Figure 6.

A, The response of CO₂ assimilation rate to intercellular pCO₂ for leaves of a control plant (○) and two T₀ transformants (▪ and ●). Solid lines are CO₂ assimilation rates (A) modeled using the equation (Farquhar et al., 1980):

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where V_cmax, K_c, and K_o, are the substrate-saturated carboxylase activity and the Michaelis constants for CO₂ and O₂, respectively. Γ_* is the CO₂ compensation point in the absence of nonphotorespiratory CO₂ release, R_d. C and O are the partial pressures of CO₂ and O₂. Rubisco kinetic parameters can only be estimated from gas-exchange measurements with plants whose Rubisco activities are low enough to limit photosynthesis under all measurement conditions. The transformants in this study fulfill this criterion and the modeled line is drawn with the following fitted parameters: V_cmax = 37 μmol m⁻² s⁻¹, K_c = 318 μbar, K_o = 55.6 mbar, Γ_* = 140 μbar, and R_d = 2.5 μmol m⁻² s⁻¹. The control plants were not limited by Rubisco activity at the higher CO₂ partial pressures. For them, the modeled line is drawn using kinetic parameters determined previously with Rubisco-deficient anti-rbcS tobacco (V_cmax = 90 μmol m⁻² s⁻¹, K_c = 404 μbar, K_o = 248 mbar, Γ_* = 37 μbar, and R_d = 3 μmol m⁻² s⁻¹ [von Caemmerer et al., 1994]). Note that Γ_* = 0.5O/S_c/o. We assumed that chloroplastic pCO₂ was equal to the intercellular pCO₂. B, The CO₂ compensation point as a function of pO₂ for leaves of the wild type (○ and □) and transformants (▪ and ●). Solid lines are the compensation point modeled using the equation (Farquhar et al., 1980):

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with the same parameter values as above.