Increased bundle sheath leakiness of <scp> CO ₂ </scp> during photosynthetic induction shows a lack of coordination between the <scp> C ₄ </scp> and <scp> C ₃ </scp> cycles

Wang, Yu and Stutz, Samantha S. and Bernacchi, Carl J. and Boyd, Ryan A. and Ort, Donald R. and Long, Stephen P. (2022) Increased bundle sheath leakiness of <scp> CO ₂ </scp> during photosynthetic induction shows a lack of coordination between the <scp> C ₄ </scp> and <scp> C ₃ </scp> cycles. New Phytologist, 236 (5). pp. 1661-1675. ISSN 0028-646X

Abstract

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.