Acclimation to low light by C4 maize: implications for bundle sheath leakiness

C4 plants have a biochemical carbon concentrating mechanism ( CCM ) that increases CO 2 concentration around ribulose bisphosphate carboxylase oxygenase ( R ubisco) in the bundle sheath ( BS ). Under limiting light, the activity of the CCM generally decreases, causing an increase in leakiness, ( Φ ), the ratio of CO 2 retrodiffusing from the BS relative to C4 carboxylation processes. Maize plants were grown under high and low light regimes (respectively HL , 600 versus LL , 100  μ E m −2  s −1 ). Short‐term acclimation of Φ was compared from isotopic discrimination (Δ), gas exchange and photochemistry. Direct measurement of respiration in the light, and ATP production rate ( J ATP ), allowed us use a novel approach to derive Φ , compared with the conventional fitting of measured and predicted Δ. HL grown plants responded to decreasing light intensities with the well‐documented increase in Φ . Conversely, LL plants showed a constant Φ , which has not been observed previously. We explain the pattern by two contrasting acclimation strategies: HL plants maintained a high CCM activity at LL , resulting in high CO 2 overcycling and increased Φ ; LL plants acclimated by down‐regulating the CCM , effectively optimizing scarce ATP supply. This surprising plasticity may limit the impact of Φ ‐dependent carbon losses in leaves becoming shaded within developing canopies.