Photosynthetic rates and related anatomical characteristics of leaves developed at three levels of irradiance (1200, 300 and 80 umol · m(-2) · s(-1)) were determined in the C4-like species Flaveria brownii A.M. Powell, the C3-C4-intermediate species F. linearis Lag., and the F1 hybrid between them (F. brownii × F. linearis). In the C3-C4 and F1 plants, increases in photosynthetic capacity per unit leaf area were strongly correlated with changes in mesophyll area per unit leaf area. The C4-like plant F. brownii, however, showed a much lower correlation between photosynthetic capacity and mesophyll area per unit leaf area. Plants of F. brownii developed at high irradiance showed photosynthetic rates per unit of mesophyll cell area 50% higher than those plants developed at medium irradiance. These results along with an increase in water-use efficiency are consistent with an increase of C4 photosynthesis in high-irradiance-grown F. brownii plants, whereas in the other two genotypes such plasticity seems to be absent. Photosynthetic discrimination against (13)C in the three genotypes was less at high than at low irradiance, with the greatest change occurring in F. brownii. Discrimination against (13)C expressed as δ (13)C was linearly correlated (r (2) = 0.81; P<0.001) with the ratio of bundle-sheath volume to mesophyll cell area when all samples from the three genotypes were combined. This tissue ratio increased for F. brownii and the F1 hybrid as growth irradiance increased, indicating a greater tendency towards Kranz anatomy. The results indicated that F. brownii had plasticity in its C4-related anatomical and physiological characteristics as a function of growth irradiance, whereas plasticity was less evident in the F1 hybrid and absent in F. linearis.