Increased bundle‐sheath leakiness of CO(2) during photosynthetic induction shows a lack of coordination between the C(4) and C(3) 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.