reactions (Carpentier et al., 1984). A microphone may detect the photoacoustic waves *via* the thermal expansion in the gas phase. This method allows *in vivo*  measurements of the photosynthetic thermal efficiency, or energy storage, and of the optical cross-section of the light harvesting systems (Carpentier et al., 1985, Buschmann, 1990).

Photoacoustic spectroscopy first emerged as a technique for photosynthesis research in the pioneering works of Cahen and Malkin (Malkin and Cahen 1979). Oxygen evolution by leaf tissue can be measured photoacoustically with a time resolution that is difficult to achieve by other methods (Canaani et al. 1988; Malkin 1996).

Photoacoustic measurements can achieve microsecond time resolution and allow determination of fast induction phenomena in isolated reaction centers, photosystems, thylakoid membranes intact cells and leaf tissue (Fig. 4).

Fig. 4. Setup for photoacoustic photosynthesis measurements in air phase on leaf discs and algae collected on filters (Cha and Mauzarall 1992). Both the sample and reference twin chambers are connected by an air passage to hearing aid microphones.

These phenomena include states of the oxygen evolving complex in leaf tissue (Canaani et al. 1988) and the earliest steps of photosynthetic electron transport in photosystems (Arata and Parson 1981; Delosme et al. 1994, Edens et al. 2000).

In the work of da Silva (da Silva et al, 1995) the photoacoustic method has been demonstrated to be suitable, efficient and reliable technique to measure photosynthetic O2 evolution in leaves.

The O2 evolution in intact undetached leaves of dark adapted seedlings was measured during photosynthesis with the objective to detect genetic differences (da Silva et al., 1995).

Photoacoustic method can also measure state photosynthesis in intact cells and leaf tissue if the measuring pulses are given in combination with continuous background light (Kolbowski et al. 1990).
