Root trait dynamics of historical canola varieties under low and high nitrogen supply

Nitrogen fertilizer is a major input cost in crop production, making it crucial to enhance nitrogen use efficiency (NUE) to reduce fertilizer dependence without compromising yields. Root system architecture (RSA) is a key determinant of nutrient acquisition, particularly under nutrient‐limited conditions. This study aimed to characterize RSA traits and their responses to nitrogen availability in 23 historically significant Brassica napus L. genotypes and one Brassica rapa L. genotype, spanning 1936–2017 and including both open‐pollinated and commercial hybrid types. Using a steady‐state hydroponic system, we assessed 21 root and shoot traits, including root elongation, branching, and biomass allocation, under high and low nitrate conditions at 7, 14, and 21 days after transfer to nutrient solution. Nitrogen limitation significantly altered RSA, increasing primary and lateral root lengths and total root area, while reducing lateral root density; root emergence and tip angles were unaffected. Low nitrate also reduced shoot and root biomass and leaf number. Commercial hybrids generally exhibited higher values across most traits under optimal nitrogen, whereas open‐pollinated cultivars showed greater trait plasticity under nitrogen limitation. Heritable traits such as total root length, primary root length, and convex hull area were identified as promising selection targets for improving NUE. Our findings highlight the importance of RSA variability and plasticity, suggesting that integrating key root traits into breeding programs can enhance productivity and sustainability in nutrient‐limited environments.