Insights into the functional relationship between cytokinin-induced root system phenotypes and nitrate uptake in Brassica napus

Root system architecture is the spatial arrangement of roots that impacts the capacity of plants to access nutrients and water. We employed pharmacologically generated morphological and molecular phenotypes and used in situ 15N isotope labelling, to investigate whether contrasting root traits are of functional interest in relation to nitrate acquisition. Brassica napus L. were grown in solidified phytogel culture media containing 1 mM KNO3 and treated with the cytokinin, 6-benzylaminopurine, the cytokinin antagonist, PI-55, or both in combination. The pharmacological treatments inhibited root elongation relative to the control. 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, due to a systemic response in the plant. This relationship was associated with changes in C and N resource distribution between the shoot and root, and in expression of BnNRT2.1, a nitrate transporter. 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 nitrate is a behaviour which integrates local N availability and the systemic N status of the plant.

Short-term 15N uptake kinetics and nitrogen nutrition of bryophytes in a lowland rainforest, Costa Rica

Though bryophytes can markedly affect the hydrological and biogeochemical cycles of tropical rainforests, virtually nothing is known on their nutritional requirements. Here, short-term 15N uptake kinetics of NO3–, NH4+ and glycine were studied in nine species of bryophytes in a lowland wet tropical forest, Costa Rica. Net uptake of all three N forms obeyed to saturation (Michaelis-Menten) kinetics between 1 and 500 µmol L–1. Mean Km (Vmax) values ranged between 21 µm (6.6 µmol g–1 DW h–1, nitrate), 94 µm (43.5 µmol g–1 DW h–1, ammonium) and 126 µm (37.6 µmol g–1 DW h–1, glycine). No significant differences were evident between epiphyllous and epiphytic bryophytes. Concentrations of nitrogenous solutes of external sources ranged between 1.7 and 35.9 µm. External nitrogen concentrations and kinetic constants of the bryophyte species allowed estimation of net uptake rates in the field. The mean uptake rates were 1.8 µmol g–1 DW h–1 for nitrate, 3.6 µmol g–1 DW h–1 for ammonium, and 3.4 µmol g–1 DW h–1 for glycine, indicating that amino acids significantly contribute to bryophyte nutrition.