**1. Introduction**

Agriculture is facing the challenge of producing more food while reducing the negative environmental impact of nitrogen (N) fertilization. Synthetic N fertilizers come with elevated prices. Such costs are not only economical but also environmental, as fertilizers cause groundwater pollution by nitrate leaching [1,2] and air pollution by nitrous oxide emission [3,4]. Hence, there is a need to reduce the environmental footprint by lowering the fertilizer inputs. Breeding crops with greater Nitrogen Use Efficiency (NUE) is one solution to achieve that goal [5]. Agronomically, it is calculated as the ratio of crop yield to N units available. The NUE is a combination of the Nitrogen Uptake Efficiency (NUpE)—the ability of the crop to take up N from the soil and the Nitrogen Utilization Efficiency (NUtE)—the ability of the crop to utilize the absorbed N for producing yield [6,7]. The root system

**Citation:** Kupcsik, L.; Chiodi, C.; Moturu, T.R.; De Gernier, H.; Haelterman, L.; Louvieaux, J.; Tillard, P.; Sturrock, C.J.; Bennett, M.; Nacry, P.; et al. Oilseed Rape Cultivars Show Diversity of Root Morphologies with the Potential for Better Capture of Nitrogen. *Nitrogen* **2021**, *2*, 491–505. https://doi.org/10.3390/nitrogen 2040033

Academic Editors: Jacynthe Dessureault-Rompré and Marouane Baslam

Received: 1 October 2021 Accepted: 8 December 2021 Published: 14 December 2021

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architecture defines the spatial distribution of roots in the soil, which translates the ability of the plant to acquire soil resources [8]. At present, root-based approaches are being developed to improve NUE in various crops [9,10]. Still, methods for phenotyping crop root system architecture can be challenging.

Oilseed rape (*Brassica napus*) has poor NUE, with a low ratio of seeds produced per N unit applied [11]. Thus, improving NUE is essential to ensure the environmental and economic sustainability of that crop production [12,13]. Breeding programs are largely focusing on NUtE in above-ground organs [14–16] and research on soil and below-ground processes are limited [17]. Yet, the root organ holds potential for NUpE improvement [18,19]. Oilseed rape offers a large genetic diversity of root morphologies [20–23]. Nonetheless, literature on the influence of N nutrition on root growth and development in that crop is still scarce compared to parented *Arabidopsis thaliana*. In the model species, a dual effect of nitrate on lateral root development is described: (i) a systemic inhibition of uniformly elevated nitrate concentrations occurs on lateral root elongation at the post-emergence developmental stage and (ii) a localized stimulation of nitrate-rich patches triggers lateral root growth in a low N environment, known as the foraging capacity [24–27]. The exploitation of Arabidopsis has unveiled some molecular networks shaping root morphology in response to nutritional factors [28–32].

Several study cases showed that root morphology optimization results in greater plant productivity and increased nutrient stress tolerance [33,34]. A rapid root proliferation may be a determining factor for accessing N from deep layers of quickly leaching or drying soils [18]. The NUpE can be improved by redesigning a more branched root system that explores a larger soil volume to prevent N leaching [18,35]. There are indications that at low N supply, high-yielding oilseed rape cultivars are characterized by expansive root growth during the vegetative stage or after the flower stem extension [36,37] and by great N uptake [38]. These observations support the hypothesis that large root phenotypes may enhance N acquisition. Nevertheless, conflicting views may be expressed regarding a possible trade-off between profuse root system, contributing to the capacity of N uptake, and metabolic costs associated with the growth and maintenance of the root organ [39].

This study explores the natural variation of root morphology in response to the nitrate supply among a diversity panel of 55 modern winter oilseed rape cultivars. The technical objective was to set up and assess the relevance of three culture systems to observe root morphology at different growth stages. Eventually, we sought to strengthen the premise that (i) root biomass production and morphological traits could be positive indicators of above-ground biomass production and (ii) larger root system size would confer greater N uptake.

#### **2. Materials and Methods**

#### *2.1. Plant Material*

Seeds of 55 winter oilseed rape cultivars were obtained from the Walloon Agricultural Research Centre (CRA-W), Gembloux, Belgium. The description of these lines is given in Table S1.
