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Estimation of Quantitative Genetic Parameters for Outcrossing-Related Traits in Barley

^a Univ. of Hohenheim, Institute of Plant Breeding, Seed Science, and Population Genetics, D-70593 Stuttgart, Germany
^b Faculty of Agriculture, Mu'tah Univ., P.O. Box 7, Karak, Jordan
^c International Center for Agricultural Research in the Dry Areas (ICARDA), P. O. Box 5466, Aleppo, Syria

^* Corresponding author (geigerhh{at}uni-hohenheim.de).

	ABSTRACT

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Unpredictable drought conditions combined with extreme heat stress are major constraints limiting the production of barley (Hordeum vulgare L.), one of the most important rain-fed crops in West Asia and North Africa. Yield and yield stability might be improved by raising levels of both heterozygosity and heterogeneity, which could be achieved through increased outcrossing in barley landrace populations. Selection for outcrossing-related traits such as high anther extrusion, large anther sizes, and vigorous stigmas may increase the level of outcrossing. This investigation was conducted to quantify the genetic variance and calculate correlation coefficients for spikelet traits related to mating characteristics. For this purpose, F₃–line populations derived from nine crosses between genotypes showing contrasting expression of anther extrusion were produced. F₃–line populations were grown in field trials at three locations in Jordan during the 2001–2002 growing season. Anther extrusion was higher in the more favorable environment than in environments with low and variable precipitation. Anther extrusion and outcrossing-related spikelet traits were influenced by genetic and environmental factors. Early heading F₃–lines within populations showed a tendency toward higher anther extrusion, presumably because of escape from drought stress expected later in the growing season. The variation of outcrossing-related spikelet traits in F₃–line populations was continuous, suggesting a polygenic inheritance, and exhibited transgressive segregation in some populations toward high anther extrusion. Most populations showed intermediate to high broad sense heritabilities for anther extrusion and anther length. However, low to intermediate heritabilities were detected for all other outcrossing-related traits. Positive genetic correlations were recorded between anther extrusion and anther and stigma size. Results suggest that outcrossing-related spikelet traits may respond to selection in particular in the case of anther extrusion and anther and stigma size.

Abbreviations: AE, anther extrusion • AL, anther length • DH, days to heading • ICARDA, International Center for Agricultural Research in the Dry Areas • SL, stigma length • SW, stigma width

	INTRODUCTION

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BARLEY IS A MAJOR cereal crop cultivated in the rainfed areas of the West Asia and North Africa (WANA) region, where drought is the most important abiotic factor limiting barley yield. Although barley shows a higher adaptability to drought than durum (Triticum durum Desf.) and bread wheat (Triticum aesitvum L.), the probability of crop failure is high in marginal areas suffering from unpredictable drought stress conditions. Heterozygosity has been demonstrated to be associated with drought tolerance in several crop species including barley (Allard and Bradshaw, 1964; Finlay, 1964; Becker and Léon, 1988; Mayer et al., 1995; Einfeldt, 1999). Considerable increases in barley grain yield have been reported because of increased heterozygosity of barley F₂–populations compared with genetically homozygous lines grown under drought conditions (Einfeldt, 1999; Mayer et al., 1995). Einfeldt (1999) reported a relative effect of heterozygosity on grain yield of 45.6% in F₂–populations of barley. For F₂–populations, we can assume an inbreeding coefficient of F = 0.5 considering the F₁ parent plant as noninbred. According to the formula for an equilibrium inbreeding coefficient F_e = 1 – t/(1 + t) (after Crow and Kimura, 1970), an outcrossing rate of t = 0.333 would result in an inbreeding coefficient of F = 0.5 relative to a panmictic base population. Assuming allele frequencies of 0.5, an F₂ population and a breeding population of 0.33 outcrossing would have the same heterozygosity level. Hence, an outcrossing rate of 0.33 would be expected to result in a considerable increase of grain yield in barley. Outcrossing rates of up to 20% have been reported for individual barley cultivars (Robertson and Deming, 1931) so that it seems reasonable to anticipate reaching a level of 33% outcrossing after several cycles of recurrent selection. Improvement in yield and yield stability under drought conditions may therefore be achieved by assembling genetically broad based synthetic cultivars of adapted genotypes with such high outcrossing rates. However, generally barley shows a very low level of natural outcrossing ranging from 0.35 to 5% as reported in different publications for H. vulgare (Doll, 1987). A range of 0 to 2.1% outcrossing (Abdel Ghani et al., 2004; Brown et al., 1978) has been reported for accessions of H. vulgare subsp. spontaneum K. Koch in the Near East. Therefore, barley is considered as a self-pollinator with a high degree of cleistogamy (Giles et al., 1974; Brown et al., 1978; Chaudhary et al., 1980; Doll, 1987; Parzies et al., 2000). Yet, variation has been observed for spikelet opening and anther extrusion at anthesis (Virmani and Athwal, 1973; Ceccarelli, 1978; Doll, 1987; Hammer, 1984; Gupta et al., 2000; Abdel-Ghani et al., 2003). The latter characteristics are positively associated with anther and stigma sizes (Hammer, 1975, 1984; Giles and Bengtsson, 1988; Battan et al., 1997). Consequently, increased outcrossing rates might be achievable by selecting for spikelet characteristics favoring open-pollination.

