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.
Highlights
Grasses are well adapted to tolerate and recover from the severe and frequent defoliation associated with grazing (Lestienne et al, 2006)
Water-soluble carbohydrates (WSCs) were profiled in order to assess the dynamics of C metabolism during the first 48 h of regrowth
WSCs with a degree of polymerization (DP) of 3–8 are referred to here as low molecular weight (LMW) WSCs, and those with DP9 to DP20 are referred to as high molecular weight (HMW) WSCs
Summary
Grasses are well adapted to tolerate and recover from the severe and frequent defoliation associated with grazing (Lestienne et al, 2006). 1570 | Guo et al. In the period immediately following defoliation, photosynthetic capacity is temporarily compromised. In the period immediately following defoliation, photosynthetic capacity is temporarily compromised This interrupts C assimilation and the capacity to assimilate newly acquired inorganic N into biomass, which is a process requiring reducing equivalents, ATP, and C skeletons generated from respiration of sucrose derived from stored C (Dawar et al, 2010; Nunes-Nesi et al, 2010). It has been established that root growth, inorganic N uptake, respiration, and nitrate assimilation decline rapidly after defoliation (Boucaud and Bigot, 1989; Ourry et al, 1989; Richards, 1993; Louahlia et al, 2008)
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