Study on nitrogen efficiency of oilseed rape (Brassica napus L.) in relation to the metabolism in leaves and roots and to the transport in phloem and xylem

In order to analyse how N assimilation, phloem and xylem transport and seed protein accumulation affect the N efficiency of oilseed rape (Brassica napus L.), these features were compared in different types of plants: - a variety of winter oilseed rape genotypes grown either under field or under greenhouse conditions - transgenic oilseed rape cultivar Drakkar expressing E. coli asparagine synthetase A under control of the CaMV 35S promoter, grown in the greenhouse - transgenic oilseed rape cultivar Drakkar overexpressing its own cytosolic glutamine synthetase under control of the CaMV 35S promoter, grown in the greenhouse - transgenic oilseed rape cultivar Drakkar overexpressing its own plastidic glutamine synthetase under control of the CaMV 35S promoter, grown in the greenhouse - transgenic oilseed rape cultivar Falcon whose expression of plastidic glutamine synthetase is reduced by a CaMV 35S-antisense construct, grown in the greenhouse Among the different genotypes of oilseed rape, a wide variation of net CO2 assimilation and biomass, metabolite contents in leaves, phloem and xylem, as well as of activities of N-assimilating enzymes was found. The genotypes that were analysed under field condition could be assigned to four different groups according to their N capacity in sink organs (seed N harvest) and source organs (amino acid content of source leaves). In young leaves (sink leaves), source leaves and roots, the amino acid content was positively correlated with the activity of nitrate reductase. However, there was no correlation between amino acid content in source leaves and in phloem sap, seed protein content and seed N harvest. The biochemical characterizations revealed that glutamate, glutamine, aspartate and serine were the major amino acids in leaves, roots and phloem sap of oilseed rape, while glutamine was the dominant amino acid in xylem sap. The reaction of eight cultivars to different nitrate concentrations was examined using hydroponically cultivated greenhouse plants. Increased nitrate concentrations led to an increase of the content of N containing compounds but to a reduction of the carbohydrate content in leaves, roots, phloem and xylem sap, and to a decrease of the N harvest index as well. Similar results were obtained in leaves of 36 different cultivars in field tests with increased nitrate fertilization. Unlike the nitrate reductase activity, the activity of glutamine synthetase in leaves and roots was not much affected by the supplied nitrate concentrations (0.5 mM or 4 mM) and by N forms (4 mM nitrate or ammonium). Compared with the wild types, no phenotypical change could be observed in all transgenic plants examined. Although the levels of expression and activity of the respective enzymes were different between the transgenic plants and the wild types, there was no significant difference regarding metabolite contents either in leaves or in phloem sap