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C4 plants show higher photosynthetic capacity and resource use efficiency than C3 plants. However, the genetic variations of these traits and their regulatory factors in C4 plants still remain to be resolved. We investigated physiological, biochemical, and structural traits involved in photosynthesis and photosynthetic water and nitrogen use efficiencies (PWUE and PNUE) in 22 maize lines and four teosinte lines from various regions of the world. Net photosynthetic rate (PN) ranged from 32.1 to 46.5 μmol m−2 s−1. PN was positively correlated with stomatal conductance, transpiration rate, and chlorophyll, nitrogen and soluble protein contents of leaves, but not with specific leaf weight. PN was positively correlated with the activities of ribulose-1,5-bisphosphate carboxylase/oxygenase and the C4-acid decarboxylases, NADP-malic enzyme and phosphoenolpyruvate carboxykinase, but not with the activity of phosphoenolpyruvate carboxylase. Leaf structural traits (stomatal parameters, leaf thickness, and interveinal distance) were not correlated with PN. These data suggest that physiological and biochemical traits are involved in the genetic variation of PN, but structural traits are not directly involved. PWUE is in the lower class of values reported for C4 plants, whereas PNUE is in the highest class of values reported for C4 plants. PNUE was negatively correlated with leaf nitrogen content but not significantly correlated with PN. PWUE was not correlated with δ13C values of leaves, indicating difficulty in using δ13C values as an indicator of PWUE of maize. In general, teosinte lines showed lower PN but higher PWUE than maize lines.

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 (PN) ranged from 19.7 to 40.5 μmol m−2 s−1. PN was positively correlated with stomatal conductance and nitrogen and chlorophyll contents of leaves and was weakly positively correlated with specific leaf weight. PN 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 PN. These data suggest that some of the biochemical and physiological traits are involved in interspecific PN 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.