Light reacclimatization of lower leaves in C4 maize canopies grown at two planting densities

C4 plants have high photosynthetic capacity but are inefficient under low light. In a canopy, lower leaves developed under high light are progressively shaded. To elucidate how lower leaves in a C4 canopy reacclimatize to low light, we investigated maize canopies differing in light environment grown at standard and low planting densities (SPD, LPD). Although upper leaves at SPD and both upper and lower leaves at LPD had light-response curves of photosynthesis of sun leaves, lower leaves at SPD had that of shade leaves. All leaves at both densities had anatomical framework of sun leaves, but the chloroplast content in mesophyll and bundle-sheath cells of lower leaves at SPD was greatly reduced to reacclimatize to low light. This study demonstrates that lower leaves at SPD reacclimatize to low light by adjusting their physiological and chloroplast traits while maintaining anatomical framework, whereas those at LPD behave as sun leaves.