Respiration in intact leaves in relation to light and anabolism
Leaves photosynthesise in the light and respire in the dark. Respiration also continues to occur in the light, against the backdrop of the much larger rates of photosynthesis, but respiration is partially suppressed by light. This suppression is highly variable and poorly understood. It appears to involve two phases: a steep decline in respiration rate (non-photorespiratory CO2 release) across a range of very low light intensities, followed by a more gradual decline that occurs across a larger range of light intensities. The end of the steep, low-light phase is manifest as the "Kok effect," a discrete decline in the apparent quantum yield of net CO2 assimilation at light intensities close to the light compensation point.

A large part of these light-related changes in CO2 release probably involves a shift from catabolic to anabolic carbon flow through glycolysis and the TCA cycle, as well as suppression of the oxidative pentose phosphate pathway in favor of the reductive pathway (a.k.a. the Calvin cycle) by light. My colleague Mark Adams and I recently created and published a mathematical model that predicts respiratory, photorespiratory and photosynthetic CO2 and O2 exchanges in relation to light and anabolic demand for carbon. I am seeking NSF funding (with another collaborator, Dr Andrew Merchant of the University of Sydney) to test this model.  My lab is currently studying the role of G6PDH-linked CO2 release from the oxidative pentose phosphate pathway in the Kok effect.

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