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I am confused about the variable "RuBisCO activity". How is it measured, and is it any different from the net assimilation rate?

Based on some background reading (e.g. Kling, 2008; Lambers et al 2011) the only difference that I can find is the units. Both would have "carbon fixed per unit time", but RuBisCO activity is expressed per unit mass of the enzyme, while assimilation rate is per unit leaf area (or less commonly, leaf mass).

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Both are measures of carbon fixation rate.

RuBisCO activity specifically refers to the rate at which the enzyme RuBisCO fixes carbon to RuBP, and is measured by isolating the enzyme from tissue samples and, usually, using radiolabelled CO2 to measure how much carbon is fixed in a set amount of time in controlled conditions and with controlled supply of substrate.

Carbon assimilation rate is generally a whole-plant or whole-leaf variable, measured on a larger scale. It is usually measured by measuring leaf sugar carbon before and after a set period, again using radiolabelling. Conditions may or may not be controlled, and it can be measured in the field (unlike RuBisCO activity).

RuBisCO activity might differ from net fixation in the short term in plants which have an intermediate carbon fixation step, such as in CAM or C4 plants, if the net fixation measurement includes the intermediates - the pool of intermediate carbon-containing molecules have technically been fixed but not by RuBisCO. However, over longer time periods this effect will disappear, as ultimately all CO2 is fixed or re-fixed through RuBisCO.

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This is a useful answer, and it does answer my question. However, I was not familiar with measuring assimilation using 14C-CO2. Almost every leaf-level measurement I have found in the literature has been measured with a LI-6400, which measures exchange of [CO2] and [H2O] between the leaf and atmosphere. In what cases is 14C used? –  Abe Jun 12 '12 at 15:21
    
Are you asking for specific cases, or a general reason why people would choose 14C? –  Richard Smith Jun 12 '12 at 19:02
    
more or less in general. It seems like a less direct measure (since the sugar pool presumably has losses to respiration as well as inputs from assimilation). It also seems more difficult in general, although perhaps less expensive. –  Abe Jun 12 '12 at 21:38
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