ABSTRACT
In general, photoinhibition is regarded as the light-dependent loss of photosynthetic efficiency, normally occurring under conditions of light harvesting antennae absorbing more excitation energy than can be dissipated by photochemistry of photosynthesis (Powles, 1984; Krause, 1988; Osmond, 1994; Long et al., 1994; Hurry et al., 1997). The primary target of damage during photoinhition is photosystem II (PS II). Further, it is also shown that specifically, the Dl subunit of PS II reaction centre is degraded, a significant consequence of photoinhibition (Ohad et al., 1984; Adir et al., 1990). The Dl protein of PS II has the highest turnover rate, particularly at high light intensities and may even be 50 to 80 fold higher than any other thylakoid proteins (Prasil et al., 1992; Andersson et al., 1994). Though the primary target of photoinhibition is photosystem II, the photoinhibition of photosystem I (PS I) is not altogether unknown. This is evident under conditions of illumination of cold sensitive plants at low temperatures. Terashima et al., (1994) and Terashima et al., (1998) have reported the inhibition of PS I in Cucumis and subsequently also in other plants such as potato (Havaux and Davaud, 1994), cucumber (Sonoike 1995), barley (Tjus et al., 1999) and spinach (Tjus et al., 2001). However, PS I photoinhibition has been much less studied (Hihara and Sonoike, 2001). Looking at a different perspective, Kato et al., (2002) have referred photoinhibition to a net inactivation of PS II which can be ascertained by a balance between gross inactivation (photoinactivation) and the simultaneous recovery of PS II through the process of Dl protein turnover.
