Calibration of Simultaneous Measurements of Photosynthetic Carbon Dioxide Uptake and Oxygen Evolution in Leaves (original) (raw)

2006, Plant and Cell Physiology

The stoichiometric ratio of O 2 evolution to CO 2 uptake during photosynthesis reveals information about reductive metabolism, including the reduction of alternative electron acceptors, such as nitrite and oxaloacetate. Recently we reported that in simultaneous measurements of CO 2 uptake and O 2 evolution in a sunflower leaf, O 2 evolution changed by 7% more than CO 2 uptake when light intensity was varied. Since the O 2 /CO 2 exchange ratio is $1, small differences are important. Thus, these gas exchange measurements need precise calibration. In this work, we describe a new calibration procedure for such simultaneous measurements, based on the changes of O 2 concentration caused by the addition of pure CO 2 or O 2 into a flow of dry air (20.95% O 2 ) through one and the same capillary. The relative decrease in O 2 concentration during the addition of CO 2 and the relative increase in O 2 concentration during the addition of O 2 allowed us to calibrate the CO 2 and O 2 scales of the measurement system with an error (relative standard deviation, RSD) of 51%. Measurements on a sunflower leaf resulted in an O 2 /CO 2 ratio between 1.0 and 1.03 under different CO 2 concentrations and light intensities, in the presence of an ambient O 2 concentration of 20-50 mmol mol À1 . This shows that the percentage use of reductive power from photochemistry in synthesis of inorganic or organic matter other than CO 2 assimilation in the C 3 cycle is very low in mature leaves and, correspondingly, the reduction of alternative acceptors is a weak source of coupled ATP synthesis.

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