CO₂ assimilation rates in tobacco^Rst and wild-type tobacco plants and their Rubisco kinetics. A, Gas-exchange measurements of CO₂ assimilation rates in response to intercellular pCO₂ measured at an irradiance of 350 (white symbols) or 950 (black symbols) μmol quanta m⁻² s⁻¹ and leaf temperature of 25°C. Measurements were made on attached leaves from two wild-type plants (wt1, 13.5 cm leaf diameter; wt2, 15 cm leaf diameter containing 33.2 and 30.6 μmol Rubisco sites m⁻², respectively) and two tobacco^Rst plants (tob^Rst1 and tob^Rst2; both 12 cm leaf diameters with 5.8 and 4.9 μmol Rubisco sites m⁻², respectively). The Rubisco limited CO₂ assimilation rates for wt1 (•) and tob^Rst2 (▪) were modeled according to Farquhar et al. (1980) using estimates of maximal ribulose-P₂ dependent carboxylase activity. B, V  _c  ^max, K  _m for CO₂ under ambient O₂ levels (K  _c  ^21%O  ₂), and CO₂/O₂ specificity (S_c/o) measured using leaf protein extracts and purified Rubisco preparations. See “Materials and Methods” for further details. V  _c  ^max measurements in bold italics are those made using leaf tissue from tissue culture grown plants; see Supplemental Figure S3 for details. Superscript a indicates data from Whitney et al. (1999) and superscript b indicates data from Kanevski et al. (1999).