Reduction, assimilation and transport of N in normal and gibberellin-deficient tomato plants


M. D. Cramer (corresponding author).


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.