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

Abstract

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.