Compensative impact of winter oilseed rape (Brassica napus L.) affected by water stress at re-greening stage under different nitrogen rates

Oilseed rape is one of the most important oil crops cultivated on over 6.5 million hectares of land in China. Although mainly cultivated in the Yangtze River Basin, rising winter temperatures in northern China in recent years have made possible to expand the cultivation area of oilseed rape northward and westward. The planting area of winter oilseed rape in Northwest China has increased year by year. Irrigation and nitrogen supply at re-greening stage is important for flower bud differentiation and branch number increase of oilseed rape. However, drought stress is usually frequent at re-greening stage in most regions of Northwest China. Local farmers apply irrigation and nitrogen to oilseed rape fields for high seed yields, but such effect has not been obvious, resulting in various environmental problems. Therefore determining an appropriate nitrogen dose under drought at re-greening stage is important for the production of oilseed rape in Northwest China. Barrel experiments, including 5 nitrogen application rates 0 g (N₀), 0.2 g (N₁), 0.4 g (N₂), 0.6 g (N₃) and 0.8 g (N₄) and two water treatments full water (W, 70%–80% of field water capacity) and deficit water (D, 50%–55% of field water capacity) at re-greening stage were conducted to measure growth and physiological index, yield components, water use efficiency (WUE) and quality traits of oilseed rape. The study used principal component analysis (PCA) to analyze and evaluate the parameters under different treatments. Results showed that aboveground dry matter, chlorophyll content, photosynthetic rate, seed yield and WUE first increased and then decreased with increasing nitrogen application rate. Then aboveground dry matter, chlorophyll content, photosynthetic rate, seed yield and WUE all reached the maximal level under N₃ treatment with the same water conditions. After drought stress and re-watering, shoot dry matter weight, chlorophyll content, photosynthetic rate, yield and yield components of all nitrogen treatments showed certain degree of compensative effect. The compensative effect intensified first and weakened then with N application rate, which was best under N₃ treatment. No significant differences were noted in growth indexes, chlorophyll content and seed yield between W and D treatments. Photosynthetic rate under D treatment was significantly higher than that under W treatment with N₀. Seed yield reduced by 2.2% and WUE increased by 3.8% in D treatment compared to that of W treatment with N₃ nitrogen application. Nitrogen partial factor productivity and seed oil content decreased with increasing nitrogen amount, while seed protein content showed the reverse trend. Compared with N₀, average partial factor productivity and seed oil content in N₃ treatment decreased by 6.2% and 13.0%, but yield and WUE increased by 87.6% and 32.9%, respectively. Based on PCA for each indicators, we found that the highest PCA score occurred in N₃D treatment. Therefore, the N₃D treatment was optimized measure for increasing WUE, yield and quality of oilseed rape in northwest China.