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Crop Science 41:670-676 (2001)
© 2001 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Agronomic Potential of Synthetic Hexaploid Wheat-Derived Populations

I.A. del Blanco*a, S. Rajaramb and W.E. Kronstada

a Dep. of Crop and Soil Science, Oregon State Univ., Corvallis, OR 97331
b CIMMYT, Apdo. Postal 6-641. 06600 Mexico DF, Mexico

* Corresponding author (isabel_delblanco{at}ndsu.nodak.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Wild relatives of wheat (Triticum aestivum L.) have primarily been used as sources of genes for biotic and abiotic stress resistance. A more direct approach to grain yield improvement has been sought by using exotic germplasm to enhance quantitative traits, including grain yield. This study was conducted to determine whether synthetic hexaploids, developed from artificial hybridization of durum wheat [Triticum turgidum ssp. durum (Desf.) Husn.] with Aegilops tauschii Coss., can enhance yield or yield components of wheat. Two hundred eighty-two BC2F2-derived lines, involving six crosses between different synthetic hexaploids and four spring wheat cultivars, were evaluated for grain yield and its components. These synthetic-derived lines were compared with their recurrent parent in field experiments conducted during the 1995 to 1996 and 1996 to 1997 crop seasons near Ciudad Obregón, Sonora, Mexico. More than 80% of the synthetic-derived lines were significantly superior to their recurrent parent for kernel weight. Eight lines had significantly higher grain yield compared with their recurrent parent. Grain yields of superior lines were up to 11% higher than those of their recurrent parents. A strong association between grains m-2, biomass, spikes m-2, and grain and biomass production rates and grain yield was observed in all populations. Path coefficient analyses of yield components revealed a strong direct effect of spikes m-2 and grains spike-1 on grain yield. Results indicate that synthetic hexaploids can be a valuable source of alleles to improve kernel weight.

Abbreviations: CIMMYT, International Maize and Wheat Improvement Center • BPR, biomass production rate per day • GPR, grain production rate per day • HI, harvest index • TKW, thousand kernel weight


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
MODERN HEXAPLOID WHEAT originated from a series of natural hybridizations between the wild diploids (2n = 2x = 14) Triticum urartu Tumanian ex Gandilyan, A genome (Chapman et al., 1976; Dvorak et al., 1988); a species closely related to Aegilops speltoides Tausch., B genome; and A. tauschii Coss., D genome (Zohary et al., 1969). The addition of the D genome had a major impact on domestication, adaptation, and end-product quality of wheat (Zohary et al., 1969).

The narrow genetic base of wheat is a concern in coping with evolving disease and insect pressures and variations in environmental stresses. However, crosses with unproductive wild relatives or primitive ancestors are not common in a wheat improvement program, because they do not result immediately in highly productive cultivars. For this reason, distantly related germplasm has been considered almost exclusively as sources of improved resistance to diseases, insects, and environmental stresses.

Synthetic hexaploid wheat, produced by crossing durum wheat (2n = 4x = 28, AB genomes) with A. tauschii Coss. (2n = 2x = 14, D genome), has been used as an intermediary for transferring resistance genes from the wild ancestor to cultivated wheat. Synthetic hexaploids have been reported as having resistance to diseases such as karnal bunt, caused by Tilletia indica Mitra (Multani et al., 1988; Villareal et al., 1994a, 1996); leaf rust, Puccinia triticina Eriks. (Kerber and Dyck, 1969; Kerber, 1987); tan spot, Pyrenophora tritici-repentis (Died.) Drechs. (Siedler et al., 1994; Riede et al., 1996); spot blotch, Cochliobulus sativus (Ito et Kuribayashi) Drechs. ex Dastur (Mujeeb-Kazi et al., 1996; Mujeeb-Kazi and Delgado, 1998); and stripe rust, Puccinia striiformis Westend. (Ma et al., 1995). Synthetic hexaploids were also found to be tolerant to abiotic stresses such as cold temperatures and saline soils (Gorham, 1990; Limin and Fowler, 1993).

