Thermoluminescence in Chloroplast Thylakoid

Luminescence occurs in almost all the materials absorbing photon energy and is a phenomenon of light emitting process. There are various types of luminescence, e.g. fluorescence, phosphorescence, delayed luminescence, chemiluminescence and thermoluminescence. In these cases, light is emitted when a photoor chomo-excited molecules is deexcited to its ground state. During this deexcitation process, radiationless internal conversion and heat dissipation occurs, which reduces the quantum yield of light emission. Luminescence can be also thermally induced and enhanced by heating the sample in the dark. This process is called thermoluminescence (TL) and describes the emission of light at characteristic temperatures from samples containing chemiluminescent active species, radical pair states or electron hole pairs (Ducruet, 2003; Misra et al., 2001). Many minerals heated at very high temperature emit luminescence and so TL has been used initially in geology, archeological dating and radiation dosimetry. The theory of charge recombination in these processes was first worked out for such minerals (Randall & Wilkins, 1945). Thermally induced photon emission by a pre-irradiated chloroplast or thylakoid or by leaf samples in darkness is known as thermoluminescence (TL) (Misra et al., 2001; Misra & Ramaswamy, 2001). This is characteristic of solid states (semi-conductors) under thermally activated recombination of electrons and positive holes that are generated by particle radiation or electromagnetic field at room or low temperature prior to their heating in dark (Chen & McKeever, 1997). TL signals were first detected in dried chloroplasts samples (Arnold & Sherwood, 1957; Tollin & Calvin 1957). Photosynthetic systems in dried chloroplasts are supposed to be severely damaged. TL emission was also recorded in intact leaves and algal cells (Arnold & Sherwood, 1957). Arnold (1966) proposed a model of recombination of free holes in PS I and of free electrons from PS II as the sources of thermally induced luminescence from algae (Chlorella) cells in darkness. However, Arnold & Azzi (1968) refuted the role of free holes from PS I. Based on further evidence on the generation of charges in irradiated chloroplast, in the new model for TL charge recombination the positive and negative charge traps were proposed to reside within PS II (Arnold & Azzi, 1968). Further, biophysical studies and availability of photosynthetic