The rapid response of the photosynthetic system to changes in light intensity (response within less than 30 min) is condidered. Variations in light intensity result in concentration changes in photosynthetic intermediates or protein states. These changes in turn affect Photosystem II (PS II) by a 'backpressure effect', resulting in the accumulation of PS II products (reduced plastoquinone, lumen protons). The product backpressure produces PS II states especially susceptible to photoinactivation and photodamage. By activation of special adaptation mechanisms, the efficiency of the photosynthetic system is optimized and photodamage is minimized. The following aspects are discussed: (1) long-term vs. short-term adaptation; (2) analysis of short-term adaptation by measurement of chlorophyll alpha fluorescence and photosynthetic oxygen evolution; (3) kinetics of the response of the photosynthetic system to changes in light intensity (induction curves, assignment of phases, time constants); (4) the 'product backpressure' on PS II (accumulation of reduced Q(A), lumen pH effect on PS II donor side reactions, thylakoid voltage effect on PS II photochemistry); (5) the molecular mechanisms of short-term adaptation (pH-dependent energy quenching, reversible inactivation of the manganese complex, light-harvesting complex (LHC) phosphorylation); (6) induction of photoinactivation and photodamage; (7) relation between product backpressure, adaptation and photodamage.
