Browsing by Author "Wan, H."

Sclerotinia stem rot is one of the most serious diseases in rapeseed (Brassica napus) due to the lack of resistance sources. A high level of resistance was reported in Brassica oleracea cytodeme, one of parental species of rapeseed. In this study, a panel of 55 resynthesized lines of B. napus (RS lines) derived from seven wild and two cultivated types of B. oleracea was evaluated for Sclerotinia resistance over 2 years. Relative to 'Zhongyou 821', a cultivar of B. napus with partial resistance against S. sclerotiorum, RS lines exhibited stronger stem resistance. Although the resistant level of RS lines was lower than that of corresponding parental B. oleracea, a moderate correlation between resistance of RS line and corresponding parental B. oleracea type was found both for leaf (r = 0.74, P = 0.02) and stem (r = 0.69, P = 0.04). Our data suggests that the RS lines are important resources to improve Sclerotinia resistance of current rapeseed. A breeding strategy is discussed to enhance the Sclerotinia resistance of rapeseed by using B. oleracea.

Many studies have examined how intensity of grazing and patterns of precipitation individually and interactively influence the spatial and temporal dynamics of grassland vegetation, such as dominance, succession, coexistence, and spatial heterogeneity. However existing models have rarely considered the diet preferences of grazers and how they interact with variation in precipitation amount and timing. We examined how plant community structure responds to the individual and combined effects of grazing intensity, selective grazing, and patterns of precipitation, based on a six-year grazing experiment with seven levels of field-manipulated grazing intensity in a typical steppe of Inner Mongolia. The palatable species, mainly forbs, were most severely damaged at intermediate levels of grazing intensity; given that these species are the major contributors to plant community diversity, a U-shaped diversity-grazing intensity relationship resulted. In contrast, spatial heterogeneity of aboveground biomass and species composition peaked at intermediate levels of grazing intensity. Cold season precipitation positively correlated with the abundance of the dominant C-3 grasses and correlated negatively with the subdominant forbs and C-4 plants. Thus, when cold season precipitation increased, plant community species diversity decreased. Grazing intensity and precipitation did not interact in their effects on species richness. These findings contrast with the predictions from current disturbance-diversity models and indicate that diet selection of grazing animals is an important factor shaping the diversity-grazing intensity relationship in semi-arid grasslands. Future grassland biodiversity conservation and management practices should take diet preference of grazing animals into account.

Degradation and decreasing productivity increasingly demand sustainable grazing management practices within Inner Mongolian steppe ecosystems. This study focuses on grazing-induced degradation processes over a wide range of stocking rates and aims to identify short-term sensitive indicators and alternative management practices. Shortterm effects of 2 grazing management systems (Mixed System and Traditional System) and 7 stocking rates (SR0, SR1.5, SR3, SR4.5, SR6, SR7.5, and SR9 for 0,1.5, 3, 4.5, 6, 7.5, and 9 sheep/ha, respectively) on yielding performance and herbage quality were measured on experimental plots in which moveable exclosures were used on areas chronically grazed by sheep. The experiment was conducted in a typical steppe ecosystem in Inner Mongolia, P. R. China. Results are presented for 2005 and 2006. Sampling time was the main factor affecting yield and quality. Stocking rate also showed considerable effects on yield. Herbage mass decreased linearly from SR0 to SR9, by 85% and 82% in 2005 and 2006, respectively. Herbage accumulation was also affected by stocking rate, and was highest at SR1.5 and clearly reduced at SR9. Grazing effects on relative growth rate indicated grazing tolerance of plants in the short-term, since up to high stocking rates, relative growth rates remained stable. Precipitation also determined plant responses to increasing levels of grazing. The year of higher rainfall generated higher grazing tolerance of plants and higher herbage growth than the drought year. Despite considerable reduction of herbage mass, consistent short-term responses of herbage quality to grazing in 2005 and 2006 were reflected only in terms of crude protein and acid detergent lignin. Herbage crude protein content was highest at SR7.5 and SR9, while lignin was lowest at SR7.5 and SR9. Neither productivity nor herbage quality was affected by the management system, suggesting that both systems may be applicable on typical steppe in the short-term.

Grassland degradation due to over-grazing causes severe ecological and economical problems in the semi-arid grasslands of Inner Mongolia, PR China. The development of sustainable management systems is required, but basic information regarding the degradation process in its formation and development is rare. In 2004 and 2005 a cutting-frequency experiment was conducted in the Xilin River Basin, Inner Mongolia, subjecting a non-degraded grassland area to 3 different cutting frequencies: single defoliation at the end of growing season and defoliations every 6 and 3 weeks. It was hypothesised that the productivity of the S. grandis/L. chinensis community will be reduced by frequent defoliations. The cumulative dry matter yield increased with increasing defoliation frequency from 204 to 277 g DM m(-2) in 2004 and from 87 to 158 g DM m(-2) in 2005. The low aboveground biomass productivity in 2005 resulted from the low amount of precipitation, which was only 50% of the long-term mean. The nitrogen yield increased from 2.9 to 6 g N m(-2) in the frequently defoliated treatments in 2004 and from 1.2 to 2.8 g N m(-2) in 2005. Total leaf area was significantly reduced by frequent defoliation, but specific leaf area increased both in S. grandis and L chinensis. Root mass (0-15 cm) was on average 929 g OM m(-2) in 2004 and 882 g OM m(-2) in 2005 and, as well as species composition, not significantly influenced by frequent defoliations. It was concluded that frequent defoliation positively affected the productivity of the S. grandis/L. chinensis community, and that the annual amount of precipitation essentially determined the biomass production of this grassland ecosystem. Because the community increased its productivity after frequent defoliation in the 2 experimental years, it was suggested that the community is resistant to defoliation stress in the short-term. (C) 2009 Elsevier B.V. All rights reserved.