Submitted to: Journal of Experimental Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/10/2003
Publication Date: 9/15/2003
Citation: LONG, S.P., BERNACCHI, C.J. 2003. Gas exchange measurements, what can they tell us about the underlying limitations to photosynthesis? procedures and sources of error. J. Experimental Botany. Interpretive Summary: In this paper, we outline the uses and sources of errors in one of the most common techniques employed by plant biologists. Gas exchange techniques are used to measure the rates of carbon assimilated through photosynthesis and the rates of water lost during photosynthesis. We identify correct, and incorrect, methods for measuring these processes. In addition, in this methods paper, we introduce the power of combining gas exchange with chlorophyll fluorescence. Chlorophyll fluorescence is a powerful tool to determine the efficiency in which plants use the light made available to them. Combining gas exchange with chlorophyll fluorescence, when done properly, provides a means to determine how much of the light made available to a plant is used in photosynthesis. This scope of this paper is directed toward younger graduate students and post-doctoral researchers, as well as scientists new to these techniques; this will also be a useful reference for anyone doing these types of measurements.
Technical Abstract: The principles, equipment and procedures for measuring leaf and canopy gas exchange have been described previously (J. Exp. Bot. 24, 253) as has chlorophyll fluorescence (J. Exp. Bot. 51, 659). Simultaneous measurement of the responses of leaf gas exchange and modulated chlorophyll fluorescence to light and CO2 concentration, now provide a means to determine a wide range of key biochemical and biophysical limitations on photosynthesis in vivo. Here the mathematical frameworks and practical procedures for determining these parameters in vivo are consolidated. Leaf CO2 uptake (A)versus intercellular CO2 concentration (Ci) curves may now be routinely obtained from commercial gas exchange systems. The potential pitfalls, and means to avoid these, are examined. Calculation of in vivo maximum rates of 1) ribulose-1,5-bishosphate (RuBP) carboxylase/oxygenase (Rubisco)carboxylation (Vc,max), electron transport driving regeneration of RuBP (Jmax), and triose-phosphate utilization (VTPU) are explained; these three parameters are now widely assumed to represent the major limitations to light-saturated photosynthesis. Precision in determining these in intact leaves is improved by the simultaneous measurement of electron transport via modulated chlorophyll fluorescence. The A/Ci response also provides a simple practical method for quantifying the limitation that stomata impose on CO2 assimilation. Previously determining the rate of photorespiratory release of oxygen (Rl) has only been possible by isotopic methods, now by combining gas exchange and fluorescence measurements, Rl may be determined simply and routinely in the field. The physical diffusion of CO2 from the intercellular air space to the site of Rubisco in C3 leaves has long been suspected a limitation on photosynthesis, but it has commonly been ignored because of the lack of a practical method for its determination. Again combining gas exchange and fluorescence provides a means to determine mesophyll conductance. This method is described and provides insights into the magnitude and basis of this limitation.