Regulation of Photosynthesis at Photosystem II

Abstract Studies have shown that the rapid turnover of the photosystem II reaction centre D1 protein is linked to the photosynthetic rate in higher plant leaves. Initially this research involved analysing the techniques that are used to assess photosynthetic activity. Leaf photosynthetic and electron transport rates and photosystem II protein turnover were measured in a series of experiments. From the rates determined, the underlying mechanisms that determine the actual rate of photosynthesis and the maximum photosynthetic capacity in leaves of higher plants were explored. In Phaseolus vulgaris, maximum photosynthetic capacity determined from CO2 fixation rates was compared with maximum electron transport rates measured via chlorophyll fluorescence. These experiments showed that the PSII electron transport rates did not reflect the maximum electron transport capacity unless light and other conditions were optimal. From the maximum ETRs and the maximum CO2 fixation rates a relationship between the two rates was derived. It was also concluded that combined gas exchange analysis and chlorophyll fluorescence derived ETRs may allow determination of non-linear electron flow. In Triticum aestivum plants under water, temperature and nitrogen stress D1 turnover and photosynthetic rates were measured. The results of these experiments suggest that D1 metabolism plays a significant role in the stress response in higher plants and that it does not limit/inhibit or restrict O2 evolution, PSII electron transport activity or PSII efficiency. D1 synthesis rates were examined in vivo from leaves that had been exposed to different concentrations of the glutamine synthetase inhibitor methionine sulfoximine. In these experiments D1 synthesis was significantly linearly related with maximum ETRs as well as O2 evolution rates and PSII efficiency. It was evident that high light and photoinhibition are not the sole triggers for changes in D1 synthesis rate. D1 turnover may be a constitutive activity, its rate related to the functional state of PSII, i.e. whether it is electron transport active or dissipative. A regulation link between D1 synthesis and photosynthetic activity is suggested. Different forms of the D1 protein generated under varying light regimes were examined in Pisum sativum. It was found that the D1 protein undergoes conformational changes or post-translational modifications in response to irradiance. Different D1 specific antibodies did not detect all the forms of D1, thus showing that Western blotting is not always a reliable technique for measuring net D1 content. Preliminary evidence suggested that the different forms of D1 may have different functional roles and may be associated with functionally distinct PSIIs. A model relating D1 structural differences and D1 metabolism is proposed. Finally, it is suggested that D1 protein metabolism, synthesis in particular, is linked to PSII electron transport activity and to feedback from various metabolic pathways. This research provides further evidence that D1 turnover plays a key role in photosynthetic regulation.