Genetic Improvement In Yield And Its Associated Traits Of Winter Wheat In China

Increasing yield potential has been one of the most important breeding objectives worldwide, especially for China which has the largest population but limited land. Knowledge on genetic gain of yield potential and its associated traits is essential for understanding yield-limiting factors and developing strategies for future variety improvement. This study was conducted from 2001 to 2004 on genetic improvements in yield and its associated traits in both north and south winter wheat growing areas. Totally, 84 historical varieties from 1970s to 2000, were included in 9 trials. They were sown in Beijing, Hebei, Shandong, Henan, Shaanxi, Jiangsu and Sichuan provinces respectively. All yield trials were treated with fungicide and pesticide and, except for Shanxi and Sichuan, where the yield trials were conducted under both high and low nitrogen regimes, were conducted with high nitrogen input. Molecular markers were used to clarify the distribution of Rht-B1b, Rht-D1b and Rht8c, and 1BL-1RS translocations. The main results are present below. Yield potential over the last 30 years has continuously increased with year of variety release. Annual genetic gain was 43.43 kg/ha or 0.70% on average nationwide,ranging from 51.47 kg/ha or 0.80% in north China to 23.32kg/ha or 0.47% in south China. They were 64.63 kg/ha, 35.40 kg/ha, 32.09 kg/ha, 72.11 kg/ha, 53.13 kg/ha, 14.20 kg/ha, 32.44 kg/ha or 1.2%, 0.54%, 0.48%, 1.05%, 0.72%, 0.31% 0.62% in Beijing, Hebei, Shandong, Henan, Shanxi, Jiangsu and Sichuan provinces respectively. The significant increase of yield potential mainly occurred at the end of 1970s or the beginning of 1980s, which was correspondent to the release of semi-dwarf varieties in both north and south and 1BL-1RS varieties in the north. Yield gain has been slowed down since 1980s and improvements afterwards were made mainly in resistance to biotic and abiotic stresses and processing quality. Yield genetic gain was primarily attributed to the increase of kernels per spike or thousand kernel weight in the north and only thousand kernel weight in the south. There was no significant change in number of spikes/m2 after 1980s in both north and south. Number of kernels/spike showed a continuous increase in the north, which was mainly attributed to the increase of fertility and number of kernels per spikelet, but no significant change in the south. Thousand kernel weight has been increased periodically and grain weight/spike increased steadily. The most significant change in traits was the reduction of plant height, reduced from above 100 cm before 1970s, to 75 to 85 cm in the north, and 85 to 95 cm in the south after 1980s. Harvest index increased remarkably, from 40% in 1970s to 45-50% after 1980s. No significant changes in maturity were observed except for northern winter wheat zone. There was increase of biomass in a few areas, which might be attributed to improvement of plant type and nitrogen uptake efficiency, though generally no significant change in biomass for most areas. Most varieties contained single dwarf gene, in which Rht-D1b was the most widely distributed, with a frequency of 30.1%, followed by Rht8c with 19.3%, Rht-B1b with 4.8%, and 8.4% of varieties might have Rht-B1d from St2422/464, Italian variety having pedigree of Saitama27, the donor of Rht-B1d. The frequencies of Rht-B1b plus Rht8c or Rht-D1b plus Rht8c, Rht-B1b plus Rht-D1b, and Rht-B1b,Rht-D1b plus Rht8c were 21.7%, 7.2% and 3.6% respectively. Distributions of different dwarf genes varied in different areas. RhtD1b was most widely used in the north and Rht8c in the south. The varieties without any dwarf gene were usually above 120 cm tall, varieties with Rht8c 100 cm tall and varieties with Rht-D1b or more than one dwarf gene 80-90 cm tall, with a few varieties less than 80 cm in height. The effects of biological conditions and genetic background on plant height were observed. 1BL-1RS translocations were the most widely used crossing parent in breeding program in north winter wheat areas in 1970s and 1980s. The percentage of varieties having 1BL-1RS translocation released after 1980, when the first 1BL-1RS variety was released in China, were 59.0%, 40.0%, 13.0%, and 20.0%, in North Winter Wheat Zone, Yellow and Huai River Reaches, Middle and Low Reaches of Yangtze River and Southwest China or 71.4%, 65.5%, 33.3% and 12.5% of the most widelygrown varieties in those areas respectively. New translocations without secalin were not found. The varieties with high dough elasticity were generally not 1BL/1RS translocations. From breeding point of view, 1BL/1RS parents are generally not recommended in the breeding for high dough elasticity, while good combination of HMW-GS should be present when 1BL/RS parent is used in the breeding for medium dough elasticity. Significant differences in nitrogen uptake, utilization and use efficiencies among varieties were observed. Varieties responded differently, mainly in uptake efficiency, in high and low nitrogen regimes. Close associations were presented between biomass and nitrogen uptake efficiency ( r=0.85, P﹤0.01) and harvest index and nitrogen utilization efficiency ( r=0.85, P﹤0.01). The results indicated that it was feasible to develop variety with both high nitrogen use efficiency and high response to high nitrogen input.