An important factor when considering this new genetic resource is whether it may offer beneficial alleles for increased yield potential. Considering that yield is a highly complex trait, it is unlikely that the best alleles for yield-related loci have been completely captured from the wild ancestors. Tanksley et al. (1996) reported the transfer of valuable alleles for quantitative traits from a wild species [Lycopersicon pimpinellifolium (L.) Mill.] to elite lines of tomato (Lycopersicon esculentum Mill.). Despite its inferior adaptation and appearance, the wild relative contributed alleles capable of enhancing important quantitative traits such as fruit size and shape. In oat (Avena sativa L.), the introduction of germplasm from related species such as A. byzantina C. Koch and A. sterilis L. resulted in grain yield improvement of cultivated oats (Lawrence and Frey, 1975; Langer et al., 1978; Rodgers et al., 1983).

Studies on the effect of A. tauschii introgression on important agronomic traits of wheat (Cox et al., 1995a, 1995b; Murphy et al., 1997) have shown that some BC2F2-derived lines from direct hybrids between wheat and A. tauschii had superior grain quality traits to those of their recurrent parents. Synthetic hexaploids have shown substantial variability for seedling vigor, straw strength, plant height, phenological cycle, grain characteristics, and also for grain yield and its components (Villareal et al., 1994b, 1994c).

The objective of this study was to determine the potential of synthetic hexaploids as a source of favorable alleles for grain yield and yield components in hexaploid spring wheat. The associations among yield components and grain yield were determined in order to define future breeding objectives with this germplasm.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A 2 yr experiment was conducted at the Agricultural Research Center for the Northwest (INIFAP) Experimental Station, Yaqui Valley, Sonora, Mexico. The Yaqui Valley is 40 m above sea level, and between 26°45' and 27°33' Latitude North and 109°30' and 110°37' Longitude West. The climate of this region is semiarid with an average annual rainfall of 266 mm distributed mainly during the summer months.

Plant Material
Six populations derived from crosses between synthetic hexaploids and common spring wheat cultivars were obtained from the International Maize and Wheat Improvement Center (CIMMYT) through the courtesy of R. Villareal. A. Mujeeb-Kazi produced the original synthetics (Mujeeb-Kazi et al., 1996). The BC2F1 populations were harvested in bulk and space-planted as BC2F2 populations. From the BC2F2 to BC2F6 generations the six populations were alternatively grown as BC2F2-derived lines near C. Obregon and El Batan, Mexico. They underwent selection to eliminate genotypes with major agronomic defects, such as non-free threshability or extreme types for maturity and plant height. They were not selected for grain yield or for yield components. Each line was harvested in bulk and space planted in each generation.

The six populations involved three durum wheat cultivars, four A. tauschii accessions, and four hexaploid wheat cultivars. Their pedigrees were: ‘Altar 84'/A. tauschii (219)//2*'Esmeralda’ (Population 1), Altar 84/A. tauschii (223)//2*'Flycatcher' (Population 2), ‘Duergand 2'/A. tauschii (214)//2*'Seri' (Population 3), Duergand 2/A. tauschii (214)//2*'Opata' (Population 4), Duergand 2/A. tauschii (214)//2*Esmeralda (Population 5), and ‘Croc 1'/A. tauschii (205)//2*Opata (Population 6). Forty-seven BC2F2:6 lines from each population were included in the experiments.

Experimental Design and Growing Conditions
The lines from the six populations were evaluated in experiments arranged in six adjacent randomized complete block designs with three replications. The recurrent parent (hexaploid) was entered as a check in each population. The sowing period was late November for both years. Seeding rate was {approx}90 kg ha-1. The soil type at this location is coarse sandy clay, mixed montmorillonitic, typic Calciorthid (Soil Survey Staff, 1975), low in organic matter with a pH of 7.7. Plots consisted of six rows, 3 m long and 0.20 m between rows. The plots were fertilized with 150 kg ha-1 N and 40 kg ha-1 P prior to planting. Six irrigations, from late November to early April, ensured adequate water availability. Preventive chemical control of weeds, diseases, and insects was applied as required. Pesticides used were clodinafop {(R)-2-[4-(5-chloro-3-fluoro-2-pyridyloxy)phenoxy]propionic acid}, bromoxinil (3,5-dibromo-4-hydroxybenzonitrile), and fluroxypyr {[(4-amino-3, 5-dichloro-6-fluoro-2-pyridinyl)oxy] acetic acid} to control weeds; tebuconazole {{alpha}-[2-(4 chlorophenyl)-ethyl]-{alpha}-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol}, and propiconazole {1-[[2-(2,4-dichlorophenyl) -4-propyl-1,3-dioxolan-2-yl] methyl]-1H,1,2,4-triazole} twice during the crop cycle to prevent diseases; methamidophos (O,S-dimethyl phosphoramidothioate), and chlorpyrifos [O,O-diethyl O-(3,5,6-trichloro-2-pyridinyl) phosphorothioate] to control insects. All pesticides were applied at recommended doses. Neither biotic nor abiotic factors had observable effects on yield in either year.

