Agronomy Journal 92:1020-1026 (2000)
© 2000 American Society of Agronomy
SNAP BEAN
An Integrated Approach of Breeding and Maintaining an Elite Cultivar of Snap Bean
Ekaterini Traka-Mavronaa,
Dimitrios Georgakisa,
Metaxia Koutsika-Sotirioub and
Theodora Pritsaa
a National Agricultural Research Foundation (NAGREF), Agricultural Research Center of Macedonia-Thrace, 570 01 Thermi, Thessaloniki, Greece
b Aristotelian Univ. of Thessaloniki, Faculty of Agriculture, Laboratory of Genetics and Plant Breeding, 540 06 Thessaloniki, Greece
koutsika{at}agro.auth.gr
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ABSTRACT
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This study is an attempt to describe a functional breeding and maintaining program of intraselection in a traditional snap bean (Phaseolus vulgaris L.) cultivar. The program was applied in three stages. The first thing examined was the existing genetic variability of source material for earliness and pod yield potential. Single-plant frequency distributions with positive skewness for earliness showed that the frequency of unfavorable alleles was high. For total pod yield, distribution was found normal. Thus, the end-target should be selection for early maturity, keeping, and stabilizing high yield. The seed shape uniformity was added as third criterion of selection. Secondly, combined pedigree intraselection, based on widely spaced single-plant performance, for the prementioned traits was applied for three successive generations. The evaluation of the third-generation families revealed progenies with high yield, earliness, and stability of performance. Thirdly, the end-product of the program applied was to restore or even improve the cultivar. The evaluation of improved selections of fourth-generation families and of the source material, at dense stand, showed that all families were the only ones producing high and stable early fresh pod harvest, even 53 d after planting (53.25–80 g/plant, compared with 0 g/plant of the control). The total pod yield of all the families was 219 to 276% superior compared with source material. Conclusively, the widely spaced single-plant combined pedigree intraselection was proved reliable and effective in restoring or even improving the local cultivar of snap bean according to update demands.
Abbreviations: DAP, days after planting
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INTRODUCTION
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WITH the advent of genetics and plant breeding, selection has been intensified for high yield potential with broader adaptation (Simmonds, 1979). The change in selection criteria resulted from increased environmental potential and reduced heterogeneity, which in turn was made possible by the increased use of agronomic inputs to reduce production constraints (Bramel-Cox et al., 1991). In general, to improve the genetic gain under low- or no-input conditions, breeders need the existing genetic diversity. Many efforts were conducted to exploit the well-adapted landraces and their possibilities of being used as source material in breeding programs. Hence, there is a need for genetic conservation to compensate for the declining native variability by deliberate maintenance.
Major breeding objectives in snap bean (Phaseolus vulgaris L.) concern the development of cultivars combining high productivity, stable yields, earliness, pest and disease resistance, tolerance to environmental stresses, and desirable agronomic–horticultural attributes (Silbernagel, 1986; Singh, 1992). The achievement of such objectives should take into account the cropping systems, the ecological conditions, and the preference of consumers in the target areas. In temperate areas of monocropping and intensive cultivation, mechanical cultivation, and industrial processing of the final product have directed efforts to breed varieties with determinate bush type, short duration, concentrated pod maturation, and uniformity in plant height, seed shape, and size.
Much of the genetic improvement of snap bean has been achieved through the selection of varieties by applying conventional breeding techniques of self-pollinated crops (Singh, 1992) such as bulk, pedigree, backcross, and their modifications (Brim, 1966) such as the single seed descent method. Conventional pedigree selection based on visual evaluations may be difficult, especially for traits with low to moderate heritability such as seed yield (Patino and Singh, 1989). Bulk breeding methods were effective for endowing genotypes with a better yield stability (Allard, 1961; Allard and Bradshaw, 1964). However, the highly competitive cultivar derived from such populations did not necessarily give the best seed production (Tucker and Webster, 1970; Tucker and Harding, 1974). Population improvement based on recurrent selection techniques has greater potential than the preceding methods for the introduction of quantitatively inherited traits, such as seed yield or horizontal resistance to pests and diseases (Baudoin, 1993). Singh (1994) showed that gamete selection for simultaneous improvement of multiple traits, including seed yield, from early generations was superior in efficiency to other methods, including conventional pedigree, single seed descent, and mass selection.