Optimizing selection strategies for the improvement of quantitative traits requires reliable estimates of the population parameters (variance components, heritability, genetic correlation coefficients) determining the selection gain. However, information about the genetic control of outcrossing-related spikelet traits in barley is fragmentary and ambiguous. Therefore, the objective of the present study was to investigate the inheritance of anther extrusion and other spikelet traits in F₃–line populations of barley derived from nine crosses between parent lines strongly diverging in the degree of anther extrusion. Data were assessed in field trials at three locations under drought conditions in Jordan.

	MATERIALS AND METHODS

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Production of F₃–Line Populations
Anther extrusion of 337 breeding lines from the International Center for Agricultural Research in the Dry Areas (ICARDA) had been evaluated at Tel Hadya, ICARDA, in the 1999–2000 growing season in both greenhouse and field trials (Abdel-Ghani et al., 2003). Four lines (259, 278, 298, and 306) showing a high level of anther extrusion and eight lines (6, 33, 43, 44, 45, 47, 66, and 125) showing almost no anther extrusion were selected (Table 1). The lines chosen behaved consistently under greenhouse and field conditions. Nine crosses between lines with contrasting expression of anther extrusion were established (Fig. 1) . Five to 10 F₁ plants per cross were self-pollinated under isolation bags to produce F₂ seeds. About 200 F₂ seeds per cross were germinated in peat substrate, vernalized at 4°C for 5 wk, and transplanted for production of F₃–lines. All steps involved in the build-up of F₃–line seed were conducted in a greenhouse at the University of Hohenheim (Stuttgart, Germany) during December 2000 to August 2001.

View larger version (34K): [in this window] [in a new window]   Fig. 1. Mean of nine F3–line barley populations at three locations for anther extrusion (%) at Rabba, Ghweer, and Shoubak, Jordan. Bars with the same letter are not significantly different at P = 0.05 for a given cross.

Character Assessment
The following data were collected: anther extrusion, length and width of anthers and stigmata, number of pollen grains per anther, heading date, and plant height. Anther extrusion was estimated as percentage of spikelets with extruded anthers per spike recorded on three successive days starting on the day of the first anthers became visible in a line, preferably before midday. All observations of the anther extrusion are likely to be underestimated, since wind may have removed part of anthers before the scores were taken. Length and width of anthers and stigmata were measured under a binocular microscope in millimeters as described by Hammer (1975). The number of pollen grains per anther was determined by the method of Joppa et al. (1968). A total of 40 anthers from up to five main spikes per plot were excised just before flowering and transferred to a 4-mL vial. Anthers were air-dried by opening the vials at room temperature for 48 h. Two milliliters distilled water were then added to each vial. Vials were shaken vigorously to ensure that all pollen grains were dislodged from the anthers. A small drop of the pollen sample was placed beneath the cover slips of a standard hemocytometer. Pollen grains were counted in eight cells of 1.0 mm² and the counts were converted to total pollen grains per anther. Heading date was recorded in days from planting date to 50% flowering i.e., when the entire spike had emerged from the flag sheath in 50% of the tillers per line. Plant height was measured in cm from ground to the tip of the spike, excluding awns. Anther extrusion, heading date, and plant height were recorded for all populations. Measurements of the length and width of anthers and stigmata and number of pollen grains per anther were only taken for the following populations: 259 x 45, 66 x 278, and 125 x 306. Stigma length was measured as the distance of one of the two-branched feathery stigmatic forks from base to tip, whereas stigma width was measured as the distance between the tips of the two forks. Plant height and days to heading were recorded on a plot basis, while all other traits were recorded on a single plant basis. The following sampling procedure was followed: number of extruded anthers and spikelets per spike were recorded on at least 5 to 10 main spikes (i.e., the first spike produced by the plant). The length and width of anthers and stigmata, were recorded from 10 to 20 individual spikelets taken from the middle part of the main spikes. Only ripe (yellow) unopened anthers were collected to record anther length and to score the number of pollen grains per anther.

