Efficient root nitrogen transport is a key factor in improving nitrogen utilization and yield of semi-dwarf rapeseed

Context

Rational allocation of nitrogen (N) application rates and planting densities is crucial for improving rapeseed yield. However, an increased N application rate reduces N use efficiency (NUE), and a higher planting density elevates the risk of lodging, thus limiting production benefits. Dwarf and compact rapeseed varieties exhibit high lodging resistance but possess low yield potential, while cultivars with conventional plant architecture possess high yield potential but are susceptible to collapse under dense planting. Limited research exists on utilizing the advantages of materials with different plant architectures to enhance N utilization and yield in rapeseed.

Methods

The experiment was conducted using the dwarf mutant HS5sca, the tall wild-type HS5, and their F1 semi-dwarf and compact hybrid HS5_+/sca all sharing the same genetic background from 2019 to 2022. The field experiment used a split-split-plot design with three different planting densities (D1, D2, and D3: 15 × 10⁴, 45 × 10⁴, and 75 × 10⁴ plants ha⁻¹) and three different N application rates (N1, N2, and N3: 120, 240, and 360 kg ha⁻¹) to find out how N application rate and planting density affected NUE and yield in different genotypes.

Results

With increasing N rate and planting density, the expression levels of nitrate transporter-encoding genes, BnaC08.NPF6.3 and BnaA07.NPF4.6, in roots increased first and then decreased in HS5_sca and HS5_+/sca, reaching the peak at N2 and D2, respectively, while in HS5 they increased with increasing N rate and increased first and then decreased with increasing planting density. Meanwhile, the activities of nitrate reductase, nitrite reductase, glutamine synthetase, and glutamate synthase in roots of the three genotypes at the flowering stage increased with increasing N rate, while these enzyme activities increased first and then decreased with increasing planting density. These enzyme activities increased N and dry matter accumulation in both roots and shoots, as well as a higher N contribution rate (NCR). Despite declines in agronomic NUE (aNUE) and the partial factor productivity of N (PFPN) with increasing N rate, the relative growth rates of root and shoot increased, positively affecting yields under most of the treatments. Conversely, NCR and aNUE decreased, while root and shoot N, dry matter accumulation, PFPN and, yield initially increased and then decreased with increasing planting density. Compared with N1, the average yield over three years for HS5_sca, HS5_+/sca, and HS5 increased by 24.5 %, 22.1 %, and 14.6 % at N2, and 8.1 %, 10.2 %, and 4.6 % under D2 compared to D1, respectively. Among the three genotypes, HS5_+/sca exhibited higher expression of nitrate transporter genes, greater N metabolism-related enzyme activities, increased shoot N accumulation, and higher NUE than both parent lines across different N application rates and planting densities.

Conclusion

At an N application rate of 240 kg ha⁻¹ and planting densities of 45–75 × 10⁴ plants ha⁻¹ , HS5_+/sca demonstrated an average yield increase of 11.2 % over three years, with greater yield potential and NUE compared to other N rate-planting density treatments.