Collection of Data
Days to heading was the number of days from sowing to 50% spike emergence. Days to physiological maturity was the number of days from sowing to when 50% of the peduncles turned yellow. Plant height was the average distance from the soil surface to spike tips, excluding awns. Grain yield was obtained from each plot and adjusted to 12% moisture. A grain sample of {approx}100 g was taken from each plot, weighed, and oven-dried at 70°C for 48 h to determine grain moisture content. Thousand kernel weight (TKW) was estimated from a sample of 250 random kernels from the dried grain sample.

Yield components were estimated as described by Sayre et al. (1997) from a subsample of 50 fertile spikes from each plot. Spikes were sampled at random at physiological maturity, and oven-dried for 48 h at 70°C. The dry sample was weighed, threshed, and the resulting grain weight recorded.

Biomass production rate was calculated as aboveground biomass divided by days from seedling emergence to physiological maturity. Grain production rate was computed as grain yield divided by grain-fill duration. Grain-fill duration was the number of days from heading to physiological maturity.

Statistical Analysis
Separate analyses of variance were performed for each of the six populations. All factors were considered random. The combined (2 yr) analysis of variance showed a significant (P<=0.05) line x year interaction for grain yield and agronomic traits. Therefore, line main effects were tested against the line x year interaction mean square, which was used to calculate Fisher's protected LSD. The distribution of the BC2F2:6 line means around their recurrent parent mean was expressed in LSD units for each trait, based on a score (Cox et al., 1995a, 1995b) as follows:

A line with a score >1 or <-1 was significantly different from its recurrent parent.

Associations among traits were calculated using the Statistical Analysis System for Windows, version 6.12 (SAS Institute Inc., 1993). The CORR procedure was used to estimate phenotypic (Pearson) correlations for line means. Correlation coefficients between yield and kernel weight, biomass, spikes m-2, and grains spike-1 were partitioned into direct and indirect effects by path analysis. Path coefficient analyses were conducted as described by Dewey and Lu (1959) and Singh and Chaudhary (1977). Path coefficients are estimated by standardized partial regression analysis, which indicates the influence of a variable upon another variable, while adjusting for multicolinearity. Biomass was included in the path analysis to better understand its influence on yield through various yield components.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Distribution of Lines
Kernel weight and plant height were the only traits where mean values of the BC2F2:6 lines exceeded the recurrent parent values (Table 1). Population means were 16% and 4%, greater than recurrent parent means for TKW and plant height, respectively. The largest difference in TKW means occurred in populations where Opata was the recurrent parent, which was the lowest in TKW among the recurrent parents. Long-term wheat yield progress is often attributed to an increase in grains m-2 (Waddington et al., 1986; Cox et al., 1988; Ledent and Stoy, 1988; Austin et al., 1989; Perry et al., 1989; Sayre et al., 1997), while TKW has remained unchanged or even reduced (Siddique et al., 1989; Slafer and Andrade, 1989; Sayre et al., 1997). Thus, this germplasm may be a source of alleles for increased kernel weight, which may enhance the yield of wheat.


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  		Table 1. Means, ranges, Fisher's LSD, and coefficients of variation (C.V.%) for agronomic traits of 282 BC2F2:6 lines in six synthetic hexaploid-derived wheat populations and the recurrent parents grown near Ciudad Obregón, Sonora, during 1995–1996 and 1996–1997.

 
Population means for grain yield averaged {approx}10% lower than recurrent-parent means (Table 1). Population means for harvest index (HI) and biomass were lower than recurrent parent means by 6% and 4%, respectively. Increased plant height did not provide greater aboveground biomass. Most population means for biomass were lower than recurrent parent means.

