ABSTRACT

The breeding of rapeseed (Brassica napus) Cultivars adapted to soils low in available boron (B) may have particular importance because the crop has a high B requirement and B deficiency is widespreed in some rapeseed-planting areas. The response of B-efficient cultivar 9118, B-inefficient cultivar 9141 and their reciprocal first generations of rapeseed to B deficiency was tested under soil culture. The results indicated that the mean of B efficient coefficient of two first generations was 0.885 which was close to 0.895 of B-efficient cultivar 9118, and was significantly higher than 0.154 of B-inefficient cultivar 9141. When available boron was deficient in soil, the variation of agronomic traits and uptake、 distribution and translocation index of boron of the first generations were consistent with that of the B-efficient cultivar 9118. It was concluded that the boron utilization of the reciprocal first generations was B-efficient, and the B-efficient trait of Brassica napus was a dominant trait. Mechanism of B efficiency of Brassica napus may be that B-efficient cultivar had a stronger capability to retranslocate boron from older organ to growing cone.

Rapeseed (Brassica napus) is one of main oil crops in China, and it requires more boron during its growth , so planting rapeseed needs applying B fertilizer soil low in available boron. But the response of various cultivars of Brassica napus to B deficiency was significantly different (Hu Qiuhui, 1990, Shen Zhenguo 1991, Xue Jianming 1995).

After studying the response of 86 rapeseed cultivars to B deficiency, we selected a B-efficient cultivar 9118 and a B-inefficient cultivar 9141. B efficient coefficient of 9118 was 0.91 which was the ratio of seed yield in soil available boron content of 0.25mg/kg to that of 1.0mg/kg, and that of 9141 was zero ( Wang Yunhua et al 1995, Xiong Shuanglian 1995). To study the mechanism and inheritance of the B-efficient trait, the paper showed the results on the response of B-efficient cultivar 9118、 B-inefficient cultivar 9141 and their reciprocal first generations to B deficiency.

Materials and Methods

All original rapeseed cultivars screened were from Rapeseed Breeding and Genitics Insititute of Huazhong Agricultural University and Oi1 Crop Research Insititute of Agricultural Academy of China, respectively, then numbered by our laborary,and B-efficient cultivar was 9118, B-inefficient cultivar 9141, and their reciprocal first generations were F₁ (9141× 9118) and RF₁ (9118×9141).

Calcareous grey-purple sand soil was introduced to the experiment, and it contained 0.13mg/kg of hot-water-soluble boron , 4.93 g/kg of organic matter , 0.25g/kg of total nitrogen ,

23.1 mg/kg of available nigtrogen , 2.05 mg/kg of available phosphorus , 46.7 mg/kg of available 

1) Address: Dept. of Resource. Environment and Agrochemistry, Huazhong Agri. Uni. Wuhan 430070 P.R. China, e-mail: brassica @ public. wh.hb.cn

potassium and pH was 8.25.

Experiment was conducted with pot culture, and each pot contained 7kg of soil . The experiment was a factorial randomized block with four replicates and consisted of two boron levels:one was 0.3 mg B/kg of boron deficient level (-B);and another was 1.00 mg B/kg of boron normal level (+B). Boron was added as boric acid dissolved in distilled water,and sufficient quantities of N, P, K, S, Mg and other trace elements were added to prevent deficiencies and mixed fully before sowing .Pots were irrigated with distilled water whenever required. Pot spot had a glass steel shed on wheels to prevent raining.

Seedling samples were harvested at the stage of 5 leaves, and bolting samples at the bolt height of 20cm. All samples harvested were divided into three parts of leaf, stem and petiol, respectively, then dried at 70℃, grounded and analyzed for B by curcuma colorimetric method. After sampling, two plants in each pot were reserve to harvest seed yield and examine agronomical traits.

