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Published online 22 January 2007
Published in Crop Sci 47:245-253 (2007)
© 2007 Crop Science Society of America
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CROP BREEDING & GENETICS

Genetic Improvement of Grain Yield and Associated Traits in the Northern China Winter Wheat Region from 1960 to 2000

Y. Zhoua, Z. H. Hea,b,*, X. X. Suic, X. C. Xiaa, X. K. Zhanga and G. S. Zhangd

a Crop Science Institute/National Wheat Improvement Center, Chinese Academy of Agricultural Sciences (CAAS), No 12 Zhongguancun South St., Beijing 100081, China
b CIMMYT China Office, C/O CAAS, No 12 Zhongguancun South St., Beijing 100081, China
c Crop Research Institute, Shandong Academy of Agric. Sciences, No 28 Sangyuan Rd., Jinan 250100, Shandong, China
d College of Agronomy, Northwest Sci-Tech Univ. of Agric. and Forestry, Yangling 712100, Shaanxi, China

* Corresponding author (z.he{at}cgiar.org)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Knowledge of changes associated with advances in crop productivity is essential for understanding yield limiting factors and developing strategies for future genetic improvement. The objectives of this study are to understand genetic gain for grain yield and associated traits in the Northern China Winter Wheat Region (NCWWR). Four trials, comprised of 47 leading common wheat (Triticum aestivum L.) cultivars from the NCWWR from 1960 to 2000, were conducted during 2001 to 2003 using a completely randomized block design of three replicates under controlled field environments. Molecular markers were used to detect the presence of dwarfing genes and the 1B/1R translocation. Results showed that average annual genetic gain in grain yield ranged from 32.07 to 72.11 kg ha–1yr–1 or from 0.48 to 1.23% annually in different provinces. The most significant increase in grain yield occurred in the early 1980s, largely because of the successful utilization of dwarfing genes and the 1B/1R translocation. There was no common trend across trials in terms of changes in spikes m–2, kernels per spike, 1000-kernel weight (TKW), or biomass. The genetic improvement in grain yield was primarily attributed to increased grain weight per spike, reduced plant height, and increased harvest index (HI). The dwarfing allele Rht-D1b was the most frequent (68.0%) among the cultivars, followed by Rht 8 (42.0%) and Rht-B1b (16.0%). The frequency of 1B/1R translocation cultivars was 42.6%. The future challenge of wheat breeding in this region is to maintain the genetic gain in grain yield and to improve grain quality, without increasing inputs for the wheat-maize double cropping system.

Abbreviations: HI, harvest index • masl, meters above sea level • NCWWR, Northern China Winter Wheat Region • TKW, 1000-kernel weight • YPT, yield potential trial


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
CHINA is the largest wheat producer in the world, with an annual sown area of around 23 million hectares and production of 94 million megagrams. There are three wheat regions in China that are further divided into 10 agroecological zones with diverse environments and cropping systems. The NCWWR, including the North China Plain Winter Wheat Zone (Zone I, true winter type) and the Yellow and Huai Valleys Winter Wheat Zone (Zone II, facultative or intermediate type), is the most important wheat producing area, and it currently produces 60 to 70% of total wheat production (He et al., 2001). This region is characterized as irrigated with an intensive wheat-maize (Zea mays L.) double cropping management systems and high yield potential. Cultivars that have good quality for bread and noodles are generally grown (He et al., 2002). In addition to breeding for tolerance to low temperatures in winter, high temperatures during grain filling, and resistance to stripe rust (caused by Puccinia striiformis Westend.) and powdery mildew (caused by Erysiphe graminis DC. f. sp. tritici Em. Marchal), the improvement of yield potential has always been a top priority, although industrial quality has recently become very important. Tremendous progress has been achieved in Chinese wheat production, and the national average yield has increased from 0.64 Mg ha–1 in 1950 to 3.90 Mg ha–1 in 2005. This has been largely due to the development of new cultivars, improvements in crop management practices, including irrigation and fertilization, and policy changes after 1980. Six to seven cultivar replacements have occurred in the NCWWR during the 40-yr period; however, no information is available on the genetic improvement of grain yield and its associated traits.

