Variations in structural,biochemical, and physiological traits of photosynthesis and resource use efficiency in Amaranthus species (NAD-ME-type C4)

C₄ plants show higher photosynthetic capacity and productivity than C₃ plants owing to a CO₂-concentrating mechanism in leaves, which reduces photorespiration. However, which traits regulate the photosynthetic capacity of C₄ plants remains unclear. We investigated structural, biochemical, and physiological traits associated with photosynthesis and resource use efficiency in 20 accessions of 12 species of Amaranthus, NAD-malic enzyme-type C₄ dicots. Net photosynthetic rate (P_N) ranged from 19.7 to 40.5 μmol m^?2 s^?1. P_N was positively correlated with stomatal conductance and nitrogen and chlorophyll contents of leaves and was weakly positively correlated with specific leaf weight. P_N was also positively correlated with the activity of the C₃ enzyme ribulose-1,5-bisphoshate carboxylase/oxygenase, but not with the activities of the C₄ enzymes phosphoenolpyruvate carboxylase and NAD-malic enzyme. Structural traits of leaves (stomatal density, guard cell length, leaf thickness, interveinal distance, sizes of mesophyll and bundle sheath cells and the area ratio between these cells) were not significantly correlated with P_N. These data suggest that some of the biochemical and physiological traits are involved in interspecific P_N variation, whereas structural traits are not directly involved. Photosynthetic nitrogen use efficiency ranged between 260 and 458 μmol mol^?1 N s^?1. Photosynthetic water use efficiency ranged between 5.6 and 10.4 mmol mol^?1. When these data were compared with previously published data of C₄ grasses, it is suggested that common mechanisms may determine the variations in resource use efficiency in grasses and this dicot group.