Statistical Analysis
Lattice analyses of variance were performed for each environment. Adjusted entry means and effective error mean squares from the lattice analyses were then used to compute the combined analyses of variance across environments (Cochran and Cox, 1957, Chap. 14.32). Tests for outlayers were conducted according to Anscombe and Tukey (1963) and sig-nificant outlayers were treated as missing values in the analyses of variance. Variance components were estimated according to Snedecor and Cochran (1980). The effects of locations were regarded as fixed, and all other effects were assumed to be random variables. A random effects model was assumed adequate for estimating population parameters although seed yield was effected by poor germination (caused by dormancy) and bird damage since it appeared unlikely that these factors would influence the genetic composition of the respective populations concerning the outcrossing-related traits under study. Heritability estimates were based on entry means and 95% confidence intervals of the estimates were calculated according to Knapp and Bridges (1987). Coefficients of genotypic correlation were estimated as described by Mode and Robinson (1959). All statistical computations were performed with the computer program PLABSTAT (Utz, 2000). To achieve normal distribution, outcrossing-related traits data were transformed: anther extrusion was transformed by arc sine, whereas all other traits were log-transformed. However, transformation did not result in noteworthy improvements of the distributions. Therefore, nontransformed data were used throughout (Steel and Torrie, 1980).

	RESULTS

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Means of Locations and Populations
Environmental conditions of the three field trial locations differed considerably. Precipitation during the growing season was higher at Rabba than at Ghweer and Shoubak. Precipitation was 60 mm below the seasonal average at Shoubak but 20 and 30 mm above the average at Rabba and Ghweer, respectively. The average temperature during the growing season was lower at Shoubak than at Ghweer and Rabba. Differences in plant height and days to heading reflect the different environmental conditions under which these experiments were conducted. Rabba received more rainfall, thus plant height was greater than at Ghweer and Shoubak. Population means for plant height ranged from 107 to 118 cm at Rabba, from 78 to 90 cm at Ghweer, and from 67 to 88 cm at Shoubak (data not shown). Because of higher temperatures at Rabba and Ghweer, plant development was faster there than at Shoubak. The growing season extended from November to mid May (5.5 mo) at Rabba and Ghweer and to mid June at Shoubak. Population means for days to heading from 1 January ranged from 118 to 131 d at Shoubak, from 84 to 95 d at Rabba, and from 89 to 103 d at Ghweer (data not shown).

Differences among locations for outcrossing-related traits were highly significant for all nine F₃–line populations. At Rabba, the percentage of anther extrusion was considerably higher (about 30%) than at Ghweer and Shoubak (Fig. 1), whereas the latter differed only slightly. Anther length was also higher at Rabba than at Ghweer and Shoubak. However, there was no such tendency for stigma length and width (Table 3). Considerable differences between populations occurred for most outcrossing-related traits, particularly for anther extrusion (Table 4). Anther extrusion was lowest in population 125 x 306, the parents of which were the only six-rowed cultivars included in the experiments. The highest anther extrusion percentage was observed in population 259 x 45.

View larger version (44K): [in this window] [in a new window]   Fig. 2. Frequency distributions of entry means in F3–line barley populations with significant genetic variance for A anther extrusion, B anther length, C stigma length, and D stigma width based on data from three locations in Jordan 2002. Triangles mark the means of the respective parent lines and of the H. vulgare subsp. spontaneum check.

	DISCUSSION

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De Vries (1971) and Sage and Isturiz (1974) reported that the degree of open flowering in wheat depends on the water supply of the lodicules. The osmotic water uptake of the lodicules from the ovary is hampered under dry conditions which impairs the swelling mechanism of the lodicules. Similarly for barley, Bossinger et al. (1992) reported that during anthesis the lodicules swell up in open-flowering, but not in closed-flowering barley cultivars, forcing the palea and lemma apart. As the filaments increase in the length at the time of lodicule's swelling, the pollen containing anthers can emerge from the floret and release their pollen. Therefore, the extent, duration, and timing of spikelet opening are determined by the availability of water in the soil. On the other hand, Sage and Isturiz (1974) and Chhabra and Sethi (1991) reported for wheat that lodicule size is genetically determined and is associated with the degree of chasmogamy. Correspondingly, Giles (1989) reported for barley that the most important varietal characteristics which influence outcrossing are lodicules which exhibit even wider variation in morphology than the lodicules of other cereals. Giles (1989) stated that genotypes with no expansive tissues with which to force the lemma and palea apart, always results in closed flowering, which prevents oucrossing. Consequently, the intermediate-to-high heritability estimates for anther extrusion observed in this study may have resulted from both uptake and retention of water by the lodicule as a function of moisture supply and genetic differences in lodicule sizes among the F₃–lines. The negative genetic correlations between outcrossing-related traits and days to heading indicate that early heading F₃–lines showed higher degrees of anther extrusion and larger anthers and stigmata, presumably because of escape from early drought stress (Blum, 1988).