Interest in investigating genetic progress in grain yield started with a report suggesting that a yield plateau was being reached in New York State (Jensen, 1978). Since then, genetic improvement in grain yield has been assessed in most major wheat producing countries such as UK (Austin et al., 1980, 1989), France (Brancourt-Hulmel et al., 2003), Italy (Canevara et al., 1994), USA (Schmidt, 1984; Cox et al., 1988; Donmez et al., 2001), Canada (Hucl and Baker, 1987; McCaig and DePauw, 1995), Australia (Perry and D'Antuono, 1989; Siddique et al., 1989), Argentina (Slafer and Andrade, 1989, 1993; Calderini et al., 1995), Mexico (Waddington et al., 1986; Sayre et al., 1997; Ortiz-Monasterio et al., 1997), and India (Kulshrestha and Jain, 1982). Two approaches were generally used: grain yield data collected over a relatively long period from regional trials including a common check cultivar or the comparison of old and modern cultivars simultaneously grown in specifically designed trials. Genetic gain in grain yield for spring wheat in favorable environments was as high as 1.5% in Mexico from 1950 to 1982 annually (Waddington et al., 1986); however, it ranged from 0.4% from 1830 to 1986 in UK (Austin et al., 1989) to 1.0% from 1920 to 1989 in Argentina for winter wheat (Slafer and Andrade, 1989). Most of these studies showed that the grain yield increase was mainly associated with increased HI and kernel number m–2 (Waddington et al., 1986; Sayre et al., 1997; Brancourt-Hulmel et al., 2003). High harvest index was the result of reduced plant height, whereas high kernel number m–2 was largely due to an increase in kernel number per spike (Slafer and Andrade, 1989) or to an increase in both kernel number per spike and spike number per unit area (Perry and D'Antuono, 1989; Donmez et al., 2001). Most studies also found no change in biomass (Austin et al., 1980; Waddington et al., 1986; Brancourt-Hulmel et al., 2003), although there were exceptions (Perry and D'Antuono, 1989; Siddique et al., 1989). Rajaram and van Ginkel (1996) reported that the utilization of landmark germplasm such as Norin 10 conferring the dwarfing genes Rht-B1b and Rht-D1b and the 1B/1R translocation from Kavkaz were the key factors for the grain yield improvement achieved by the CIMMYT wheat program based in Mexico.

The objective of this study was to assess the genetic gain in grain yield and associated traits from 1960 to 2000 in the NCWWR, the most important wheat region in China. This could provide valuable information for further improving grain yield and determining future strategies for breeding programs in China and elsewhere.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Plant Materials and Experimental Design
During the 2001–2002 and 2002–2003 crop seasons, four yield potential trials (abbreviated as YPT)—YPT1 contained 10 cultivars selected from Beijing, YPT2 had 11 cultivars from Hebei Province, YPT3 had 15 cultivars from Shandong Province, and YPT4 had 11 cultivars from Henan Province—were sown in Beijing (lat. 39°48'N, long. 116°28'E, 31.2 meters above sea level, masl), Shijiazhuang (lat. 38°04'N, long. 114°26'E, 81.8 masl), Jinan (lat. 36°41'N, long. 116°59'E, 51.6 masl), and Zhengzhou (lat. 34°43'N, long.113°39'E, 110.4 masl). Each trial consisted of the leading cultivars from the 1960s to the present in each province. Only cultivars sharing more than 20% of wheat area in the respective province were included in the trials. All leading cultivars in Zone I were released by various breeding programs based in Beijing, and germplasm from Beijing was also widely used by breeding programs in Zone II. Therefore, YPT1 was conducted in Beijing. The provinces of Hebei, Shandong, and Henan share more than 75% of the wheat area in Zone II and generate over 50% of total national wheat production. Breeding programs have been well established and leading cultivars were released in Shijiazhuang, Jinan, and Zhengzhou, respectively. These three locations are considered the representative location for provinces of Hebei, Shandong, and Henan, respectively. Therefore, YPT2, YPT3, and YPT4 effectively represent the breeding advances and wheat production in Zone II. Detailed information on all tested genotypes is given in Table 1.


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  		Table 1. Cultivars tested with year of release, dwarfing gene(s), and status of the 1B/1R translocation.

