Plants are photosynthetic organisms that use light to reduce carbon dioxide, nitrate and sulfate to synthesise the organic molecules that are the building blocks of all life. The reductants are produced in the photosynthetic linear electron transfer chain in which two photosystems, PSII and PSI, operate in series. Excitation of PSII extracts electrons from water and the electrons are transferred to PSI for a second excitation, after which the electrons are potent reductants for anabolic reactions. Both photosystems require light to function, but they can also be damaged by light. This phenomenon is called photoinhibition. Plants have several overlapping mechanisms to prevent photoinhibition, but too large or rapid changes in environmental conditions can overwhelm the capacity of these protective mechanisms.

The first two papers included in the thesis, investigated the effects of photoinhibition of either PSI or PSII on the regulation of photosynthesis in Arabidopsis (Arabidopsis thaliana), a common model organism. The observed changes occurred in the phosphorylation of light-harvesting antenna proteins, which regulate the allocation of light to PSI and PSII. The detected changes, previously associated with acclimation to fluctuations in light intensity and quality, are able to restore the functional balance between the photosystems after photoinhibition. PSI photoinhibition also induces the accumulation of ATP synthase and cytochrome b6f complex in the thylakoid membrane. Chronic PSI photoinhibition also alters the redox regulation of enzymes involved in light reactions and carbon metabolism. These mechanisms have previously been linked to acclimation to changes in environmental conditions. My results show that the same mechanisms are also important in minimising the adverse effects of photoinhibition.

The third paper included in the thesis, examined how a 10°C drop in temperature alters high light acclimation in lettuce (Lactuca sativa). Under these conditions, lettuce is extremely efficient at quenching excitation energy to heat and protecting the PSII photoinhibition repair cycle from photodamage. This is proposed to occur through the concerted function of phosphorylation of the minor antenna protein, LHCB4, and accumulation of the light-harvesting-like protein, SEP2. Further analysis also revealed that PSII repair is regulated at the maturation stage of the reaction centre protein D1 under these conditions. The distinct regulatory mechanisms identified in lettuce show that plants have diverse mechanisms to protect photosynthesis, depending on the plant species and the environmental stresses to which they are exposed. The molecular characterisation of these different mechanisms paves the way for improving the stress tolerance and productivity of crop species.