Cold Acclimation: A Complex Interaction Of Low Temperature, Light And The Redox State Of Photosystem Ii

The process of cold acclimation refers to the physiological processes by which cold-tolerant plants such as cereals, when developed at low temperature, acquire freezing tolerance. Photosynthesis provides the energy required for these complex metabolic changes to take place. However, photosynthetic organisms must maintain a balance between energy input as a result of photochemical reactions and energy consumption through intersystem electron transport and the various biochemical reactions of cellular metabolism. Imbalances may be sensed by changes in the redox state of photosystem II (PSII), as this represents the rate-limiting step of photosynthetic electron transport. The redox state of PSII is sensitive to both temperature and irradiance. I demonstrate in this thesis that the photosynthetic adjustments which occur as a result of cold acclimation are responses to the redox state of PSII and not low temperature per se. In addition, plant growth habit and the mRNA accumulation of a nuclear gene associated with the acquisition of freezing tolerance (Wcs19) are also modulated by the redox state of PSII. Thus, changes in the environment which perturb the redox state of PSII, act to generate a chloroplastic redox signal, which may represent the first component in a sensing/signalling pathway which acts synergistically with other transduction pathways to elicit the appropriate responses to all environmental stresses