Balancing light capture with distributed metabolic demand during C₄ photosynthesis

In C₃ leaves, photosynthetic electron transport is closely coupled to the carbon reduction cycle in each chloroplast due to the small pools of nicotinamide adenine dinucleotide phosphate (NADP) and adenosine triphosphate (ATP). In contrast, in C₄ leaves, photosynthetic electron transport occurring in two different cell layers is buffered via extensive metabolite exchange and larger pools of metabolites. The demand for NADPH and ATP in the mesophyll and bundle sheath cells depends on the decarboxylation type. At one extreme, NADP malic enzyme (ME) species such as Zea mays produce almost no NADPH by linear electron flux in the bundle sheath. The NADPH required by the photosynthetic carbon reduction (PCR) cycle is either transferred into the bundle sheath via malate or phosphoglyceric acid (PGA) is cycled into the mesophyll for reduction. At the other extreme, nicotinamide adenine dinucleotide (NAD)-ME species such as Panicum miliaceum only need to produce ATP in the mesophyll to supply the C₄ cycle. These two extremes restrict linear electron flux to either the mesophyll (NADP-ME) or the bundle sheath (NAD-ME), with the remaining ATP requirement being generated from cyclic electron flux in either cell type. Biochemical diversity within C₄ species means that intermediate solutions with some linear electron flux in both mesophyll and bundle sheath cells also exist. Additional flexibility is also required for any given decarboxylation type because the requirements change depending on the leakiness of the bundle sheath to CO₂. Leakiness tends to increase at lower irradiance and under fluctuating light. To maximize quantum yields, the different cellular locations for linear electron flux between the decarboxylation types require different distributions of light absorption between the mesophyll and bundle sheath cells. The majority of chlorophyll is co-located within the cells with linear electron flux. However, light absorption is not simply proportional to chlorophyll distribution because of the complex leaf anatomy. We visualized profiles of light absorption through leaves of Flaveria bidentis and Z. mays by imaging chlorophyll fluorescence emerging from the transverse face of a cut leaf. Green light was absorbed throughout the leaf. In contrast, blue light was strongly absorbed near the surface, with little light penetrating the bundle sheath. This resulted in the rate of CO₂ assimilation under blue light being half that under green light of the same photon irradiance. The decline in the rate of CO₂ assimilation after switching from green to blue light occurred over 100 s and represented a change in metabolite pool size of 400 µmol m^-2. We predict that leakiness is greater under blue light than under green light for a given photon irradiance. Engineering a single-cell CO₂-concentrating mechanism would be simpler than a Kranz-type C₄ system as it would require little cellular-specific adjustment to thylakoid composition and function.