Jeremy Harbinson - Quantum yields, and the coordination and regulation, of photosynthesis in C3 leaves

Photosynthesis light-use efficiency is fundamentally constrained by the light-limited quantum yield for carbon dioxide fixation, which under non-photorepiratory conditionsis as good as light-use efficiency gets. Normally assumed to be 0.125 - the magic number in photosynthetic quantum yields, it is actually less than this,and shows wavelength dependency. Despite its importance, this limiting quantum yield is not much studied or quantitatively understood. Some years ago we analysed the spectral-dependency of the light-limited yield for carbon dioxide fixation in cucumber, and we have repeated and extended this work for tomato.In addition to simple absolute quantum yield measurements we also, for tomato,examined the impact of state-transitions on carbon fixation yields, something which has not so far (to the best of our knowledge been) done. In addition to carbon dioxide fixation under light-limited conditions we also measured parameters associated with PSII and PSI efficiency. These results will be reported.  We have also developed equipment that allows us under a range of controlled conditions (varying irradiance, carbon dioxide and oxygen concentrations, and temperature) to measure, more or less concurrently, parameters associated with efficiency and regulation of light-use efficiency by PSI and PSII, and carbon dioxide fixation. With this we can build quite a complete picture of how photosynthesis in vivo proceeds and is regulated as a multicomponent system.Some of the results we have obtained do not seem to be consistent with the simple model(s) of regulation that have been developed over the last few years.Data will be presented to illustrate some of the anomalies we feel we have uncovered.