**1. Introduction**

62 Oilseeds

Qian, P., Schoenau, J. J., and Huang, W. Z. 1992. Use of ion exchange membranes for the routine soil testing. Comm. Soil Sci Plant. Anal. 23(15 & 16): 1791-1804

SAS Institute. 2008. Version 9.2. SAS Inst. Inc., Cary, NC.

N fertilizer application can guarantee the high yield of crop; it is a general method to improve the yield of crop (Zhang et al., 2010). However, not only N use efficiency was declined, but also environment contamination was serious day by day because of N fertilizer over application (Zhang et al., 2010). So, control and reduction in the amount of N fertilizer application is very important, it is necessary to dredge up the potential of N absorption and N use efficiency of crop. Oilseed rape (Brassica napus L.) is an important oil production crop in China. However, the N use efficiency and N efficiency of oilseed rape are very low (Schjoerring et al., 1995). The N application amount has reached 200-330kgN per ha (Schjoerring et al., 1995; Wiesler et al., 2001a) and it is increasing year by year. But the studies on differences of N efficiency in different oilseed rape varieties and breeding research of the oilseed rape with high N efficiency were relatively slower than other cereal (Liu et al., 2009). Oilseed rape requires high amounts of N for growth, but the N efficiency (seed yield per unit of accumulated N in plant) is very low. Consequently, it is necessary to improve the N efficiency (Rathke et al., 2006). In generally, N efficiency has two components: N uptake efficiency and N utilization efficiency (Sattelmacher et al., 1994). The differences of N efficiency between oilseed rape genotypes are significant (Wiesler et al., 2001b; Christian et al., 1999). Kessel et al., 1999 has been measured N concentration of organs in different oilseed rape genotypes, results suggested that oilseed rape possess the 2 physiological characteristics (low N concentration in dropped leaves, high N harvesting

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Nitrogen Efficiency in Oilseed Rape and Its Physiological Mechanism 65

(2001) studies of *Brassica napus* L. suggested that large amounts of endogenous N were redistributed from the leaves to the stems to the roots tap tissues which acted as a buffering storage compartment and later used to supply the reproductive tissues. About 15% of total N cycling through the plant was lost through leaf fall and 48% had been remobilized from vegetative tissues and finally recovered in the mature pods (Thomas et al., 2007). It can be concluded that, the accumulation and distribution characters of N in vegetative and reproductive organs are important factors that affected the crop production (Zhang et al., 2006), the higher yield production is not only dependent on higher N absorption, but also depend on higher N distribution efficiency (Wang et al., 2003). Studies on the regular of N absorption, distribution and redistribution at different growth stages will supply scientific foundation for rational application of N fertilizer, improving N harvesting index and increasing crop production. However, most of these studies mainly focused on the redistribution of N in cereal crops such as wheat (Anderson et al., 2005; Dalling et al., 1985), maize (Pommel et al., 2006) and rice (He et al., 2002). A few of them (Rossato et al., 2002; Malagoli et al., 2005) looked at oilseed rape. Moreover, studies on the relationship between N redistribution and crop production were fewer. In this study, two oilseed rape varieties (X-36 and X-50) with different N use efficiencies were grown in sand culture with complete nutrient solution and normal N supply. To better understand the N dynamics during later growth stages, the relationship between N redistributed from the vegetative organs to the grain and grain yield was studied using

**2.1 Comparison of Nitrogen (N) efficiency between oilseed rape genotypes** 

The experiment was conducted at Agricultural Resources and Environmental College Experiment Field in HuNan Agricultural University during Sep. 2004 to May. 2005. 16 oilseed rape varieties were used as plant materials, the numbers are shown in Table 2, plant materials were supplied by China National Oilseed Crop Improvement Center, Hunan Branch. The soil used was an alluvial for vegetable cultivation derived from river flow alluvial material, containing organic matter 27.79 g/kg, total N 1.90 g/kg, and total P 0.79 g/kg, total K 19.76 g/kg. The NaOH hydrolyzed N was 108.31 mg/kg of soil, Olsen-P 15.85 mg/kg of soil, available K 19.76 g/kg of soil with pH 5.67. Urea was used as N fertilizer, calcium magnesium phosphate as P fertilizer (containing P2O5 12%) and potassium chloride

The experiment had two N treatments: N application and no N application, 16 oilseed rape varieties used as plant materials, 32 treatments, 3 replicates, 96 districts, 10.5 m2 per district, randomized block. N application treatment: 225kg N, 75kg P2O5 and150 kg K2O per hectare, 50% N fertilizer used as basal fertilizer, 20% N fertilizer used as added fertilizer during winter, 30% N fertilizer used at stem elongation stage, all P and K fertilizer were used as basal fertilizer; no N application treatment: the same with N application treatment except of non-N fertilizer applied. Seedling on Sep. 25 2004, transplanted on Nov. 2 2004, transplanted

density was 100 thousand plants per ha, management as normal.

the 15N labeling method.

