Nitrogen (N) is an essential macronutrient that limits plant yield and productivity. In order to increase crop yield, considerable amounts of nitrogenous fertilizers are applied to agriculture systems each year. However, about 25-70% of the applied fertilizer in ecosystem has been leached and released to the environment, in the form of NO, N₂O and NH₃, aggravating environmental pollution. Therefore, increasing nitrogen use efficiency in agriculture systems is essential to maintain the food production while alleviating the deleterious environmental effects of applied N.

The mechanisms linking C/N balance to N uptake and assimilation are central to plant responses to changing soil nutrient levels. Defoliation and subsequent regrowth of pasture grasses both impact C partitioning, thereby creating a significant point of interaction with soil N availability. Using defoliation as an experimental treatment, the dynamic relationships between plant carbohydrate status and NO₃⁻-responsive uptake systems, transporter gene expression and N assimilation were investigated in Lolium perenne. High- and low-affinity NO₃⁻ uptake were reduced in an N-dependent manner in response to a rapid and large shift in carbohydrate remobilization triggered by defoliation. This reduction in NO₃⁻ uptake was rescued by an exogenous 1% glucose supplement, confirming the carbohydrate-dependence of NO₃⁻ uptake. The regulation of NO₃⁻ uptake in response to the perturbation of plant C/N was associated with changes in expression of the nitrate high-affinity transporter LpNRT2.1b. Furthermore, NO₃⁻ assimilation appears to be regulated by the C/N balance, implying a mechanism that signals the availability of C metabolites for NO₃⁻ uptake and assimilation at the whole-plant level. This study also shows that cytokinins are involved in the regulation of nitrogen acquisition and assimilation in response to the changing C/N ratio.

Root architecture is also a crucial component that impacts the capacity of plants to access nutrients and water. By using the recently developed package RootNav, comprehensive morphological changes in root system architecture in response to different N sources were investigated in Brassica napus. In order to avoid a light-induced morphological and physiological responses affecting whole plant growth, an existing solid agar vertical-plate system was modified so that to allow roots to be shielded from light without sucrose addition and the emerging shoot to be grown without direct contact with the medium, thereby mimicking more closely the environmental conditions in nature. The results of 10-days-old B.

napus seedlings showed that total root length, LR density and root exploration area decreased with increasing external NO₃⁻ concentrations from 0.5 mM to 10 mM. The application of 0.5 mM NO₃⁻ induced more branching in the root system relative to the treatments with higher N concentrations (5 mM and 10 mM). The proportion of biomass allocation occupied by roots was greater in the low NO₃⁻ treatment relative to the high NO₃⁻ treatments, reflecting the fact that plants invested more resources in their roots when nutrient uptake from the environment was limited. In treatments of increasing NH₄⁺ concentration from 0.5 mM to 10 mM, primary root length, total root length, LR branching zone, LR density and root exploration area were reduced. These results indicated that NH₄⁺ toxicity usually leads to a stunted root system in B. napus, whereas a low concentration of NH₄⁺ is an optimal nitrogen resource for plant growth. Increasing L-glutamate concentration from 0.01 mM to 0.1 mM suppressed primary root length, whilst the LR branching zone did not change in the different L-glutamate treatments, suggesting that L-glutamate even at micromolar level could arrest primary root growth and LR branching in B. napus. By using in situ ¹⁵N isotope labelling, morphological and molecular phenotypes generated pharmacologically were employed to investigate whether the impacts of contrasting root traits are of functional interest in relation to N acquisition. Brassica napus L. were grown in solid medium containing 1 mM KNO3 and treated with cytokinin, 6-benzylaminopurine, the cytokinin antagonist (PI-55), or both in combination. The contrasting root traits induced by PI-55 and 6-benzylaminopurine were strongly related to ¹⁵N uptake rate. Large root proliferation led to greater ¹⁵N cumulative uptake rather than greater ¹⁵N uptake efficiency per unit root length. This relationship was associated with changes in C and N resource distribution between the shoot and root, and in expression of BnNRT2.1. The root/shoot biomass ratio was positively correlated with ¹⁵N cumulative uptake, suggesting the functional utility of root investment for nutrient acquisition. These results demonstrate that root proliferation in response to external N is a behaviour which integrates local N availability and systemic N status in the plant. In conclusion, using two major economic forage species, L. perenne and B. napus, this thesis illuminates the impacts of carbon and root system architecture on N uptake. This work contributes to our understanding of the mechanisms regulating N uptake and will help further in efforts to improve nitrogen use efficiency.