Genetic Dissection Of Nitrogen Use Efficiency In Brassica Rapa

Nitrogen (N) is the most essential nutrient for growth and development of plants. N fertilizer input is considered an effective way to enhance the yield of agricultural crops. However, the excessive use of N fertilizers in crop cultivation has reduced nitrogen use efficiency (NUE) and increased the input cost and serious environmental pollution. Developing crop cultivars with better NUE is essential for sustainable agriculture that is more economically valuable and environmental friendly. Brassica. rapa is an important crop, comprises several plant species including vegetables, oilseeds and fodders. The knowledge on the genetic improvement of N uptake and utilization in B. rapa is lacking. By exploring the genetic potential for the improvement of NUE in B. rapa crops may contribute to reduction of N fertilizers globally as well as in China. The objectives of current study were to identify quantitative trait loci (QTL) underlying morphological traits associated with NUE and to assess the genetic variation in B. rapa under varying N levels.To characterize the natural variation of shoot and root response under different N conditions,45accessions belonging to five cultivar groups of B. rapa were screened. Wide genetic variation was observed for shoot and root traits under both N conditions. In general, plants grown under HN conditions significantly produced higher values for most of the traits. However, accessions differed significantly in the extent of growth reduction under N limiting conditions. Both tolerant and sensitive accessions to LN condition were identified. Shoot traits including plant height (PH), plant width (PW), number of leaves (NOL) and shoot dry biomass (SDB) responded differently between LN and HN levels, among accessions. The SDB of most of accessions was significantly reduced under LN conditions. On average, the SDB of the accessions grown under LN was reduced by32%compared to HN. Accessions also showed highly varied response to root traits under both N conditions. Root traits including root dry biomass (RDB), Primary root length (PRL), root thickness (RT) and root density (RD) revealed significant differences between LN and HN conditions. However, TRL was almost unaffected by N nutrition, whereas range of variations existed among accessions. In most accessions, root traits including RDB, RT and RD were significantly increased at HN condition. In contrast, PRL and root to shoot ratio (R:S ratio) were decreased under same condition. The wide natural variation for shoot and root traits provided attractive features for the genetic dissection of these traits by QTL analysis and developing efficient breeding strategy for improved NUE. The coefficient of correlation showed significant and positive correlations for various traits. The most significant correlation was observed between RDB, TRL and SDB under both N levels. The strong correlation between RDB, TRL and SDB suggested that greater vigor of root and shoot is essential for efficient uptake of N in B. rapa.Quantitative trait loci (QTL) analysis is one of the most powerful and valuable tools to identify the genetic factors controlling complex traits. In the present study,103doubled haploid (DH) lines from a crossing between a Chinese cabbage line and a rapid cycling line were grown hydroponically under LN and HN conditions to identify the QTLs related to the morphological and yield traits of B. rapa. Four shoot traits,(PH, PW, NOL, SDB) and three root traits (RDB, TRL, RD) were investigated. The correlation analysis revealed highly significant and positive correlations for most of the traits. Strong correlation was found between SDB, RDB and TRL, under both N conditions, showing that these traits shared common mechanism. In the light of inter-relationship between SDB and root traits, it is likely that N uptake may be improved by developing better root system of B. rapa that ultimately enhance the shoot biomass. Among18indentided QTL, ten and eight were confined to low N (LN) and high N (HN) treatments, respectively. The total phenotypic variation explained by individual QTL ranged from12.8-26.9%under LN level, and13.4-22.3%under HN levels, respectively. The most significant QTL was detected for PH under LN condition, accounting26.9%of phenotypic variation. Two common QTLs, one for PH on chromosome A05, and another for SDB on A04were identified under low N and high N conditions simultaneously. Most of the identified QTL were treatment specific, revealing that these morphological traits were regulated by different genes under various N environments. In both trials, no major QTL (which explained more than30%of the phenotypic variance) was identified that indicated the complex genetic control of NUE, mainly due to the influence of several loci each with minor effect to N uptake and utilization in B. rapa. Moreover, this revealed that genetic improvement of NUE by conventional breeding will be complicated requiring combination of several genes contributing to the NUE. Further genetic analysis of additional population for large effect QTL is essential through the evaluation of shoot and root morphological traits under different N levels.The research reported here showed the existence of substantial genetic variation for N uptake and metabolism in B. rapa, and led to identification of QTLs associated with shoot and root morphological traits in B. rapa. These findings enhanced our deep understanding of the genetics of NUE in B. rapa and may prove beneficial for practical genetic improvement.