The photosynthetic characteristics (responses to CO₂ and light), ribulose-1,5-bisphosphate carboxylase (Rubisco) properties, and the size and number of cells of the mesophyll of Nicotiana tabacum L. leaves of genotypes selected for survival at low atmospheric CO₂ concentrations are described. When grown in the greenhouse with nutrient solutions, the total dry matter production of the selected genotypes was 23% greater than that of the parent genotype; this increase was related to a greater number of mesophyll cells of smaller size in the selected plants compared to the parent. However, it was not related to changes in the photosynthetic characteristics nor to Rubisco properties. These results suggest that the increased dry matter accumulation of the selected genotypes is not due to a reduction in photorespiration nor an increase in the CO₂ assimilation rates. Rather, the selection of haploid tobacco plantlets in low CO₂ has resulted in plants with greater leaf area (shown in previous work), due to the production of more cells of smaller size and to lower respiration rates per unit of leaf dry mass (previous work), thus increasing light capture, reducing the loss of assimilates and increasing total plant dry matter production.