C4 plants have a biochemical carbon concentrating mechanism (CCM) that increases CO2 concentration around Rubisco in the bundle sheath (BS). Under limiting light, the activity of the CCM generally decreases, causing an increase in leakiness, (Φ), the ratio of CO2 retrodiffusing from the BS relative to C4 carboxylation processes. Maize plants were grown under high and low light regimes (respectively HL, 600 vs LL, 100 μE 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 (JATP ), allowed us use a novel approach to derive Φ, compared to 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 CO2 overcycling and increased Φ; LL plants acclimated by downregulating the CCM, effectively optimising scarce ATP supply. This surprising plasticity may limit the impact of Φ-dependent carbon losses in leaves becoming shaded within developing canopies.