The Operation of Two Decarboxylases, Transamination, and Partitioning of C4 Metabolic Processes between Mesophyll and Bundle Sheath Cells Allows Light Capture To Be Balanced for the Maize C4 Pathway 1[W]

The C4 photosynthesis carbon-concentrating mechanism in maize (Zea mays) has two CO2 delivery pathways to the bundle sheath (BS; viamalate or aspartate), and rates of phosphoglyceric acid reduction, starch synthesis, and phosphoenolpyruvate regeneration also vary between BS andmesophyll (M) cells. The theoretical partitioning of ATP supply betweenM and BS cells was derived for thesemetabolic activities fromsimulatedprofiles of light penetration across a leaf,with apotential 3-folddifference in the fraction of ATP produced in the BS relative to M (from 0.29 to 0.96). A steady-state metabolic model was tested using varying light quality to differentially stimulate M or BS photosystems. CO2 uptake, ATP production rate (JATP; derived with a low oxygen/chlorophyll fluorescencemethod), and carbon isotope discriminationweremeasured on plants under a low light intensity, which is considered to affect C4 operating efficiency. The light quality treatments did not change the empirical ATP cost of gross CO2 assimilation (JATP/GA). Using themetabolic model, measured JATP/GAwas comparedwith the predicted ATP demand asmetabolic functions were varied betweenMandBS.Transaminationandthe twodecarboxylasesystems(NADP-malic enzymeandphosphoenolpyruvate carboxykinase) were critical for matching ATP and reduced NADP demand in BS and M when light capture was varied under contrasting light qualities.