**2. Materials and methods** 

(containing K2O 60%) as K fertilizer.

**2.1.1 Plant materials and experiment design** 

index) has higher N efficiency also. The development of N efficient genotypes and improvement of N management will require to understand the relationship between physiological processes and biomass, yield formation of crop under no N application conditions (Dreccer et al., 2000). Mahmoud et al., (2005) compared the differences of root growth and nitrate N exhaust in cultivar culture between high N efficiency and low N efficiency oilseed rape; larger amount of root biomass, higher root uptake activity and higher exhausted nitrate N amount were found in the high N efficiency oilseed rape; Nitrate-N uptake from soil depends on root growth and uptake activity, the amount of N depleted from the compartments significantly correlated with root-length density (Mahmoud et al., 2005). Seiffert et al., (2004) suggested that, the N use efficiency can be increased significantly by strengthen the activities of asparagine synthetase and glutamine synthetase through transgenic methods. Obviously, it is necessary to 'system study' on N uptake and N use efficiency in different oilseed rape. The differences of yield, N absorption and N use efficiency in different oilseed rape genotypes should be compared, and preliminarily discussion the contribution of N absorption efficiency and N use efficiency to N efficiency under different N application levels, in order to supply scientific basic and plant materials for the future study.

The differences of concentration and distribution of N in crop is depending on the differences of organs and growth stages, and N redistribution in different organs will occur at different growth stages; these differences are related to the transfer of growth center (Peoples et al., 1998; Zhang et al., 2008). Studies in wheat (Andersson et al., 2005), oilseed rape (Malagoli et al., 2005) and pea (Séverine et al., 2005) showed that the leaves and stems of crop have become important N "sources" after the flowering stage, grain N does not only come from root N uptake during the later growing period, but also from that redistributed from the vegetative organs (Zhang et al., 2010). The latter was one of the key factors that guarantee crop N requirement during the later growing period. Nutrient competition between different organs of the crop during the later growing period is fierce; the uptake ability of the root declines at this time and soil mineral nutrients are exhausted (Dong et al., 2009). In oilseed rape, all leaves drop off at harvesting stage and N in the leaves is lost. To achieve high yield and high N use efficiency in crop production, N must be redistributed from the roots to the grains, residue N in uneconomic tissues must be reduced, and N harvesting index of economic tissues must be increased (Zhang et al., 2010). Research on N redistribution in cereals reached the following conclusions. The redistribution proportion of N from the leaves, stems and reproductive organs to the grains was more than 60% in wheat (Palta et al., 1995a, 1995b). Dalling et al., (1985) showed that, although the soil can supply adequate N after the crop flowering stage, at least 50% of the N in grains is still redistributed from the vegetative organs. The effect of environmental conditions on N redistribution was considerable e.g., low N in barley (Przulj et al., 2001) and drought stress in wheat (Barbottin et al., 2005; Palta et al., 1994) can accelerate N redistribution in plants, but the process can be restrained when plants are damaged by diseases and insects e.g., wheat (Dimmock et al., 2002). However, only a few studies looked at N redistribution in oilseed rape. Malagoli et al., (2005) found that in oilseed rape the N of flowers and pods was mainly from endogenous N, comprising 64 and 73% of total N content in these tissues, respectively. The N requirements of seed filling were mainly satisfied by N mobilized from the vegetative parts. Rossato et al., (2001) studies of *Brassica napus* L. suggested that large amounts of endogenous N were redistributed from the leaves to the stems to the roots tap tissues which acted as a buffering storage compartment and later used to supply the reproductive tissues. About 15% of total N cycling through the plant was lost through leaf fall and 48% had been remobilized from vegetative tissues and finally recovered in the mature pods (Thomas et al., 2007). It can be concluded that, the accumulation and distribution characters of N in vegetative and reproductive organs are important factors that affected the crop production (Zhang et al., 2006), the higher yield production is not only dependent on higher N absorption, but also depend on higher N distribution efficiency (Wang et al., 2003). Studies on the regular of N absorption, distribution and redistribution at different growth stages will supply scientific foundation for rational application of N fertilizer, improving N harvesting index and increasing crop production. However, most of these studies mainly focused on the redistribution of N in cereal crops such as wheat (Anderson et al., 2005; Dalling et al., 1985), maize (Pommel et al., 2006) and rice (He et al., 2002). A few of them (Rossato et al., 2002; Malagoli et al., 2005) looked at oilseed rape. Moreover, studies on the relationship between N redistribution and crop production were fewer. In this study, two oilseed rape varieties (X-36 and X-50) with different N use efficiencies were grown in sand culture with complete nutrient solution and normal N supply. To better understand the N dynamics during later growth stages, the relationship between N redistributed from the vegetative organs to the grain and grain yield was studied using the 15N labeling method.
