| As the development of light and simple cultivation technique, direct seeding expands rapidly in winter oilseed rape production of Yangtze Basin, in which the seed emergence and young seedling growth are easily subjected to water-logging and nitrogen(N) uptake and utilization differ from that in the conventional production. It is of vital to make clear the nitrogen requirements at different stages and the sensitive period to water-logging during seed emergence and young seedling growth in direct-seeded oilseed rape. In this dissertation, we conducted a study to identify the most representative leaf for seedling assessment, compare N uptake and utilization of various cultivars, and investigate appropriate N application rate at different stages of winter oilseed rape. Moreover, the effects of water-logging on seed emergence, young seedling growth and nutrient uptake at different stages during seed germination and seedling period were studied in oilseed rape. The objectives of this study are to provide experimental information for developing reasonable cultivation technique to manage N and combat water-logging in direct-seeded oilseed rape. The main results are as follows:1. The morphological and physiological characteristics of various functional leaves were compared with that of whole plant at seedling stage using two oilseed rape cultivars viz., Huaza 9 and Huaza 62 as plant material, under the same plantation density and fertilizer application. The results showed that, the photosynthetic rate, the contents of non-structural carbohydrates, nutrients and soluble protein in 4th and 5th leaf of a plant were more close to those of the whole plant. The area and dry weight of 5th leaf was highly correlated with those of whole plant. It is therefore recommended that the 5th leaf could be a most suitable leaf to represent whole plant for the evaluation of plant growth and assimilate accumulation for winter oilseed rape at seedling stage. The following regression equations can be used to predict the dry matter accumulation and the total leaf area of whole plant: 1) y = 3.32 x + 1.51(R2 = 0.88, P < 0.001) where y = dry matter(g) of whole plant and x = dry matter(g) of the 5th leaf; 2) y = 1.24 x + 222.69(R2 = 0.67, P <0.001), where y = total leaf area(cm2) of whole plant; x = length(cm) × width(cm) of the 5th leaf.2. Twelve cultivars that are widely grown in the Yangtze Basin were used to compare the differences in plant dry matter accumulation, N uptake and utilization among cultivars. Clustering analysis using minimum distance method divided the 12 cultivars into 3 types(R2 = 0.905) according to the differences in plant dry weight, N uptake amount and N utilization efficiency(NUE). Dry matter accumulation and yield formation of Huaza 9, a relatively higher N efficiency cultivar and Nonghuayou 101, a relatively lower N efficiency cultivar were compared under two N supply levels. The results showed that, plant dry matter and N uptake were promoted more in Huaza 9 than in Nonghuayou 101 by N application. Under the same N application rate, the seedling period of Nonghuayou 101 was longer than that of Huaza 9; while, the pod growth period of Huaza 9 was longer than that of Nonghuayou 101. Under N applied condition, dry matter accumulation rate of Huaza 9 was lager than that of Nonghuayou 101 at all stages. At vegetative growth stage, Nonghuayou 101 had lower dry matter accumulation and lower N partition ratio in leaves than Huaza 9, but no difference in dry matter partition ratio in leaves were detected among two cultivars. At reproductive stage, dry matter accumulation of Nonghuayou 101 was greater than that of Huaza 9, but partition ratios of dry matter and N in reproductive organs were higher in Huaza 9 than those in Nonghuayou 101. The differences of pod growth period, dry matter accumulation rate and partition ratios of dry matter and N in reproductive organs may contribute to different seed yield and NUE among cultivars.3. Huaza 9 was used as material to study N uptake and utilization of direct-seeded oilseed rape under different rates of N applied before sowing, at seedling stage and at initiation of stem elongation stages, respectively. The results showed that, N application as basal fertilizer had a lager impact on seedling growth period, N application at seedling stage had a lager impact on both seedling and pod growth periods, while, N application at initiation of stem elongation affected the flowering period more. The effects on morphological development of oilseed rape was shown as: N application as basal fertilizer > at initiation of stem elongation > at seedling stage. Higher N application as basal fertilizer not only increased N partition ratio in leaves before initiation of stem elongation, but also extended the period of fast dry matter accumulation and improved dry matter accumulation rate, therefore, increased plant dry matter accumulation, which contributed to the construction of reproductive organs at late growth stage. Appropriate N application at seedling stage and at initiation of stem elongation could prevent leaves from senescence and falling off at earlier stage after flowering, which contributed to pod growth and development and seed yield increase. Under the N application mode by farmers, the appropriate N application amount was 90.0, 37.5 and 90.0 kg N/hm2 before sowing, at seedling stage and at initiation of stem elongation, respectively, as shown in this study. The SPAD value of the 4th leaf form the uppermost of a plant reflected plant N nutrition status, which can be used as an indicator for N diagnosis at seedling stage of oilseed rape.4. The tolerance of 8 hybrid cultivars of oilseed rape to water-logging at radicle growth was tested. The investigations on seedling emergence rate, toproot and hypocotyl length, SPAD value of 1st leaf from the bottommost and seedling dry weight showed that Huaza 62 had a relatively higher tolerance but Fengyou 737 mostly sensitive to water-logging among all cultivars. Nevertheless, N content was affected more but the balance between potassium and sodium kept well in seedlings of Huaza 62 under water-logging, in comparison with Fengyou 737.5. Two cultivars, viz., Huaza 62 and Fengyou 737, were used as materials to study the effects of water-logging on seed emergence, young seedling growth and nutrient uptake of oilseed rape. The results showed that, seedling emergence and growth were seriously inhibited by water-logging during seed germination. Seedling growth was also inhibited but seedling survival was not by water-logging during seedling period. Seedling emergence was delayed by water-logging at imbibition stage; hypocotyl was elongated by water-logging at cotyledon expansion stage, which resulted in high seedlings. Radicle growth was most sensitive stage to water-logging during seed germination. Seedling emergence decreased significantly under water-logging at this stage. During young seedling growth, water-logging at 2-leaf and 3-leaf stages had more serious impacts on seedling growth. All seedling emergence, leaf number and size, SPAD value, seedling dry weight and the ratio of N/P decreased, but the ratio of K/Na increased after water-logging during seed germination or young seedling growth, compared to the control. In comparison with Fengyou 737, Huaza 62 showed less impacted in seedling emergence, leaf number, leaf SPAD value, seedling dry weight and the ratio of K/Na by water-logging during seed germination and young seedling growth. |
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