Depletion of carbohydrate reserves limits nitrate uptake during early regrowth in Lolium perenne L.

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 grasses both impact C partitioning, thereby creating a significant point of interaction with soil N availability. Using defoliation as an experimental treatment, we investigated the dynamic relationships between plant carbohydrate status and NO(3)(–)-responsive uptake systems, transporter gene expression, and nitrate assimilation in Lolium perenne L. High- and low-affinity NO(3)(–) uptake was reduced in an N-dependent manner in response to a rapid and large shift in carbohydrate remobilization triggered by defoliation. This reduction in NO(3)(–) uptake was rescued by an exogenous glucose supplement, confirming the carbohydrate dependence of NO(3)(–) uptake. The regulation of NO(3)(–) uptake in response to the perturbation of the plant C/N ratio was associated with changes in expression of putative high- and low-affinity NO(3)(–) transporters. Furthermore, NO(3)(–) assimilation appears to be regulated by the C–N status of the plant, implying a mechanism that signals the availability of C metabolites for NO(3)(–) uptake and assimilation at the whole-plant level. We also show that cytokinins may be involved in the regulation of N acquisition and assimilation in response to the changing plant C/N ratio.