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 NO3--responsive uptake systems, transporter gene expression, and nitrate assimilation in Lolium perenne L. High- and low-affinity NO3- 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 NO3- uptake was rescued by an exogenous glucose supplement, confirming the carbohydrate dependence of NO3- uptake. The regulation of NO3- uptake in response to the perturbation of the plant C/N ratio was associated with changes in expression of putative high- and low-affinity NO3- transporters. Furthermore, NO3- assimilation appears to be regulated by the C-N status of the plant, implying a mechanism that signals the availability of C metabolites for NO3- 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.