A fast-growing normal and a slow-growing gibberellin-deficient mutant of Lycopersicon esculentum (L.) Mill. cv. Moneymaker were used to test the hypothesis that slow-growing plants reduce NO3− in the root to a greater extent than do fast-growing plants. Plants that reduce NO3− in the root may grow more slowly due to the higher energetic and carbon costs associated with root-based NO3− reduction compared to photosynthetically driven shoot NO3− reduction. The plants were grown hydroponically with a complete nutrient solution containing 10 mM NO3− and the biomass production, gas exchange characteristics, root respiratory O2 consumption, nitrate reductase activity and translocation of N in the xylem were measured. The gibberellin-deficient mutants accumulated more total N unit−1 dry weight than did the faster-growing normal plants. There were no significant differences between the genotypes in the rates of photosynthesis expressed on a leaf dry weight basis. The plants differed in the proportion of photosynthetic carbon available to growth due to a greater proportion of daily photo-synthate production being consumed by respiration in the slow-growing genotype. This difference in allocation of carbon was associated with differences in the specific leaf area and specific root length. In addition, a greater leaf weight ratio in the fast-growing than in the slow-growing plants indicates a greater investment of carbon into biomass supporting photosynthetic production in the former. We did not find differences in the activity or distribution of nitrate reductase or in the N composition of the xylem sap between the genotypes. We thus conclude that the growth rate was determined by the efficiency of carbon partitioning and that the site of NO3− reduction and assimilation was not related to the growth rate of these plants.