Regulation of Photosynthesis in plants under abiotic stressMost plants complete their life cycle in a single location and therefore are affected by the changing environment. As a result, plants have evolved physiological and developmental adaptations to overcome stress. The work presented in this thesis has examined the regulation of photosyntheticelectron transport in barley, rice and Thellungiella salsuginea.Barley is considered as a crop which is comparatively tolerant to soil salinity. The focus of this study was to evaluate the physiological responses of photosynthesis in barley under salinity and to characterize traits responsible for the regulation of photosynthesis. At low salt concentrations, barley plants protect PSII centres from excitation pressure by down-regulating the electron transport chain and maintaining DeltapH, by cyclic electron transport associated with PSI, to support nonphotochemical quenching (NPQ). However, at the highest concentration of salt examined, this regulation starts to fail. The failure might result from a specific loss of PSI, resulting in reduced cyclic electron flow, or an increase in the leakiness of the thylakoid membranes, resulting in loss of DeltapH.The effects of salinity on the regulation of electron transport through Photosystem I and Photosystem II have been studied in two rice varieties from Sri Lanka. The regulation of photosynthesis in the salt-tolerant At-354 is more prominent than in the salt-sensitive Bg-352 when plants are exposed to salt. Exposure of Bg-352 to salt resulted in a substantial decrease in gas exchange, PSII photochemistry, leaf area and loss of chlorophylls. The decrease in the photosynthesis in AT-354 is caused by stomatal limitations, which restrict the CO2 entry into the plants, whereas the decrease of photosynthesis in Bg-352 is caused by non-stomatal limitations. Results suggest that At-354 protects PSII centres from excitation pressure by down-regulating the electron transport chain and maintaining DeltapH by cyclic electron transport associated with PSI to support NPQ. At high salt concentration, this regulation starts to fail in Bg-352.Tolerance to abiotic and biotic stress has evolved in many wild plant species, termed extremophiles.These plants contain essential genes which may used to improve crop production in changing environments. Thellungiella salsuginea is an extremophile, able to grow and reproduce in extreme environments. Stepien and Johnson (2009) identified a protein, known as the plastid terminal oxidase (PTOX) which acts as an alternative electron sink in T. salsuginea under salt stress. The current study showed that, in addition to salt, T. salsuginea showed increases in PTOX protein content and activity when exposed to drought, different growth irradiances and cold with high light. Semi-natural conditions also triggered the activity of PTOX. This study also showed that salt caused an up-regulation of PTOX gene transcripts in the leaves of salt treated T. salsuginea plants compared to control plants. Direct electron transport from PSII to PTOX and then to oxygen via the PQ pool accounted for up to 30% of total PSII electron flow in T. salsuginea (Stepien and Johnson, 2009). Efficient electron flow from PSII to PTOX would however, probably require co-location of these complexes in the same thylakoid fraction. To examine the location of PTOX in the thylakoid membrane, immunoblot analyses were performed, to test for changes in other protein complexes which may be associated with PTOX. In addition blue-native polyacrylamide gel electrophoresis and immunoblots were performed to isolate and detect the PTOX protein with any associated complexes. Although immunoblot analysis showed a prominent signal, mass spectrometry data did not allow identification of PTOX. This results suggests that further studies are needed to identify the precise localisation of the PTOX protein in the thylakoid membranes in T. salsuginea.