Differences in the expression of outcrossing-related traits among populations were observed, corroborating several studies which showed that outcrossing-related traits may differ considerably among the various wild and cultivated forms of barley (Hammer, 1975, 1977, 1984; Giles and Bengtsson, 1988; Battan et al., 1997; Gupta et al., 2000). Doll (1987) concluded that variation in the outcrossing rate was associated with variation in the degree of chasmogamy.

The significant genetic variances and moderate-to-high heritability values found in this study for anther extrusion and anther length imply good perspectives to increase open flowering in barley by recurrent selection. Heritabilities for the length and width of stigmata were low to moderate and inconsistent among the three tested F₃–line populations because of small or nonsignificant genetic variances. Therefore, selection for these traits does not seem to be rewarding. In agreement with our results, Virmani and Athwal (1973) reported high heritability estimates for anther and stigma length as well as for percent of exserted stigmata in rice. However, Sage and Isturiz (1974) found a low heritability coefficient for anther extrusion in wheat.

The distribution of outcrossing-related spikelet traits in F₃–line populations was continuous and, considering the moderate to high heritability estimates, suggests a polygenic control for these characters. In contrast, Ceccarelli (1978) reported that anther extrusion in barley is controlled by one single gene with complete dominance for extruding genotypes. These apparently conflicting results may be due to the scoring method (presence or absence of extrusion in Ceccarelli's, as opposed to percentage of extrusion in this study), the genetic material, and/or the environmental conditions under which the experiments were conducted. Anther extrusion in wheat was shown to be under the control of few, possibly only two, genes with additive gene effects (Sage and Isturiz, 1974). No other investigations about the inheritance of outcrossing-related floral traits in predominantly self-pollinated cereals were found in the literature. In some of our populations, transgressive segregation was exhibited toward the high parent for anther extrusion and anther length, suggesting that both parents contributed favorable alleles to their offspring and that there are good chances to achieve superior gene combinations by selection.

The positive genetic correlation between anther extrusion and both anther and stigma length detected in this study indicate that selection for anther extrusion will lead to concurrent increases in anther and stigma lengths. Positive phenotypic correlations were reported earlier between different outcrossing-related traits in barley (Hammer, 1984) and rice (Oryza sativa L., Virmani and Athwal, 1973). High anther extrusion may be considered as an important spikelet trait for enhancing the outcrossing rate. Anther extrusion depends on two preconditions: spikelets must open and the filaments need to elongate to expose the anthers before the spikelets close again. Among mature spikes, dry anthers are frequently found trapped at the top of closed spikelets of nonextruding barley genotypes, suggesting that anther extrusion is more restricted by the degree of spikelet opening than by filament elongation (Rajki, 1960, 1962; Sage and Isturiz, 1974; Ceccarelli, 1978). Anther extrusion can be considered as a partial indicator of spiklet opening in barley, and it has been suggested to define the trait as the degree of spikelet opening, rather than as anther extrusion (Ceccarelli, 1978).

In summary, the present results suggest that selection for anther extrusion is a promising approach to improve the outcrossing rate in barley. In wheat, visual mass selection for anther extrusion in F₃ and F₅ generations of fertility restorer bulks effectively improved the level of anther extrusion (Ghiasi et al., 1982). Relative to the original F₃–bulk mean, the increase was 19% anther extrusion per cycle of selection. Recurrent selection was very successful in improving various agronomic and quality traits in triticale (xTriticosecale Wittm., Geiger et al., 1994) and should be effective in barley as well. Improved open-flowering is expected to raise outcrossing, thus allowing the development of barley cultivars with increased heterozygosity. As shown by Mayer et al. (1995) and Einfeldt (1999), heterozygosity may significantly enhance both yield ability and stability of barley in the drought-prone WANA region.

	ACKNOWLEDGMENTS

 
The authors are very grateful to Dr. Ayed Omary from Mu'tah University, Jordan and the personnel of the National Center for Agricultural Research and Technology Transfer (NCARTT), Jordan, in particular eng. Ismail Twassi (director of Shoubak Agriculture Station), eng. Abdel-Raheem Boualize (director of Ghweer Agriculture Station), and eng. Yacoub Al-Hijazeen for providing all necessary resources for the field and laboratory experiments and for very valuable advice and their cooperation. Adel Abdel-Ghani is a "Deutscher Akademischer Austausch Dienst (DAAD)" Ph.D. fellowship holder. The authors wish to thank "Deutsche Forschungsgemeinschaft (DFG)" and DAAD for financial support.

Received for publication January 8, 2004.

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