Root or shoot nitrate assimilation in terrestrial vascular plants – does it matter?

Background: Most terrestrial vascular plants can assimilate soil obtained NO₃^- in their root and shoot. Scope: Data from the literature are collated and analysed with respect to genotype and environmental effects on the partitioning of NO₃^- assimilation between root and shoot of terrestrial vascular plants. Conclusions: Temperate evergreen woody species in the Ericaceae and Pinaceae carry out most of their NO₃^- assimilation in the root when growing in low (0.5 mM) up to at least 5 mM soil NO₃^-. The root is the main site of NO₃^- assimilation for temperate deciduous woody species and perennial and annual herbaceous legume species at 0.5–1 mM NO₃^- but for many, shoot assimilation increases in importance with increased NO₃^- supply. Temperate perennial grasses and annual non-legume species and tropical/ sub-tropical species regardless of life-form, carry out a substantial, usually major proportion of their NO₃^- assimilation in shoots at NO₃^- concentrations above 0.5 mM. Furthermore, high NH₄⁺ supply, mycorrhizal infection and infection by parasitic plants can increase the proportion of total plant NO₃^- assimilation carried out in the shoot while abiotic stress and elevated atmospheric [CO₂] can cause this to decrease. Shoot NO₃^- assimilation is an advantage under non-stress conditions due to its positive effect on leaf expansion but can be a disadvantage under freezing and chilling stress conditions. Increased reliance on root NO₃^- assimilation at elevated CO₂ was associated with increased and conversely decreased plant growth and NO₃^- assimilation depending on study. Resolution of these different findings across studies is an important area for further research.