REDUCTION, ASSIMILATION AND TRANSPORT OF N IN NORMAL AND GIBBERELLIN-DEFICIENT TOMATO PLANTS

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 photosynth etically driven shoot NO3- reduction. The plants were grown hydroponic ally with a complete nutrient solution containing 10 mM NO3- and the b iomass production, gas exchange characteristics, root respiratory O-2 consumption, nitrate reductase activity and translocation of N in the xylem were measured. The gibberellin-deficient mutants accumulated mor e total N unit(-1) dry weight than did the faster-growing normal plant s. There were no significant differences between the genotypes in the rates of photosynthesis expressed on a leaf dry weight basis. The plan ts differed in the proportion of photosynthetic carbon available to gr owth due to a greater proportion of daily photosynthate production bei ng consumed by respiration in the slow-growing genotype. This differen ce in allocation of carbon was associated with differences in the spec ific leaf area and specific root length. In addition, a greater leaf w eight ratio in the fast-growing than in the slow-growing plants indica tes a greater investment of carbon into biomass supporting photosynthe tic production in the former. We did not find differences in the activ ity or distribution of nitrate reductase or in the N composition of th e xylem sap between the genotypes. We thus conclude that the growth ra te was determined by the efficiency of carbon partitioning and that th e site of NO3- reduction and assimilation was not related to the growt h rate of these plants.