Gas Exchange & Fluorescence

Photosynthesis is an orchestration of multiple biochemical and biophysical processes that convert light energy into chemical energy (i.e., the light reactions) and subsequently assimilate carbon dioxide (CO2) from the atmosphere into plant metabolites (i.e., the Calvin reactions). Both the light and Calvin reactions have been explored experimentally for decades with increasing precision and resolution as technology has developed. While measurements of both types of reactions are informative, the combination of the two can provide additional information about plant resource allocation, utilization efficiency and overall health.

Typical gas exchange measurements are used to quantify CO2 and water fluxes across a leaf surface. The yield of chlorophyll fluorescence can be experimentally manipulated to provide information about how absorbed light energy is partitioned between photochemical, (e.g. electron transport), and non-photochemical (photoprotective) processes. These fluxes and yields of fluorescence can be measured under ambient environmental conditions (survey) or in response to changes in various environmental factors (response curves). Simultaneous chlorophyll fluorescence and gas exchange measurements on the same leaf area can be used to assess quantum (of light) efficiency of electron transport (φPSII) versus quantum efficiency of carbon fixation in the Calvin reactions (φCO2). From theory, there is a theoretical minimum


requirement of quanta necessary to precisely match the energetic output of the light reactions with that of CO2 assimilation; information that can be obtained by comparison of φCO2 and φPSII. Diversion from the theoretically predicted relationship can be indicative that chemical energy from the light reactions is being utilized for 'sinks' other than CO2 assimilation. In response curves, the relationship of electron transport to carbon fixation allows for calculation of additional parameters such as mesophyll conductance (gm, steady state electron transport and known intercellular CO2) and triose phosphate limitations (TPU, decreasing assimilation with no change in electron transport). The parallel measurements can help develop a more complete understanding of the partitioning and utilization of energy and metabolites in plants.

Laisk A, Loreto F. 1996. Determining photosynthetic parameters from leaf CO2 exchange and chlorophyll fluorescence. Ribulose-1,5-bisphosphate carboxylase/oxygenase specificity factor, dark respiration in the light, excitation distribution between photosystems, alternative electron transport rate and mesophyll diffusion resistance. Plant Physiology 110, 903-912.

LI-COR Photosynthesis Systems:

The LI-6400XT and LI-6800 Portable Photosynthesis Systems are compact, rugged, field portable instruments able to provide researchers with detailed information on plant responses, including the ability to measure chlorophyll fluorescence and gas exchange simultaneously over the same leaf area. The LI-6800 is our newest system, featuring a touch-screen interface, improved fluorometer, better gas analyzer precision, better control over chamber conditions, and more.

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