 
The trials were seeded in a completely randomized block design with three replications. Each plot consisted of six rows of 7.0-m length and 1.4-m width. Seeding rate and field management were determined according to local practices, with about 270 seeds m–2 in Beijing and Shijiazhuang, and 180 seeds m–2 in Jinan and Zhengzhou, respectively. The trials were sown in early October and harvested in early to mid June, depending on the location. The soil types in the four locations are all sandy clay, high in organic matter and slightly alkaline (pH 8.2, 8.0, 8.1, and 8.2, corresponding to each site, respectively). Before planting, ammonium phosphate (about 170 kg ha–1 of P, 67 kg ha–1 of N), urea (about 13 kg ha–1 of N), and potassium chloride (about 75 kg ha–1 of K) were broadcasted and incorporated. An additional 120 kg ha–1 of N was top-dressed at stem elongation (Zadoks Stage 30). Irrigations were provided at the tillering stage (before winter), stem elongation, booting, and grain-filling stages. Nets with a mesh size of approximately 20 x 20 cm, through which the young plants grew, were used to prevent lodging. Fungicides and pesticides were applied at shooting, booting, and grain filling to prevent diseases and pests. Weeds were controlled by hand.

Plant Sampling and Observations
Heading date was recorded as 50% spike emergence from boot. Plant height was measured from the soil surface to the top of the spike (excluding awns) at physiological maturity. After physiological maturity, two 0.5-m-long rows in the middle of each plot (subsamples) were cut with a sickle at the soil level. The remaining four central rows (after cutting 25 cm from each end) were hand harvested, threshed, dried, and weighed to give grain yield in megagrams per hectare at the 140 g kg–1 moisture level. The subsamples were used to record total above-ground biomass, HI, spikes m–2, kernels per spike, kernels m–2, and TKW. No winterkill or cold damage was observed at any location.

Determination of 1B/1R Translocation and Dwarfing Genes
The presence of the 1B/1R translocation was determined by the presence of Glu-B3j (null) allele by SDS-PAGE (Gupta and Shepherd, 1992) and further confirmed by sequence-characterized amplified regions (SCARs) marker analysis on a 1.5% (w/v) agarose gel (Fransis et al., 1995) with the primer combination AF1 (GGA GAC ATC ATG AAA CAT TTG) and AF4 (CTG TTG TTG GGC AGA AAG). Rht 8 gene was identified with SSR marker Xgwm 261 following the method described by Korzun et al. (1998) and further confirmed by pedigree information. Rht-B1b and Rht-D1b alleles were detected on the basis of the report of Ellis et al. (2002) with minor modification, i.e., the forward primer NH-BF.2 (5'-TCTCCTCCCTCCCCACCCCAAC-3') and reverse primer WR1.2 (5'-CCATGGCCATCTCGAGCTGC-3') were used for the detection of wild-type allele (Rht-B1a) at Rht-B1 locus and the combination of primers NH-BF.2 and MR1 (5'-CATCCCCATGGCCATCTCGAGCTA-3') for the detection of mutant (Rht-B1b) at Rht-B1 locus; the primers DF (5'-CGCGCAATTATTGGCCAGAGATAG-3') and MR2 (5'-CCCCATGGCCATCTCGAGCTGCTA-3') were employed for the detection of the allele Rht-D1b at Rht-D1 locus. Norin 10 (Rht-B1b and Rht-D1b), Janz (Rht-B1b), and Kukri (Rht-D1b) were used as controls in each gel.

Statistical Analysis
In each trial, significance of cultivar effects was determined by the analysis of variance with cultivar effects considered to be fixed; year and interactions involving year and replications were considered to be random. The Statistical Analysis System (SAS Institute, Inc., 1997) procedure PROC MIXED was used to carry out the analysis. Fisher's F-protected least significant difference (LSD) method was used to separate cultivars means. Means and standard errors (SE) were determined by PROC MEANS, and phenotypic correlation coefficients were taken from the results of PROC CORR.