The snap bean cultivar Zargana Kavalas is registered as Greek in the National Catalogues of agricultural and vegetable plant species since 1985, with the Agricultural Research Center of Macedonia-Thrace being the breeder and maintainer. Maintenance of the cultivar was performed by discarding off-types and once-over harvesting the dry seed production. Recently, the cultivar showed a diversion from its original type related to late maturity and certain pod and seed abnormalities. Sources of genetic variation in a self-pollinated crop, such as bean, may be cultivars' heterogeneity, mutations, or insect cross pollination (Silbernagel et al., 1993). Pearson (1956) and Riley et al. (Tokatlidis et al., 1998) reported in wheat (Triticum aestivum L.) that homozygosity enhances the frequency of chiasma formation and recombination. Silbernagel et al. (1993), taking into account the difficulties in snap bean seed selection and multiplication, suggest that the breeders and maintainers must reselect a new breeders' seed stock of most cultivars every 2 or 3 yr. Aiming to restore or even improve the local cultivar Zargana Kavalas, we applied an intraselection breeding program. This study is an attempt to describe a functional breeding and maintaining program in a simple objective manner, so that other newly established similar programs could benefit from our experiences. The present program was applied gradually, starting with the study of the existing genetic variability for early maturity and pod yield potential, continuing with combined pedigree intraselection, based on widely spaced plant performance for the prementioned traits and additionally for seed shape uniformity, and ending with the derived families according to update market demands evaluation.
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Materials and methods
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Experimental Material
All experiments were conducted at the Agricultural Research Center of Macedonia-Thrace farm, located near Thessaloniki. A polyethylene-covered greenhouse of 500 m2, heated by a gas oil air furnace, was used to provide an isolation environment.
As source material, the traditional snap bean cultivar Zargana Kavalas was used. It is a determinate bush type and well adapted to intensive farming systems in a wide range of environmental conditions. The fresh pods are attractive in appearance, bright green, large, fleshy, stringless, smooth, slightly oval in cross section, with little carpel separation, or skin sloughing. Additionally, the plant has a relatively slow rate of seed, fiber development, or both once it reaches maturity, giving some holding ability to harvest operations if delayed by weather conditions. The pods are picked five to eight times during the season and are consumed mainly as fresh green.
Experimental Procedure
The breeding and maintaining program lasted five growing seasons and consisted of three stages: First stage—prebreeding experiment for one growing season, spring of 1997; Second stage—breeding and maintaining experiments for three growing seasons, summer of 1997 and spring and summer of 1998; Third stage—testing experiment in spring of 1999 (Fig. 1)
. Descriptions in detail of the experiments of the three stages are as follows:
First Stage
The source population included 798 plants. The seeds were sown in the greenhouse on 17 Mar. 1997, following plant-to-plant and row-to-row distances of 70 and 80 cm, respectively. Each hill was overplanted and later thinned to one seedling. Thus, the land area per plant was equal to 0.6 m2. The prementioned plant-to-plant and row-to-row distances remained unchanged during the breeding and maintaining procedure. The phenotypic variation of earliness and productivity was estimated by counting the number of pods of each plant: (i) during two subsequent samplings for earliness, 70 and 78 d after planting (DAP), and (ii) at the end of fruit-setting (92 DAP) for total pod yield. Combined selection, based on single-plant performance, for earliness and productivity at 12.5% selection pressure was applied. One hundred plants were selected and consisted of the first-generation selected families. The seed shape of these families was controlled by examining their uniformity in shape index and by discarding families declining from the original type.
Second Stage
The first eight in ranking of the selected families, as well as a random sample of eight selected families from the rest, were subsequently grown in the same isolation environment, following a randomized complete block design, with two replicates, each consisting of 25 plants. The sowing date was 30 July 1997. Single-plant measurements were taken on two subsequent dates, 60 DAP for earliness and 76 DAP for yield. Combined pedigree intraselection on single-plant performance for earliness, productivity, and seed shape uniformity, at 2.6% selection pressure, was applied and resulted in 21 plants. Selected second-generation progenies were subsequently established on 13 Mar. 1998, following an R-21 replicated honeycomb design (Fasoulas, 1993), with a total number of 840 plants (40 plants/entry). Single-plant measurements were taken on two subsequent dates, 68 DAP for earliness and 88 DAP for yield. Combined pedigree honeycomb intraselection on single-plant performance for earliness, productivity, and seed shape uniformity, at 2.6% selection pressure, was applied and resulted in 21 plants. The evaluation of the third-generation derived progenies was conducted in the same environment, following the previous experimental design (R-21). The sowing date was the 23 July 1998. Single-plant measurements were taken on three subsequent dates, 56 and 69 DAP for earliness and 89 DAP for yield. Combined pedigree honeycomb intraselection on single-plant performance for earliness, productivity, and seed shape uniformity was applied and resulted in selected offsprings of eight superior families for test evaluation and further breeding purposes.
