Light stress provokes plastic and elastic modifications in structure and function of photosystem II in camellia leaves

Adaptation of plants to a continuously changing environment may be approached in terms of an optimisation strategy that is dictated by the thermodynamic demand for minimal entropy production. Any change in environmental input is a stressor in the sense that it disturbs optimality. Stress is defined by this disturbance and is therefore relative, with non-stress serving as the reference condition. Because of the thermodynamic demand for optimality, suboptimality forces the system to undergo a state change to a new optimal state. These state changes bring about the adaptation of the system. Any physical or chemical change that occurs during the adaptation processes is denoted as strain. In the present study, the polyphasic rise in chlorophyll (Chl) a fluorescence (OJIP) is used to investigate the behaviour of photosystem II in camellia leaves under different physiological steady states established by adaptation to different light regimes at both room temperature and 10°C. Analysis of the fast fluorescence rise according to the JIP-test allows establishment of structural and functional parameters, providing a quantification of the system's behaviour. The calculated functional parameters are the specific (per reaction center, RC) and phenomenological (per cross section of the sample, CS) energy fluxes for absorption (ABS), trapping (TR) and electron transport (ET). The flux ratios or the yields (the maximum quantum yield of primary photochemistry, φ_p0, the efficiency with which a trapped exciton can move an electron into the electron transport chain, ψ₀, or the probability that an absorbed photon will move an electron into the electron transport chain, ψ_E0 are structural parameters provided by this test, as well as an expression of the concentration of the reaction centers (RC/CS). The photochemical (k_p) and nonphotochemical (k_N) de-excitation rate constants are also calculated. The present study focuses mainly on the response of photosystem II to cyclic environmental changes, denoted as the residual strain. The study investigates the adaptive processes in relation to the regulation of the different functional and structural parameters, as well as reversibility (elasticity) and irreversibility (plasticity). Having screened camellia leaves for several environmental changes, we observed that the various structural parameters undergo modification that differ in both their extent and degree of elasticity, which indicates that different survival strategies are employed in response to stress.