SAS PROC REG was used to estimate absolute (grain yield gains in kilograms per hectare per year) or relative (the percentage grain yield gain per year) genetic gain of grain yield and related traits by the following equations: yi = a + bxi + u (1) or ln(yi) = a + bxi + u (2), where yi is the mean grain yield for the three replications in each trial of cultivar i, ln(yi) is the natural log of yi, and xi is the year in which cultivar i was released. The intercept of both equations is estimated by a, while b measures absolute (1) or relative (2) grain yield gains. The residual error is estimated by u (SAS Institute, Inc., 1997; Ortiz-Monasterio et al., 1997).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Analysis of variance indicated that cultivar effects for all traits were significant in all four trials with regression of grain yield on year of release accounting for 41 to 77% of the sums of squares for entries. The regression of harvest index on year of release accounted for 52 to 82% of the sums of squares for entries (data not shown).

Grain Yield
The mean yield of YPT1 was 5.39 Mg ha–1; yield ranged from 3.72 Mg ha–1 for Nongda 139 released in 1968 to 7.01 Mg ha–1 for CA9722 released in 1999 (Table 2). The annual genetic gain in grain yield was 1.23% (R2 = 0.76, P < 0.01) or 64.27 kg ha–1yr–1 (Fig. 1a ). Two groups of cultivars were clearly distinct; Beijing 10, Nongda 139, and Dongfanghong 3 comprised the first group, and the remaining cultivars that were released after 1980 were the second group. There was a significant increase in grain yield with the release of Fengkang 8 in 1980; Fengkang 8 carried the dwarfing gene Rht-D1b and the 1B/1R translocation. Jing 411, Jingdong 6, and Jingdong 8 are derivatives of Fengkang 2 and Fengkang 8. The three of them had early maturity and slightly reduced plant height, but only Jing 411 showed a significant increase in grain yield compared with Fengkang 8. However, CA9722 and Nongda 3291 showed a significant reduction in plant height, and the grain yield of CA9722 was significantly higher than all cultivars.


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  		Table 2. Average grain yield and agronomic traits of tested cultivars in YPT1 in Beijing during 2001–2002 and 2002–2003.

 

Figure 1
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  		Fig. 1. Regression of grain yield on year of release in the four trials. (a) YPT1 in Beijing, (b) YPT2 in Hebei, (c) YPT3 in Shandong, and (d) YPT4 in Henan. * and ** represent significance at the 0.05 and 0.01 probability levels, respectively.

 
The mean grain yield of YPT2 was 6.84 Mg ha–1; yield ranged from 5.97 Mg ha–1 for Jimai 7 released in 1977 to 7.51 Mg ha–1 for both Shi 4185 and Heng 4041, released in 1994 and 1995, respectively (Table 3). The annual genetic gain in grain yield was 0.48% (R2 = 0.41, P < 0.05) or 32.07 kg ha–1yr–1 (Fig. 1b). Two groups of cultivars were clearly evident, viz., Shijiazhuang 54, Jimai 3, and Jimai 7 in the first group, and the remaining cultivars in the second group, and all were released after 1982, and each had the 1B/1R translocation except Jimai 38. Shijiazhuang 54 was released in 1964 and yielded significantly higher than both Jimai 3 and Jimai 7. Shijiazhuang 54 was replaced largely because of its high susceptibility to stripe rust. Heng 4041 and Shi 4185, the two current leading cultivars, showed a significant increase in grain yield over Shijiazhuang 54, indicating that Shijiazhuang 54, Heng 4041, and Shi 4185 were the three landmark cultivars for grain yield improvement in Hebei.


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  		Table 3. Average grain yield and agronomic traits of tested cultivars in YPT2 in Hebei Province during 2001–2002 and 2002–2003.

 
The mean grain yield of YPT3 was 6.55 Mg ha–1; yield ranged from 5.60 Mg ha–1 for Taishan 4 released in 1969 to 7.40 Mg ha–1 for Lumai 7 released in 1981 (Table 4). The annual genetic gain in grain yield was 0.48% (R2 = 0.42, P < 0.01) or 32.09 kg ha–1yr–1 or (Fig. 1c). Taishan 1 was the most widely grown cultivar in the NCWWR in the 1970s. Jinan 13, released in 1977, showed a significantly higher yield than all cultivars released before 1980. Lumai 7, released in 1981, showed a significant higher yield than Lumai 14, but it was not significantly different from the cultivars released after 1987. Five cultivars including Lumai 1, Lumai 7, Lumai 14, Jinan 16, and 95(6) 161 carry the 1B/1R translocation. Jinan 17 and Jimai 19, the current leading cultivars, have good quality for pan bread and Chinese noodles, respectively.


