Ⅰ. INTRODUCTION

Rapeseed (Brassica napus L.) is a globally important oilseed crop widely cultivated as a source of edible vegetable oil, biodiesel feedstock, and high‑protein meal for livestock feed (Abdallah et al., 2010). In grassland-based and mixed crop– livestock systems, rapeseed meal is a key protein supplement, and rapeseed can also function as a rotational break crop that supports nutrient cycling and farm resilience. Beyond its economic role, rapeseed contributes to diversified crop rotations and provides significant ecosystem services through improved soil structure and floral resources for pollinators (Wang et al., 2025). Crop productivity in rapeseed is determined not only by total aboveground biomass but also by the efficiency with which assimilates are partitioned to the seeds, where seed yield components such as pod number, seeds per pod, and thousandseed weight are tightly linked to source–sink balance. Harvest index (HI), defined as the ratio of economic yield (typically seed yield) to total aboveground biomass, has therefore been widely used as a key integrative indicator of productivity in cereal and oilseed breeding, including rapeseed (Dai et al., 2016;Dwivedi et al., 2023).

As a member of the Brassicaceae, rapeseed is characterized by relatively high sulfur (S) demand compared with many other field crops (Castro et al., 2004;Aghajanzadeh et al., 2014). Sulfur is indispensable for the synthesis of S‑containing amino acids, the formation and stabilization of storage and structural proteins (Saito, 2000), and the production of sulfur‑rich secondary metabolites such as glucosinolates that influence plant defense and seed quality (Borpatragohain et al., 2019). Nitrogen (N) is the primary macronutrient determining vegetative growth, sink capacity, and seed protein concentration in oilseed rape, yet the crop is often described as having high N uptake efficiency but relatively low N utilization efficiency (Rathke et al., 2005). Interactions between N and S nutrition are particularly important in rapeseed, because S deficiency can impair protein synthesis and alter glucosinolate profiles (Duboussetet al., 2010), whereas adequate S supply can enhance the efficiency of N uptake and assimilation (Fismes et al., 2000). In this context, sulfur harvest index (SHI) and nitrogen harvest index (NHI)—the proportions of total plant S and N recovered in the seeds—provide informative metrics for assessing nutrient use efficiency and the degree of nutrient remobilization to the reproductive organs.

Substantial genotypic variation has been reported in rapeseed for biomass production, HI, and N‑related efficiency traits, indicating that carbon and nutrient partitioning patterns are amenable to genetic improvement (Xu et al., 2012;Weese et al.,2015). However, organ-specific biomass allocation and N and S harvest index have rarely been evaluated simultaneously across Brassica napus cultivars with contrasting yield potential and end-use. Therefore, this study aimed to provide fundamental information for cultivar improvement by comparing organspecific biomass partitioning patterns together with sulfur and nitrogen harvest indices across eight Brassica napus cultivars.

Ⅱ. MATERIALS AND METHODS

1. Plant materials and growth conditions

Eight Brassica napus cultivars (Akela, Capitol, Colosse, Naehan, Pollen, Saturnin, Sparta, and Tamra) were cultivated under field conditions at Chonnam National University (Gwangju, Republic of Korea) until physiological maturity. Seeds were sown into bed soil in a tray with 50 blocks. One seedling was transplanted into 0.7 m × 0.7 m plots. Seedling were cultivated with chemical fertilizers which were applied at rates of 200 kg N ha⁻¹ and 35 kg S ha⁻¹. The experiment was arranged with three biological replicates per cultivar.

2. Biomass partitioning

At physiological maturity, plants were harvested and separated into seeds, pods, leaves, fallen leaves, branch and stem, and roots. Each organ was dried in an 80°C oven until constant weight was achieved, and dry weight was measured.

3. Sulfur and nitrogen content

Dried samples were ground to a fine powder. Total S and N concentration were determined using macro elemental analyzer (vario MACRO cube, Elementar, Germany).

4. Harvest index calculations

Harvest indexes were calculated on an above ground basis using following formulas:

5. Statistical analysis

Data from each cultivar (n = 3 replicates) were subjected to analysis of variance (ANOVA). Differences among cultivar means were assessed using Duncan’s multiple range test. All analyses were performed using SAS 9.4 (SAS Institute Inc., Cary, NC, USA).

Ⅲ. RESULTS AND DISCUSSION

1. Biomass Partitioning among Cultivars

Organ-specific dry weights showed substantial variation among the eight cultivars (Table 1). Seed dry weight was highest in Colosse at 35.7 ± 6.7 g DW and lowest in Tamra at 5.8 ± 0.3 g DW. Branch and stem dry weight was highest in Akela at 49.7 ± 5.9 g DW and lowest in Tamra at 7.5 ± 1.0 g DW. Leaf and root dry weights showed relatively smaller differences among cultivars. Total biomass was classified into three groups: a high-biomass group (Colosse and Akela), a medium-biomass group (Naehan, Pollen, Saturnin, and Sparta), and a low-biomass group (Capitol and Tamra). The harvest index (HI) was high in Colosse, Capitol, and Pollen, but low in Akela and Sparta.

The present study revealed substantial genotypic variation in biomass partitioning among eight oilseed rape cultivars, highlighting that total biomass production per se is a poor predictor of nutrient recovery in seeds. Despite Colosse and Akela exhibiting the highest total biomass, only Colosse combined high biomass with a high HI, whereas Akela produced a large vegetative sink with a low HI. This decoupling between biomass and HI supports the view that ideotypes for efficient oilseed rape should prioritize assimilate allocation to reproductive organs rather than maximal vegetative growth (Rathke et al., 2005;Berry et al., 2010), as previously proposed for N-efficient genotypes that exhibit superior HI and nitrogen utilization efficiency (Bouchet et al., 2016).