Population means were lower than corresponding recurrent parent means for spikes m-2 (11%), grains m-2 (27%), and grains spike-1 (15%) (Table 1). Population means for days to heading and physiological maturity were similar to recurrent parent means. Growth rates of synthetic-derived lines were lower than those of recurrent parents, averaging 11% less for grain production rate per day (GPR), and 4% less for biomass production rate per day (BPR). These lower growth rates were expected from populations having lower grain and biomass yields but similar phenological periods compared with their corresponding recurrent parents.

Classification of lines based on mean score (Table 2) illustrates the number of lines falling within 0, 1, 2, and 3 LSD values of their recurrent-parent mean. Thirty-nine percent of the BC2F2:6 lines were within one LSD value of their recurrent parents across all traits. Individual trait scores were positively skewed for TKW with 83% of the lines superior to their recurrent parent. The distributions were negatively skewed for grain yield and other yield components. Phenological traits, such as days to heading and maturity, were slightly skewed towards earlier development. Plant height was positively skewed, indicating that the majority of synthetic-derived lines were taller than their recurrent parents. Grain and biomass production rates were negatively skewed with {approx}42% of the lines having equal GPR as their recurrent parent, but only 5% were superior. Sixty-eight percent of the lines had the same BPR compared with the recurrent parent, but only 7% were superior.


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  		Table 2. Distribution of 47 BC2 F2.6-lines relative to their recurrent parents for grain yield and agronomic traits in six synthetic hexaploid-derived wheat populations grown near Ciudad Obregón, Sonora, during 1995 to 1996 and 1996 to 1997.

 
Characteristics of High-Yielding Lines
Eight synthetic-derived BC2F2:6 lines with higher grain yield than their recurrent parent were observed in populations 1, 2, 3, and 6 (Table 3). Seven lines had higher TKW compared with the recurrent parent. These lines had higher kernel weight without being the highest lines for that trait, and they were able to maintain acceptable levels of biomass and grains m-2. Five of the eight lines were taller, and three had higher biomass than the recurrent parent. Harvest index for the superior lines was equal to the recurrent parent, suggesting that the increased yield of these lines was proportional to their increased biomass and plant height. Grains m-2 and grains spike-1 were equal or lower than those of the recurrent parent. The highest-yielding lines in Population 3 had a shorter phenological period compared with their recurrent parent, Seri. In other populations, phenological periods of superior lines varied relative to their recurrent parents. Five lines had similar number of spikes m-2, one had higher, and two had lower compared with the recurrent parent. Six lines had equal, and two higher, GPR; four lines had higher BPR compared with the recurrent parents.


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  		Table 3. Agronomic traits of synthetic-derived wheat lines, with grain yield one Fisher's LSD value higher than the recurrent parent, grown near Ciudad Obregón, Sonora, during 1995 to 1996 and 1996 to 1997.

 
Cox et al (1995a)(1995b), using backcross populations derived from direct hybrids between wheat and A. tauschii, found some lines with grain yield and TKW superior to the recurrent parent. However, disease infection in those studies might have had some confounding effects on agronomic measurements. Villareal et al (1994c) found some outstanding genotypes for grain yield, biomass, TKW, and spikes m-2 among 50 synthetic hexaploids, compared with the cultivar Seri 82. The highest yielding genotype was Duergand/A. tauschii 214, the synthetic parent for populations 3, 4, and 5 in this study.

Associations among Traits
Biomass, grains m-2, and rates of biomass and grain production had the highest positive correlations with grain yield in all six populations (Table 4). Spikes m-2 was significantly correlated with yield for all populations. Harvest index was correlated with grain yield in five populations. Four of six associations between grain yield and TKW were positive and significant, although correlation coefficients were less than 0.56. Lines with the highest TKW were not the highest yielding lines, even though highest yielding lines had TKW superior to those of the recurrent parents. Grains spike-1 was positively correlated with grain yield in three populations. Heading and maturity dates tended to be negatively associated with grain yield; therefore longer cycle genotypes had lower yield. Temperatures were very high at the end of the cropping season; hence, long-cycle lines did not have time for adequate grain filling. Plant height had a positive association with grain yield in four populations. Straw strength of synthetic-derived lines was similar to that of common wheats, with lodging ratings near zero. Only a few lines suffered considerable lodging, most of them in Population 5, and some in Population 2 (data not shown).


