Does the in vitro assay underestimate Rubisco activity?
Following a literature search, the in vitro measurements of Rubisco activity were used in WIMOVAC to predict A. The predicted A was c. 65% lower than the observed A, clearly demonstrating that the observed A was mechanistically impossible if the in vitro estimate of Rubisco activity was correct. It has been suggested that the low values of Rubisco activity reported using the NADH spectrophotometric assay are due to a loss of activation associated with the extraction of Rubisco (Sage et al., 1993; Theobald et al., 1998). However, when the fully activated in vitro values for Rubisco activity were used in the model instead of the initial values, the predicted A was still c. 50% lower than the observed A (Table 2). Even when maximal activity (the activity of the fully activated enzyme in the absence of inhibitors; Parry et al., 1997) was measured in spring wheat, a value was reported that was still c. 20% lower than that required to support the A observed in the same plants (Theobald et al