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  		Table 4. Average grain yield and agronomic traits of tested cultivars in YPT3 in Shandong Province during 2001–2002 and 2002–2003.

 
The mean grain yield of YPT4 was 7.02 Mg ha–1; yield ranged from 5.99 Mg ha–1 for Boai 7023 released in 1970 to 7.77 Mg ha–1 for Yumai 34 released in 1993 (Table 5). The annual genetic gain in grain yield was 1.05% (R2 = 0.77, P < 0.01) or 72.11 kg ha–1 yr–1 (Fig. 1d). Boai 7023 was a reselection of the Italian cultivar Funo, and Zhengyin 1 was an introduction from Italy. Yumai 2, released in 1979, gave a significant increase in grain yield over the earlier cultivars. It was characterized by closed plant architecture, short and erect leaves, high spike fertility, fast grain filling, and greater leaf longevity. It was also considered an ideotype cultivar for the high input conditions in the Yellow and Huai Valleys. Yumai 18, characterized by early maturity, short stature, rapid grain filling, tolerance to diseases, and broad adaptability, ranked first in sowing area in China for more than 7 yr after 1995. Yumai 34 was a landmark cultivar since it combined high yield potential, broad adaptation, and excellent quality characteristics for pan bread and Chinese noodles.


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  		Table 5. Average grain yield and agronomic traits of tested cultivars in YPT4 in Henan Province during 2001–2002 and 2002–2003.

 
The above genetic gains in grain yield were achieved without extending the cropping season. Significant advances in earlier heading date were observed in YPT1 and YTP4 with annual genetic gain of –0.12% (P < 0.01) and –0.22% (P < 0.05), respectively.

Yield Components
On average, spikes m–2 decreased from YPT1 (741) in Beijing, to YPT2 (643) in Shijiazhuang, YPT3 (568) in Jinan, and YPT4 (591) in Zhengzhou, whereas kernels per spike, TKW, and kernel weight per spike increased from YPT1, YPT2, and YPT3 to YPT4. However, notable variation for grain yield components was found in all four trials. For example, spikes m–2, kernels per spike, and TKW of four cultivars released after 1990 in Beijing ranged from 651 (Jingdong 8) to 733 g (Nongda 3219), 19.1 (Jingdong 6) to 26.7 g (CA9722), and 43.6 (Jingdong 6) to 37.6 g (CA9722).

There was no clear common trend across the four trials in terms of changes for spikes m–2, kernels per spike, and TKW, with the exception of kernel weight per spike. YPT1 in Beijing was characterized by a significant increase in TKW (1.30%, P < 0.01), decreased spikes m–2 (–0.65%, P < 0.05), and increased kernels spike–1 (0.60%, P > 0.05). YPT2 in Hebei was characterized by a significant increase in kernels per spike (0.99%, P < 0.01), reduction of spike m–2 (–0.79%, P < 0.05), and a slight increase in TKW (0.06%, P > 0.05). YPT3 in Shandong was characterized by significantly reduced spikes m–2 (–0.74%, P < 0.05), increased kernels per spike (0.54%, P < 0.05), and slightly increased TKW (0.35%, P > 0.05). YPT4 in Henan was only characterized by significantly increased spikes m–2 (0.59%, P < 0.05), increased TKW (0.51%, P > 0.05), and slightly decreased kernels per spike (–0.10%, P > 0.05). However, increases in kernel weight per spike were observed in all four trials, i.e., YPT1 (1.79%, P < 0.01), YPT2 (1.00%, P < 0.01), YPT3 (0.78%, P < 0.01), and YPT4 (0.54%, P = 0.18). Kernels m–2 was slightly increased in all four trials (0.05–0.47%, P > 0.05, data not shown).

The simultaneous increase of TKW and kernel number spike–1 observed in YPT1, YPT2, and YPT3, or the shift of negative relationships between them observed in YPT4, resulted in an increased kernel weight spike–1, and thus led to the increased grain yield. An increase in the number of kernels per spikelet, rather than the numbers of spikelets, seemed on the basis of observations in YPT3 (data not shown) to play a more important role in increasing kernels spike–1. This is further supported by the significant positive correlations between grain yield and kernel weight per spike, ranging from 0.73 to 0.95 (P < 0.05) observed in YPT1, YPT2, YPT3, and YPT4 (data not shown).

