All photons absorbed by the photosynthetic pigments result in excited chlorophyll, but at irradiance levels beyond the linear, light-limited region of the light-response curve of photosynthesis, not all excited chlorophyll can be used in photochemistry (Figs. 8, 15). The fraction of excitation energy that cannot be used increases with irradiance and under conditions that restrict the rate of electron transport and Calvin-cycle activity such as low temperature and desiccation. This is potentially harmful for plants, because the excess excitation may result in serious damage, if it is not dissipated. To avoid damage, plants have mechanisms to safely dispose of this excess excitation energy. When these mechanisms are at work, the quantum yield of photosynthesis is temporarily reduced (minutes), a normal phenomenon at high irradiance. The excess excitation energy, however, may also cause damage to the photosynthetic membranes if the dissipation mechanisms are inadequate. This is called photoinhibition, which is due to an imbalance between the rate of photodamage to PS II and the rate of repair of damaged PS II. Photodamage is initiated by the direct effects of light on the O2-evol-ving complex and, thus, photodamage to PS II is unavoidable (Nishiyama et al. 2006). A reduction in quantum yield that is re-established within minutes to normal healthy values is referred to as dynamic photoinhibition (Osmond 1994); it is predominantly associated with changes in the xantho-phyll cycle (Sect. 3.3.1). More serious damage that takes hours to revert to control conditions leads to chronic photoinhibition; it is mostly related to temporarily impaired D1 (Sect. 2.1.1; Long et al. 1994). Even longer-lasting photoinhibition (days) can be referred to as sustained photoinhibition (Sect. 7.2). A technique used for the quantification of photoinhibition is the measurement of quantum yield by means of chlorophyll fluorescence (Box 2A.4).
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