ABSTRACT

Photoprotection in plants generally refers to a multi-component process, evolved to overcome a potential damage arising from absorption of light energy by photosystem II antenna system in excess of consumption during photosynthesis (Horton et al., 1996; Niyogi, 1999; Demmig-Adams and Adams, 1996; Muller and Niyogi, 2001; Mullineaux and Karpinski, 2002; Ort and Baker, 2002). The light energy absorbed by plants in excess of that which can be utilized for photosynthesis is understood to be excess excitation energy (EEE) (Niyogi, 2000; Mullineaux and Karpinski, 2002). Leaves, when exposed to full sunlight, are unable to use all the absorbed light energy to perform photosynthesis (Demmig-Adams and Adams, 1996b). Under relatively optimal environmental conditions, the intercepted light energy by photosynthetic pigments is utilized for carbon assimilation. However, it is thought that photoprotection involves the availability of several alternative electron acceptors under C 0 2 limited environment, the detoxification of reactive oxygen molecules and various repair cycles in order to prevent acute photodamage (Ort, 2001). Photoprotection could also be viewed as one of the two components of the overall process of photoinhibition, the other being photodamage. It is understood that photoprotection is dependent on the action of xanthophyll cycle which functions during a shorter time period of less than one hour, while the photodamage results in the inactivation of reaction centre Dl protein of photosystem II and its subsequent repair (Demmig-Adams and Adams, 1992; Long et al., 1994; Thiele et a l , 1996; Kitao et al., 2000).