Increased grain yield can also be achieved in different ways in the same location. For example, Lumai 7 in YPT3 was characterized by average number of spikes m–2 (553), increased kernels spike–1 (36.0), and relatively low TKW (40.8 g), whereas Jimai 19 had a lower number of spikes m–2 (484), but greater kernels spike–1 (38.8) and TKW (45.4 g). Lumai 22 is called a large-spiked wheat cultivar by farmers and is yet another example. It had less spikes m–2 (399), but more kernels per spike (45.0) and large TKW (47.9 g), i.e., large and heavy spike (1.81 g); although, it is generally believed that a certain number of spikes is needed for stable yield performance across environments (Zhuang, 2003).

Plant Height, Biomass, and Harvest Index
Plant height was significantly reduced during the 40-yr period across four trials, i.e., YPT1 (–0.83%, P < 0.01), YPT2 (–1.33%, P < 0.01), YPT3 (–0.52%, P < 0.05), and YPT4 (–1.51%, P < 0.01). A negative association between grain yield and plant height was observed in all four trials: YPT1 (–0.76, P < 0.05), YPT2 (–0.55, P > 0.05), YPT3 (–0.29, P > 0.05), and YPT4 (–0.83, P < 0.01). All cultivars released after the 1980s in YPT2, YPT3, and YPT4 are 75 to 85 cm tall, but the leading cultivars such as Jingdong 8 in Zone I are still relatively tall (103 cm). The new cultivars, such as CA 9722 and Nongda 3291 with heights of 85 cm, are not widely cultivated.

Harvest index significantly increased over the 40-yr period across four trials, i.e., YPT1 (0.92%, P < 0.01), YPT2 (0.42%, P < 0.01), YPT3 (0.46%, P < 0.01), and YPT4 (0.50%, P < 0.05). For example, HI increased from 0.28 to 0.44 in YPT1, while it increased from 0.42 to 0.50 in YPT4. Significantly positive associations between grain yield and HI were observed across trials; these ranged from 0.67 to 0.95 (P < 0.05). This gave an indication that further increases in HI may continue to contribute to grain yield improvement.

Only slight changes in biomass were observed in YPT1, YPT2, and YPT3, but YPT4 was an exception. Significantly positive correlations between grain yield and biomass were observed in YPT1, YPT2, YPT3, and YPT4, these ranged from 0.70 to 0.79 (P < 0.05). This indicates the importance of improving, or at least maintaining, current biomass in future cultivar development.

Contributions of Dwarfing Genes and 1B/1R Translocation
The improvement of grain yield and HI, due mainly to a reduction in plant height (r between HI and plant height ranged from –0.71 to –0.82, P < 0.01), are closely associated with incorporation of dwarfing genes into the leading cultivars. As shown in Table 6, Rht-D1b was the predominant dwarfing gene (68.0%), followed by Rht 8 (42.0%) and Rht-B1b (16.0%). However, substantial variation was observed in the different provinces; for example, Rht-B1b was not found in YPT3 in Shandong, whereas Rht-B1b was relatively high in YPT2 in Hebei. This is primarily due to the specific germplasm used in the different institutes. It is also assumed that other major dwarfing genes are present in Henong 326 (71 cm), Lumai 14 (81 cm), and Yannong 15 (75 cm) since Rht-B1b or Rht-D1b was not identified in them. As shown in Tables 2GoGo to 5, genotypes with Rht 8 alone had plant heights from 93 to 122 cm. Plant heights ranging from 71 to 112 cm were achieved with the integration of Rht-D1b, Rht-B1b, or with combinations involving Rht 8. Other minor dwarfing genes are probably present in the tested genotypes since notable variations in plant height among entries with the same genotype occurred in all four trials.


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  		Table 6. Frequency (%) of Rht-B1b, Rht-D1b and Rht 8 in four Provinces.

