Yu Wang, Samantha S. Stutz, Carl J. Bernacchi, Ryan A. Boyd, Donald R. Ort

Photosynthetic energy conversion efficiency (εc), the efficiency with which crops convert intercepted radiation into biomass, is a major limitation to the yield potential for both C3 and C4 crops (Zhu et al., 2008, 2010; Long et al., 2015). The εc of C4 species has the intrinsic advantage of minimizing energy loss to photorespiration under most conditions, compared with C3 species (Long & Spence, 2013). Although only 3% of species use the C4 pathway, they account for 23% of terrestrial gross primary productivity (Sage et al., 2012). C4 species are also overrepresented in agricultural production in which just three C4 crops (maize, sugarcane and sorghum) account for 32% of global production (Long & Spence, 2013; FAO et al., 2020). All three are from a single C4 evolutionary clade, tribe Andropogoneae, and use the NADP malic enzyme (ME) for decarboxylation in the bundle sheath. Despite high productivity, even under optimum conditions, these C4 crops still fall well short of the theoretical maximum energy conversion efficiency of 6% in the field (Zhu et al., 2008, 2010; Dohleman & Long, 2009). Understanding the limitations to realizing the theoretical maximum in field conditions is key to increasing the productivity of C4 crops.