2. Sulfur harvest index

Sulfur harvest index (SHI) varied from 25.6% to 46.5% among cultivars (Table 2). Capitol showed the highest SHI at 46.5 ± 0.7%, while Naehan exhibited the lowest at 25.6 ± 1.6%. Seed sulfur content was highest in Colosse at 185.5 ± 10.1 mg S and lowest in Tamra at 25.3 ± 1.0 mg S. Total aboveground sulfur content was high in Colosse and Akela at 444.4 ± 41.3 mg S and 436.2 ± 35.8 mg S, respectively. Interestingly, cultivars with high seed sulfur content did not show high SHI. For example, Akela had high seed sulfur content but showed a low SHI of 27.3 ± 4.6%, presumably due to its high vegetative sulfur content.

The marked differences in SHI among cultivars indicate divergent sulfur partitioning and remobilization strategies. Capitol and Pollen showed the highest SHI values (>45%), whereas Naehan and Akela exhibited SHI below 30%, despite Akela accumulating large amounts of sulfur in aboveground biomass. This pattern agrees with studies reporting that genotypes differ not only in sulfur uptake but also in their ability to remobilize sulfur from leaves and stems to seeds, which strongly influences sulfur recovery in grain (D'Hooghe et al., 2014). The combination of high total sulfur and low SHI in Akela suggests a genotype with substantial vegetative sulfur retention, analogous to lines in which sulfur is stored predominantly in structural tissues rather than redirected to seeds under both adequate and limiting sulfur supply (Ahmad and Abdin, 2000). In contrast, the high SHI of Capitol and Pollen indicates more efficient sulfur remobilization to seeds, a trait that is expected to favor the maintenance of sulfur-rich storage proteins and glucosinolate profiles important for seed quality and plant defense (Wittstock and Halkier, 2002).

3. Nitrogen harvest index

Nitrogen harvest index (NHI) showed greater variation among cultivars than SHI (Table 3). Capitol showed the highest NHI at 74.3 ± 0.4%, while Akela exhibited the lowest at 39.8 ± 8.5%. Seed nitrogen content was remarkably the highest in Colosse and lowest in Tamra. Total aboveground nitrogen content was highest in Colosse and lowest in Tamra. Pollen, Capitol, and Colosse cultivars showed high NHI values exceeding 68%, indicating efficient nitrogen translocation to seeds. In contrast, Akela and Naehan showed relatively low NHI values, suggesting greater nitrogen retention in vegetative organs.

Variation in NHI among cultivars was even larger than that observed for SHI, underscoring the key role of nitrogen remobilization in determining genotypic differences in nitrogen use efficiency. Capitol, Pollen, and Colosse exhibited high NHI values (≥68%), whereas Akela, Naehan, and Sparta showed substantially lower NHI, with Akela reaching only about 40% despite the highest aboveground nitrogen content. These results mirror findings that N-efficient oilseed rape genotypes typically combine higher HI and NHI with reduced nitrogen retention in vegetative organs, thereby achieving greater nitrogen utilization efficiency for grain production (Malagoli et al., 2005). The profile of Akela—large vegetative biomass, high total nitrogen, but low NHI—resembles N-inefficient ideotypes in which a significant proportion of fertilizer nitrogen remains in crop residues, increasing the risk of post-harvest losses and reducing overall nitrogen use efficiency (Rathke et al., 2006). In contrast, Colosse represents a more favorable ideotype, with high total nitrogen accumulation, high seed nitrogen content, and high NHI, partly overcoming the commonly reported limitation that only about half of applied nitrogen is recovered in oilseed rape seeds (Rathke et al., 2005;Berry et al., 2010).

4. Relationship between sulfur and nitrogen harvest indexes

The relationship between SHI and NHI is noteworthy. In most cultivars, NHI was higher than SHI, reflecting the tendency for sulfur to remain more in vegetative organs compared to nitrogen. This difference is presumably attributed to the distinct physiological functions and remobilization mechanisms of these two nutrients. Capitol and Pollen combined high SHI and NHI, pointing to a generally efficient remobilization system for both elements, whereas Colosse showed high NHI with intermediate SHI, and Akela showed low values for both indexes. The partial decoupling of SHI and NHI observed among cultivars suggests that S and N remobilization are related but not fully co-regulated processes (Dubousset et al., 2010).

Ⅳ. CONCLUSIONS

This study demonstrated substantial genotypic variation in biomass partitioning, SHI and NHI among eight Brassica napus cultivars. The decoupling of total biomass from harvest index, as exemplified by Colosse (high biomass, high HI) versus Akela (high biomass, low HI), reveals that vegetative vigor alone is insufficient for high seed productivity. Cultivars with high SHI and NHI despite moderate biomass, such as Capitol and Pollen, represent promising ideotypes for sustainable production that maximize nutrient recovery while minimizing fertilizer inputs. Future breeding programs should prioritize simultaneous improvement of HI, NHI, and SHI through marker-assisted selection and focus on identifying the genetic basis of these traits across diverse environmental conditions. These efforts will be essential for developing cultivars that combine high productivity with enhanced nutrient use efficiency in sustainable agricultural systems.