 
From the late 1970s to the early 1990s, wheat breeding in the NCWWR focused on the utilization of the 1B/1R translocation from introductions such as Lovrin 10, Lovrin 13, and Neuzucht, which showed resistance to rusts, powdery mildew, and heat stress after anthesis (He et al., 2001). Fengkang 8 in Beijing, Jimai 24 in Hebei, Lumai 7 in Shandong, and Yumai 13 in Henan all have 1B/1R and were developed in 1980, 1982, 1981, and 1987, respectively. The current leading cultivars in Beijing and Hebei continue to have 1B/1R. The low frequencies of 1B/1R in Shandong and Henan are largely due to the intensive selection for processing quality; thus, good quality cultivars such as Jinan 17, Jimai 19, and Yumai 34, which lack 1B/1R, are the primary cultivars being grown.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The improvement in grain yield has always been the top priority for Chinese wheat breeding programs because of China's growing population. The present study indicated that annual genetic gain in grain yield ranged from 0.48% in Shandong and Hebei Provinces to 1.23% in Beijing, with an average of 0.81%. During the same period, average crop yields have increased more than 5% annually in all four provinces, and national yield has increased more than 4% per year (unpublished data from our lab). Crop management must play an important role in increasing wheat production. Increased grain yield was achieved without extending the cropping season. In fact, the combination of tolerance to late sowing and early maturity has been a major factor for the wide adoption of the current wheat-maize double cropping system (Zhuang, 2003). This study showed that the significant progress in genetic improvement in grain yield achieved in the NCWWR was comparable to achievements in other countries (Austin et al., 1980, 1989; Cox et al., 1988; Donmez et al., 2001; Brancourt-Hulmel et al., 2003; Waddington et al., 1986; Sayre et al., 1997). It is interesting to observe that during the same period, both the utilization of dwarfing genes and 1B/1R translocation were the key factors for grain yield improvement in the NCWWR and CIMMYT spring wheat breeding program in Mexico (Rajaram and van Ginkel, 1996) even though they were operated separately and dealt with different types of wheat (winter and spring, respectively). Our study also indicated that the combination of high grain yield and excellent industrial quality was possible, as exemplified by Jimai 19 and Yumai 34. However, Chinese wheat breeding faces great challenges in the future. Demand for high grain yield is continuing because of reduced wheat growing areas and the still increasing population. In addition to powdery mildew and yellow rust, Fusarium head blight [caused by Gibberella zeae (Schwein.) Petch] is now endemic to the NCWWR, and sharp eye spot (caused by Rhizoctonia cerealis Van der Hoeven) and take-all [caused by Gaeumannomyces graminis (Sacc.) Arx & D. Olivier var. tritici J. Walker] are also present. Water resources in China are sharply declining, and the expense of wheat production must remain competitive with that of other high-value cash crops that increasingly compete with wheat. Quality has become more and more important. Continued genetic gains in grain yield must, therefore, be combined with improved processing quality, good resistance to various diseases, and better resource management (Zhuang, 2003).

There was no clear trend across trials in terms of changes in spikes m–2, kernels per spike, TKW, and biomass. For example, as grain yield increases, spikes m–2 decreases in YPT1, YPT2, and YPT3, but increases in YPT4. This may reflect different breeding strategies used in different provinces. For Beijing, Hebei, and Shandong, locally developed cultivars were characterized as having a large number of spikes, and reduction of spike number should reduce lodging. However, in Henan Province, the leading cultivars were introduced from Italy and had fewer spikes; lodging was not a serious problem, and thus an increase in spike number would benefit yield in this province. The significant genetic improvement in grain yield was primarily attributed to increased kernel weight per spike, reduced plant height, and increased HI. The direct contribution of kernel weight per spike to grain yield progress has not been widely reported because most studies found a direct contribution of increased kernel number m–2 to grain yield (Waddington et al., 1986; Sayre et al., 1997; Brancourt-Hulmel et al., 2003). Increased grains/spike weight ratios rather than increased partitioning to the spikes has also been documented (Abbate et al., 1998). The importance of improving TKW in YPT1 is also worth mentioning since it is not commonly reported. Large kernel size is generally preferred in China, and it is associated with rapid grain filling under high temperatures, which are very common in north China after anthesis. Therefore, TKW is an important selection criterion for the Chinese wheat breeding programs. The increase in TKW could also be due to the use of the 1B/1R translocation, which could contribute to tolerance to high temperatures during grain filling (personal communication, Q.S. Zhuang). A positive effect of 1B/1R on TKW was also reported by Moreno-Sevilla et al. (1995). Considering the limited potential to further increase TKW in Chinese environments and the trends of reducing spike number per unit area, it is generally believed that further increases of kernels per spike (even 2–3 kernels) could offer an opportunity for increasing grain yield potential. Experience in developing new cultivars such as CA 9722 and CA0175 in Beijing has supported this approach.

