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An effective procedure to maintain a bread wheat cultivar

Ioannis S. Tokatlidis1, Ioannis Xynias2, John T. Tsialtas3 and Ioannis Papadopoulos4

1 Department of Agricultural Development, Democritus University of Thrace, Pantazidou 193, Orestiada 68200 Greece.
www.duth.gr
Email itokatl@agro.duth.gr
2
Technological Education Institute of Kalamata, 24100 Kalamata, Greece. www.teikal.gr Email ioannis_xynias@hotmail.com
3
Hellenic Sugar Industry SA, Larissa factory, 41110 Larissa, Greece. www.ebz.gr Email i.tsialt@ebz.gr
4
Technological Education Institute of W. Macedonia, 53100 Florina, Greece. www.teikoz.gr Email yannisstmp@hotmail.com

Abstract

Honeycomb selection within a bread wheat (Triticum aestivum L.) cultivar, for single-plant yield and under the very low density of 1.2 plants/m2 (LD), was applied for two successive generations and resulted in 10 and 20 lines, respectively. Progeny evaluation of the lines was conducted under both LD and the typical crop density of 500 plants/m2 (TD) in two locations. Compared with the original cultivar, five out of the 10 initially selected lines had significantly higher grain yield per plant (by 26-38%) under LD, and two of them had higher grain yield per plot (by 19 and 22%) under TD. Fifteen out of the 20 lines, derived through selection within the initially formed lines, outperformed by 18-53% the original cultivar under LD, whereas under TD four of them exhibited by 17-20% higher grain yield per plot than their original cultivar. On average, productivity gain in the 1st generation was estimated to be 20 and 4% under LD and TD, respectively, whereas in the 2nd generation 24 and 9% under LD and TD, respectively. Results evidenced that the selection procedure used could serve an effective method for cultivar maintenance.

Media summary

Honeycomb selection within a bread wheat cultivar, for single-plant yield and under very low density, constitutes an effective way for cultivar maintenance.

Key words

Intra-cultivar variation, Non-stop selection, Cultivar degeneration

Introduction

“Elite” gene pools have inherent mechanisms to provide a continuing source of new genetic variability (Rasmusson and Phillips 1997). The within a cultivar variation has been reported in several species: in a bread wheat for grain yield (Fasoula 1990); in inbred lines of sunflower using the RFLP analysis (Zhang et al. 1995); in rice cultivars by RFLP and microsatellite analysis (Olufowote et al. 1997). Peng et al. (1999) reported that maximum yield of the rice cultivar named IR8, developed by International Rice Research Institute (Philippines), has been reduced by about 2 Mg/ha (20%) during the past 30 years. Consequently a process for cultivar maintenance constitutes a worthy consideration issue.

When a bread wheat cultivar is released, it is common practice to maintain the breeder’s seed by the “ear to row” procedure. Seed reproduction under high densities, however, may favour cultivar degeneration because the rate of genotypes characterised as strong competitors-low yielders increases at the expense of weak competitors-high yielders, as a result of the inverse relationship between competitive and yielding ability (Fasoula and Fasoula 2000). Therefore, Fasoula and Fasoula (2000) stated that the non-stop selection under very low densities, so that competition between plants is minimised, is important for exploiting newly derived variation, eliminating deleterious mutations, and securing breeder’s seed of optimal quality. In a recent work Tokatlidis et al. (2004) found significant differentiation between lines derived from the bread wheat cultivar named Nestos for four grain characters (yield, protein content, carbon isotope discrimination and ash content). So the objective of this study was to investigate if the within cv Nestos variation is exploitable in order to upgrade the cultivar’s productivity.

Methods

The bread wheat cultivar named Nestos was used in the study. This cultivar has been developed by the Cereal Institute of National Agricultural Research Foundation, Greece, was written in the Greek National catalogue and the catalogue of the European Union in 1988, where the “ear to row” method is used to conserve the breeder’s seed.

Two density conditions were used: 1) the very low density of 1.2 plants/m2 (LD), with plants spaced 100x100 cm in a triangular grid according to honeycomb designs (Fasoulas and Fasoula 1995), and 2) the typical crop density of 500 plants/m2 (TD) in randomised complete block trials consisted of plots replicated 3 times, each plot including 8 rows of 5 m length and 17 cm between rows. The LD was used for both selection and progeny evaluation purposes, while the TD was used for progeny evaluation. Experimentation was conducted in two locations of northern Greece: Technological Education Institute Farm of Florina (Site1), and Agricultural Research Station Farm of Nea Zoi (Site2). During 1998-99 growing season foundation seed of cv Nestos was used to establish in Site1 a non-replicated (NR-0) honeycomb trial, with a total of 1054 plants being grown under LD. Moving-circle selection (Fasoulas and Fasoula 1995) was applied to select 10 high yielding plants. Seed of each selected plant constituted a separate line. During 1999-00 the 10 lines along with their original cv Nestos were evaluated in both Sites under LD in a R-21 replicated honeycomb design, with each entry being represented by 55 plants. In Site1 the four best plants from the five best lines were selected, forming thus 20 lines, whereas in Site2 seed obtained from all plants of each entry were mixed and screened. During 2000-01, in both Sites, progeny evaluation was conducted under LD for the 20 lines obtained from Site1 (R-21 honeycomb design, 55 plants per entry), as well as under TD for the 10 initially formed lines by using the mixed seed obtained from Site2. Mixed seed from all plants of each entry in R-21 of Site1 constituted the material for a new progeny evaluation under TD in both Sites during the following season (2001-02). Mean grain yield per plant under LD, and mean grain yield per plot (of the six internal rows) under TD, were measured. For comparison purposes, z-test for independent samples and different standard deviations was used under LD, and Least Significance Test was used under TD (P<5%).

