Abstract

Leaves are naturally exposed to variable light conditions; changes in intensity are often accompanied by changes in quality. In C4 plants, where bundle sheath (BS) cells are surrounded by more superficial mesophyll (M), changes in light quality affect ATP partitioning between BS and M. Low light intensity is known to affect leakiness (Φ), the relative amount of CO2 retrodiffusing from the BS, thus the coordination between the C4 and the C3 activity.

To investigate the effect of changing ATP partitioning on C4 / C3 coordination we estimated Φ under different light quality at low irradiance. Φ was not influenced by light quality, suggesting that efficient biochemical mechanisms were coping with the changing ATP partitioning. These mechanisms were modelled to take into account the functional and spatial differentiation between BS and M. ATP demand was studied by incrementally allocating ATP sinks (starch synthesis, PEPCK , and PGA reduction) to BS. The ‘window’ of ATP demand in BS relative to M ranged between 0.27 and 0.8. ATP production in BS was estimated by modelling light penetration in a maize leaf and resulted in a wider range (min 0.29; max 0.96). Variable transamination rates were functional to allowing the uncoupled delivery of NADPH and CO2 to BS.