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  		Table 4. Pearson phenotypic correlations for grain yield and kernel weight (TKW) with harvest index (HI), biomass, spikes m-2, grains m-2, grains spike-1, days to heading (Heading), days to maturity (Maturity), plant height (Height), grain production rate (GPR), and biomass production rate (BPR) of 47 BC2F2:6 lines in six synthetic-derived wheat populations grown near Ciudad Obregón, Sonora, during 1995 to 1996 and 1996 to 1997.

 
The correlation between TKW and HI varied among populations from –0.26 to 0.49. Significant associations of TKW with biomass and BPR were observed for four populations. A negative or zero association was observed for TKW with grains m-2, spikes m-2, and grains spike-1. This was expected, since these traits represent competitive sinks. Kernel weight was lower in longer cycle genotypes that suffered late season heat stress. Grain production rate had low, or no association, with TKW.

Genetic gains in grain yield may be achieved in the future by increasing TKW while maintaining or, if possible, augmenting number of grains m-2. It is accepted that these two yield components are negatively associated; however, their phasic developments overlap minimally (Slafer et al., 1996). Therefore, the common explanation that a higher number of grains m-2 reduces assimilate availability per individual grain may be true if source limitations occur during grain filling. Yet, wheat, particularly under optimum conditions, has been reported to be sink limited (Borojevic, 1979; Thorne et al., 1979; Borghi et al., 1986; Bindraban, 1997). In the case of barley, TKW is higher than in wheat even when number of grains m-2 is the same and the duration of grain filling is shorter (Richards, 1996). Synthetic hexaploids could offer a valuable source of alleles for increased kernel weight.

Path-Coefficient Analysis
A path coefficient analysis was performed to identify direct and indirect effects of biomass and yield components on grain yield (Table 5). The direct effect of kernel weight on grain yield was higher than suggested by the correlation between these two traits in all populations. Some negative indirect effects through spikes m-2 and grains spike-1 account for the difference. Biomass had no direct effect on grain yield, but it had a positive indirect effect through spikes m-2 and kernel weight. Spikes m-2 had a strong direct effect on grain yield, though it also had a negative indirect effect on kernel weight and grains spike-1, which reduced the final values of the correlation. Grains spike-1 also had a strong positive direct effect on grain yield. Nevertheless, the negative indirect effects on kernel weight and spikes m-2 greatly decreased the final values of the correlation between these traits.


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  		Table 5. Direct and indirect effects of kernel weight, biomass, spikes m-2, and grains spike-1 on grain yield of BC2F2:6 lines in six synthetic-derived wheat populations grown near Ciudad Obregón, Sonora, during 1995 to 1996 and 1996 to 1997.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Introgression of alleles from synthetic hexaploids into wheat did modify the expression of quantitative traits such as grain yield and its components. Eight synthetic-derived lines had significantly higher grain yield than their recurrent parents, suggesting that introgression of germplasm from A. tauschii, durum, or a combination of both into wheat may result in positive transgressive segregation for yield. Eighty-three percent of the synthetic-derived lines had heavier kernels compared with their recurrent parents. Furthermore, outstanding lines for grain yield had superior kernel weight and, in several cases, also had similar number of grains m-2 compared with the recurrent parents. High-yielding lines also had increased biomass, while maintaining their grain m-2 and HI. Therefore, source and sink were simultaneously increased in these lines. Hence, synthetic hexaploids are a promising source to improve quantitative traits in wheat.


   ACKNOWLEDGMENTS
 
The authors thank the Wheat Program at CIMMYT for all the field operations, and Dr. Reynaldo Villareal for kindly providing the populations for this study. The comments and suggestions of all reviewers are greatly appreciated.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
I.A. del Blanco present address: Dep. of Plant Sciences, North Dakota State Univ., Fargo, ND 58105. Technical Paper no. 11547 of the Oregon State Univ. Agric. Expt. Stn.

Received for publication July 29, 1999.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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