Increased bundle sheath leakiness of CO₂ during photosynthetic induction shows a lack of coordination between the C₄ and C₃ cycles

Use of a complete dynamic model of NADP-malic enzyme C ₄ photosynthesis indicated that, during transitions from dark or shade to high light, induction of the C ₄ pathway was more rapid than that of C ₃, resulting in a predicted transient increase in bundle-sheath CO ₂ leakiness (ϕ). Previously, ϕ has been measured at steady state; here we developed a new method, coupling a tunable diode laser absorption spectroscope with a gas-exchange system to track ϕ in sorghum and maize through the nonsteady-state condition of photosynthetic induction. In both species, ϕ showed a transient increase to > 0.35 before declining to a steady state of 0.2 by 1500 s after illumination. Average ϕ was 60% higher than at steady state over the first 600 s of induction and 30% higher over the first 1500 s. The transient increase in ϕ, which was consistent with model prediction, indicated that capacity to assimilate CO ₂ into the C ₃ cycle in the bundle sheath failed to keep pace with the rate of dicarboxylate delivery by the C ₄ cycle. Because nonsteady-state light conditions are the norm in field canopies, the results suggest that ϕ in these major crops in the field is significantly higher and energy conversion efficiency lower than previous measured values under steady-state conditions.