The balance in cellular energy flow between the energy source and metabolic sinks is termed photostasis. An imbalance in energy flow may be a consequence of the cumulative impact of changes in either, or both, light and temperature and is sensed by photoautotrophs as a change in excitation pressure, a measure of the relative redox state of quinone A of photosystem II reaction centres which reflects the reduction state of the intersystem electron transport chain. Changes in excitation pressure constitute an ‘operational’ retrograde redox signal which mediates changes in nuclear and plastid gene expression required to optimize photosynthesis and avoid the oxidative damage associated with excessive light energy absorption during abiotic stress. Restructuring of the photosynthetic apparatus in response to environmental change is accomplished by the coordination of retrograde operational signalling pathways with the photoreceptor-mediated sensing and signalling pathways which regulate chloroplast biogenesis and photomorphogenesis. Phenotypic and photosynthetic responses to abiotic stresses in green algae, cyanobacteria and terrestrial plants are compared with respect to acclimation to excitation pressure.
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