Summary Plants grown at elevated pCO2 often fail to sustain the initial stimulation of net CO2 uptake rate (A). This reduced, acclimated, stimulation of A often occurs concomitantly with a reduction in the maximum carboxylation velocity (Vc,max) of Rubisco. To investigate this relationship we used the Farquhar model of C3 photosynthesis to predict the minimum Vc,max capable of supporting the acclimated stimulation in A observed at elevated pCO2. For a wide range of species grown at elevated pCO2 under contrasting conditions we found a strong correlation between observed and predicted values of Vc,max. This exercise mechanistically and quantitatively demonstrated that the observed acclimated stimulation of A and the simultaneous decrease in Vc,max observed at elevated pCO2 is mechanistically consistent. With the exception of plants grown at a high elevated pCO2 (> 90 Pa), which show evidence of an excess investment in Rubisco, the failure to maintain the initial stimulation of A is almost entirely attributable to the decrease in Vc,max and investment in Rubisco is coupled to requirements.

No abstract

Rubisco activity can be measured using gas exchange (in vivo) or using in vitro methods. Commonly in vitro methods yield activities that are less than those obtained in vivo. Rubisco activity was measured both in vivo and in vitro using a spectrophotometric technique in mature Pinus taeda L. (loblolly pine) trees grown using free-air CO2 enrichment in elevated (56 Pa) and current (36 Pa) pCO2. In addition, for studies where both in vivo and in vitro values of Rubisco activity were reported net CO2 uptake rate (A) was modelled based on the in vivo and in vitro values of Rubisco activity reported in the literature. Both the modelling exercise and the experimental data showed that the in vitro values of Rubisco activity were insufficient to account for the observed values of A. A trichloroacetic acid (TCA) precipitation of the protein from samples taken in parallel with those used for activity analysis was co-electrophoresed with the extract used for determining in vitro Rubisco activity. There was significantly more Rubisco present in the TCA precipitated samples, suggesting that the underestimation of Rubisco activity in vitro was attributable to an insufficient extraction of Rubisco protein prior to activity analysis. Correction of in vitro values to account for the under-represented Rubisco yielded mechanistically valid values for Rubisco activity. However, despite the low absolute values for Rubisco activity determined in vitro, the trends reported with CO2 treatment concurred with, and were of equal magnitude to, those observed in Rubisco activity measured in vivo.