The slight change in biomass, significant reduction in plant height, and increase in harvest index in Chinese winter wheats are in agreement with other reports (Cox et al., 1988; Brancourt-Hulmel et al., 2003; Sayre et al., 1997).

The significant increase in grain yield mainly occurred in the early 1980s, largely because of the successful utilization of dwarfing genes and the incorporation of the 1B/1R translocation. Breeding semidwarf wheat in China started in the late 1950s. Although in Youbaomai, the first improved semidwarf cultivar was released in 1964, premature dying was a major constraint, and progress was slow until the 1980s. All leading cultivars now have plant heights around 80 cm. Chinese experience also indicates that breeding for semidwarf was more difficult in Zone I compared with Zone II. This might be due to the short grain-filling period (30–35 d), rapid temperature rise after anthesis, and poor tillering ability of semidwarf cultivars or lines (Zhuang, 2003). The release of Nongda 3291, CA9722, and Lunxuan 987 with plant heights around 85 cm, gives an indication of the progress in reducing of plant height in Zone I during the last few years. However, it is very unlikely that further reduction in plant height in the NCWWR will benefit yield progress. It is generally believed on the basis of the experience of Chinese wheat breeders (Zhuang, 2003) that the optimum plant height is around 80 cm in Zone II.

Our study indicated that Rht-D1b was the most frequent dwarfing gene (68.0%), followed by Rht 8 (42.0%) and Rht-B1b (16.0%). In Chinese wheats Rht-B1b was derived from two sources, Norin 10 and the Italian germplasm St2422/464 with Rht-B1b and Rht 8. Suwon 86, carrying both Rht-B1b and Rht-D1b, and Chinese cultivars Huixianhong from Henan and Youbaomai from Shandong are primary sources of Rht-D1b in Chinese wheats (Zhang et al., 2006). Huixianhong and St 2422/464 were widely used in Henan, as was Youbaomai in Shandong (Zhuang, 2003). Most current leading cultivars in the NCWWR have plant heights around 75 to 85 cm, suggesting that combinations of Rht-B1b or Rht-D1b with Rht 8 confer optimal plant height for this region.

As already indicated, significant yield gains were achieved with the release of cultivars carrying the 1B/1R translocation in the early 1980s. In the early 1970s, resistance to yellow rust was the major breeding objective, largely because of the frequent breakdown of resistance of cultivars. Therefore, Lovrin 10, Lovrin 13, and Neuzucht with the 1B/1R translocation were primarily used as resistance donors in breeding, although their agronomic traits were also acceptable. The excellent yield performance and tolerance to high temperature at late growth stages were observed in both the early generations and yield trial stages. Therefore, it is believed that the 1B/1R translocation played a major role in the improvement of grain yield in China in the early 1980s, although this cannot be confirmed with the data from the present study, probably because of the small number of entries used. This is in agreement with CIMMYT experience on the release of Veery ‘S’ (Rajaram and van Ginkel, 1996). Although no consistent results were reported regarding the effects of 1B/1R on grain yield (Villareal et al., 1991; Carver and Rayburn, 1994), its positive contributions to broad adaptation has been confirmed (Graybosch, 2001). At present, 1B/1R remains frequently present in cultivars and advanced lines in Zone I and Hebei Province, but its negative effects on pan bread and Chinese noodle quality need to be considered in the breeding programs (He et al., 2005).


   ACKNOWLEDGMENTS
 
The authors are grateful to Prof. R. A. McIntosh and Prof. Edward Souza for kindly reviewing this manuscript. This project was supported by the Ministry of Agriculture (2003-Q01 and 05-02-01A).

Received for publication March 16, 2006.


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TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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