*2.5. Statistical Treatment*

Analysis of variance for the in vitro and hydroponic culture dataset was performed using a linear model implemented with the lmer function in the {lme4} package in R statistical language. The model consisted of fixed effects for cultivar and nitrate treatment and their interactions. Pearson correlations were calculated for all combinations of phenotypic traits in every nitrate condition, using R and XLSTAT. The principal component analyses of the in vitro and hydroponic culture dataset were conducted with XLSTAT. The Tukey's Honesty Significant Difference (HSD) test for the nitrate uptake assay was performed using R.

#### **3. Results**

Biomass production and root morphology were examined in a set of 55 winter oilseed rape cultivars (Table S1). Three culture systems were employed to examine the root system in response to the nitrate supply. In a pilot screen, the full set grew in vitro (Figure 1a). Eventually, a dozen of cultivars with contrasting root morphologies were further characterized in hydroponics (Figure 1b), and four of them upon soil culture (Figure 1c).