Results

Progeny performance of the 1st generation’s lines

Figure 1 includes data from progeny evaluation under both LD and TD of the 10 lines formed through selection in the original cultivar. Under LD, the original cv Nestos gave 24.75 g/plant, whereas the average yield of the selected lines was 29.81 g/plant and ranged from 24.67 to 34.15 g/plant. The least significant difference was approximately over 4.12 g/plant and five lines had significant superiority over Nestos, which ranged from 26 to 38%. Under TD, the original cv Nestos yielded 1580 g/plot, and the selected lines yielded on average 1638 g/plot. Yield of the lines ranged from 1336 to 1922 g/plot. Three out of the 10 lines yielded lower than the original cultivar, but the differences were not significant, since the least significant difference was over 276 g/plot. On the other hand, two selected lines exhibited significantly higher yield than the original cultivar (by 19 and 22%).

Figure 1. The performance of the initially formed lines (1-10), expressed % of the respective yield of the original cv Nestos, including the mean grain yield per plant at the low density of 1.2 plants/m2 (LD) and the mean grain yield per plot at the typical crop density of 500 plants/m2 (TD).

Figure 2. The performance of the lines derived through selection within the initially formed lines {e.g., code 2(1-4) represents the average performance of the 4 lines derived from line 2}, expressed % of the respective yield of the original cv Nestos, including the mean grain yield per plant at the low density of 1.2 plants/m2 (LD) and the mean grain yield per plot at the typical crop density of 500 plants/m2 (TD).

Progeny performance of the 2nd generation’s lines

Under LD, the original cv Nestos gave 19.08 g/plant, whereas the average yield of the selected lines was 23.66 g/plant, and ranged from 18.03 to 29.26 g/plant. One out of the 20 lines yielded lower than Nestos but the difference was not significant, since the least significant difference was approximately over 2.48 g/plant. Fifteen out of the 20 lines had significant superiority over Nestos, which ranged from 18 to 53%. Highest grain yield per plant resulted from the group consisted of the four lines derived from line 2 {i.e., group 2(1-4)}, being by 40% higher than grain yield per plant of cv Nestos (Fig. 2). Under TD, the original cv Nestos yielded 1516 g/plot, and the selected lines yielded on average 1652 g/plot. Yield of the lines ranged from 1408 to 1818 g/plot. Three out of the 20 lines yielded lower than the original cultivar, but the differences were not significant, since the least significant difference was over 232 g/plot. Four selected lines exhibited significantly higher yield than the original cultivar (by 17 till 20%). The 10(1-4) group, consisted of the four lines derived from line 10, gave the highest grain yield per plot, which was by 15% higher than that of cv Nestos (Fig. 2).

Figure 3. The average % superiority of the selected lines over the original cv Nestos in the two generations, under the low density of 1.2 plants/m2 (LD) and the typical crop density of 500 plants/m2 (TD).

Data of Figure 3 illustrate the productivity gain obtained in both generations. The average superiority of 10 initially selected lines over cv Nestos was 20% under LD and 4% under TD. The 20 lines, derived from the initially formed lines, yielded on average 24% (LD) and 9% (TD) higher than the original cultivar.

Results depicted exploitable intra-cultivar variation. Slight phenotypic differences are expected to represent the within cultivar variation, and therefore only an appropriate way could identify them. To achieve such a goal, the method used in this work proved to be effective. Cultivar upgrading by honeycomb selection has been also reported by Fasoula (1990) in bread wheat, and by Traka et al. (2000) in snap bean. Low density, so that widely spaced plants exclude any plant-to-plant interference, facilitates identification of superior genotypes, because heritability is improved by increasing the share of genetic variance at the expense of environmental variance, as well as phenotypic expression and differentiation is maximised (Fasoula and Fasoula 2000). If the increased gain observed in the 2nd generation (Fig. 3) was actual or resulted by chance, deserves further consideration. In any case, maintenance of breeder’s seed under low density and selection of the highest yielding plants constitutes an alternative and effective method to avoid gradual cultivar degeneration. Non-stop selection after the release of a cultivar, has been suggested by Fasoula and Fasoula (2000), in order to eliminate deleterious mutations and exploit any positive source of existing and newly derived variation.

Conclusion

The development of a cultivar, even in autogamous species like bread wheat, does not ensure that cultivar remains constant in the long term. Existing or newly derived variability must be manipulated in such a way that a cultivar will not gradually degenerate. Slight differences between individual plants of a cultivar, however, are recognisable only when they are fully expressed. Maximum phenotypic expression and differentiation presupposes the lack of any plant-to-plant interference, in other words the lack of intra-cultivar competition. Obviously a very low density, so that single-plant yield is maximised, fulfils such a condition, as results of this work indicated. Ntanos and Koutroubas (2002) reported data in rice indicating that low density of 80cm x 80cm was preferable than that of 15cm x 15cm in the aim to maintain a cultivar’s purity. Consequently, the optimum density at which breeder’s seed should be maintained deserves reconsideration.

References

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Fasoula VA and Fasoula DA (2000). Honeycomb breeding: Principles and applications. In Plant Breeding Reviews (Ed. J Janick) 18, 177-250.

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Ntanos DA and Koutroubas SD (2002). The effect of variety maintenance systems on the seed purity of rice. In Book of Abstracts of Dissemination Conference of Current European Research on Rice, Torino, Italy, June 6-8, 2002, pp. 21-22.

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Peng S, Cassman KG, Virmani SS, Sheehy J and Khush GS (1999). Yield potential trends of tropical rice since the release of IR8 and the challenge of increasing rice yield potential. Crop Science 39, 1552-1559.

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