Results and Discussion

No boron deficient symptoms for the parents and their first generations were found at seedling stage in boron deficient treatment. In the condition of boron deficiency in soil at bolting stage B-inefficient parent 9141 showed distinct symptoms: spliting stem, and flowering but no seeding and its seed yield decreased significantly(Table 1) . But boron-efficient cultivar9118 didn’t show boron deficient symptom. The result in table 1 showed that in the absence of added B the plant height, number of branch, number of silique per plant, weight of thousand seeds and seed yield per plant of 9141 were affected severely, which decreased 42.4%, 40.0%, 72.0%, 15.6%, 34.8% and 84.6%, respectively, and its boron efficient coefficient was 0.154. But the decrease of the six agronomic traits of 9118 was only 8.4%, 10.0%, 6.0%, 4.9%, -1.2% and 10.5%, respectively, and the boron efficient coefficient reached 0.895. In contrast with the parents, response of the reciprocal first generations to boron deficiency was consistent with B-efficient parent 9118. The boron efficient

Table 1 Effect of B deficiency on parents and their reciprocal first generations

coefficient of F₁ and RF₁ were 0.879 and 0.890, respectively. It can be concluded that boron efficiency of reciprocal first generations crossed betwecn B-efficient cultivar 9118 and B-inefficient cultivar 9141 showed B-efficient, and the trait of B-efficiency of Brassica napus was a dominent trait, but the genetic law of B-efficient trait should be studied further.

It can be seen from table l that the growth duration and the bolting duration of 9141 were longer than that of 9118, and F₁ and RF₁ for the two agronomic traits were near to 9118. It showed that growing period might be linked to boron efficiency, B-efficient caltivar had shorter growing period and bolting earlier, and the growing period of B-inefficient cultivar was longer and bolting later. But the linked relationship betwen growing period and boron efficiency must be confirmed further by F₂ generation population, which may be useful as one index to screen boron-efficient germplasm resource of Brassica napus, especially in the fields which have normal available boron content. The growing period of Brassica napus was less affected by boron level.

When soil boron was deficient (Fig.1 A), the B content of 9141 in the oldest leaf(L₁) was lower than that of RF₁ and higher than 9118 and F₁,and in the youngest leaf(L_y), it was the lowest among them. As compared with Fig.1 A, the boron content of 9141 in all leaves was higher than that of 9118、 F₁ and RF₁ at boron normal level(Fig.1B). It seemed to mean that 9141 could absorb more boron, but its retranslocation ability of boron from old leaf to young leaf was influenced by boron deficiency.

According to the method of Shelp B. J. (1987) and Shen zhenguo(1992) , the retransloction index were calculated, which was the ratio of boron contents in youngest leaf(L_y): oldest leaf(L₁) (table 2). The data showed that boron retranslocation was different at the different growth stage, and also different in the two different B level. When soil boron was deficient, the retranslocation index decreased at the seedling stage, and increased at the bolting stage. But the variation of boron retranslocation index between B-inefficient cultivar 9141 and B-efficient culltivar 9118、 F₁ and RF₁ was significantly different. The decrease of B retranlocation index of 9118、 F₁ and RF₁ was remarkablly less than that of 9141 at the seedling stage in boron deficient level, and the increase of B retranslocation index of 9118、 F₁ and RF₁was much higher than 9141 at the bolting stage. This indicated that boron retranslocation and utilization of 9141 were severely influenced by boron deficiency, and showed that boron retranslocation ability of B-efficient cultivar was much stronger than that of B-inefficient cultivars,which may be a mechanism of B efficient utilization of Brassica napus.

Figure 2A and B were the curve of absorption and distribution of boron of 9141,9118 F₁ and RF₁ at the bolting stage in two different boron level. It indicated that the boron content of 9141 in youngest leaf(L₁) decreased significantly which was consistent with the result at seedling stage(Fig.2 A). To compare Figure 2A with B, boron content of the four materials in upper-stem and

Table 2 Effect of boron deficiency on boron retranslocation index in Brassica napus

The data were the mean ± SD of four replications.

down-stem decreased due to B deficiency, but the reduction of 9141 was the most severious, and the B contents of 9118、 F₁ and RF₁ in stem were relatively higher,which may be another aspect of B efficient utilization of Brassica napus. Becasse boron was one of immobile nutrition elements in plant, and it was difficult in reuse in plant. But relative higher B content in stem may be useful to promote B retranslocation from elder organ to growing cone and satisfy plant requirement of boron nutrition.

Acknowledgements

This research was supported by National Natural Science Fundation of China. The authors gratefully acknowledge Rapeseed Breeding and Research Institute of Huazhong Agricultural University and Oil Crop Research Institute of Agricultural Academy of China for providing original materials, Wu Nianyuan for pot management and Zheng Qizhen for typing the manuscript.

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