The decreased maximum quantum yield of primary photochemistry (Fv/Fm), interpreted as photodamage (Powles, 1984; Torzillo et al., 1998; Maxwell & Johnson 2000), may be due to an inactivation of PSII reaction centers, an inhibition of electron transport at both donor and acceptor sides of PSII, and a distribution of excitation energy transfer in favor of PSI (Lu & Vonshak, 1999; Lu et al., 1999; Lu & Vonshak, 2002), increasing the cyclic electron flow around PSI (Gilmour et al., 1982; Joset et al., 1996). This decrease is often identified as an adaptive acclimation process of down regulation of PSII, a protective mechanism that helps dissipate excess energy from.