Third Stage
The evaluation of the improved selections of the fourth-generation families compared with the source material was conducted in the open field, at dense stand, during spring 1999, following a randomized complete block design, with four replicates. The experimental unit was a two-row plot, 2 m long, with 36 plants, 11 cm apart, i.e., plant density equal to 9 plants/m2. We made eight mixtures, with an equal number of seeds from each offspring plant, tracing back to eight third-generation selected families. Measurements for earliness (53 and 60 DAP) and productivity (73 DAP) were recorded on fresh pods, which were hand-harvested and transported to the laboratory, where they were counted and weighted. All measurements were taken at plot level.
The selection and evaluation procedure used is presented in Fig. 1. Normal cultural practices for each experiment were followed for irrigation, fertilizer, and pesticide application.
Statistical Analyses
The existing genetic variability in the cultivar for early maturity and pod yield was estimated. We examined single-plant frequency distributions of pod number of 798 plants, sampled on three subsequent dates by applying the Shapiro W normality test (Sall and Lehman, 1996). Analyses of variance and tests of significance for early maturity and pod yield were performed in the randomized complete block design experiments. We analyzed replicated honeycomb designs by applying a microcomputer program, which was constructed specially for plant selection and analysis of honeycomb designs (Batzios and Roupakias, 1997). To test the hypothesis that pairs of entries grown in the R-21 honeycomb designs were equivalent for mean yield and early maturity, the Z-test was applied considering number of observations per entry as the mean number of replicates of all entries (Batzios and Roupakias, 1997).
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Results
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Single-plant frequency distribution of early maturing pods of source material, 70 DAP, is significantly nonnormal (p = 0.0000) according to the Shapiro W normality test, with skewness of 1.014, kurtosis of 0.96, and CV of 81% (Fig. 2)
. The corresponding single-plant distribution, 78 DAP (Fig. 3)
, is also not normal (p < 0.0001), but less asymmetrical than the previous one, with skewness of 0.016, kurtosis of -0.47, and CV of 49%. The total pod yield distribution, 92 DAP, followed a normal distribution (p = 0.136), with CV even reduced to 34% (Fig. 4)
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Fig. 2 Single-plant frequency distribution of early maturing pods of the source material 70 d after planting (DAP) in the isolation environment
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Fig. 3 Single-plant frequency distribution of early maturing pods of the source material 78 d after planting (DAP) in the isolation environment
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Fig. 4 Single-plant frequency distribution of pod yield of the source material 92 d after planting (DAP) in the isolation environment
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The data of the successive three generations of applied selection are depicted in Tables 1, 2, and 3
. From the results of the first-generation progenies (Table 1), we found that the first group of families gave an average of 18.52 early pods 60 DAP, compared with 14.14 for the second group. Total yield was lower (42.66 pods/plant) in the first group, compared with the second group with the random sample of families (46 pods/plant). The evaluation of the second-generation families (Table 2) revealed five progenies (pedigrees 4-1, 1-17, 1-10, 1-15, and 8-7) being the last in early maturity (5.90–8.68 pods/plant), but with the best yield performance (48.50–54.20 pods/plant). The remaining 16 families combined earliness (11.29–17.38 pods/plant) with slightly lower productivity (33.74–39.57 pods/plant). The evaluation of improved selections from the selected third-generation families (Table 3) has shown that yield was not significantly differentiated in 20 families (35.24–40.42 pods/plant), while early harvest, 69 DAP, was not significant in 19 families (31.19–36.85 pods/plant). Seven families (pedigrees 4-13-14, 2-19-18, 4-20-19, 8-18-15, 8-18-16, 8-18-17, and 3-21-20) originating from four of the first group of the first-generation families, possessed high-yielding capacity (38.53–40.42 pods/plant), early maturity (34.66–36.27 pods/plant), and stability of performance (CV = 24–30%). These families have also shown better stability in early harvest even 56 DAP. Of these families, two progenies, the pedigrees 4-13-14 and 4-20-19, were the best in stability of early and total pod yield. One more family, the pedigree 7-6-7, was the first in early harvest 56 and 69 DAP, and fourth in total pod yield.
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Table 1 Number of early maturing pods 60 d after planting (DAP) and total pod yield 76 DAP of the first-generation 16 progenies in the isolation environment (pods/plant)
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Table 2 Number of early maturing pods 68 d after planting (DAP) and total pod yield 88 DAP of the second-generation selected progenies in the isolation environment (pods/plant)
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Table 3 Number of early maturing pods 56 and 69 d after planting (DAP) and total pod yield 89 DAP of the third-generation selected progenies in the isolation environment (pods/plant)
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The evaluation of eight mixtures of improved selections of the fourth-generation families and of the source material (Table 4) , at dense stand, revealed that all selected families were the only ones producing high and stable early fresh pod harvest, even 53 DAP (8–12.50 pods/plant or 53.25–80 g/plant compared with 0 g/plant of the source material). The subsequent harvest, 60 DAP, has also shown almost zero production in the control and high production in the selected families. The total pod yield of the eight mixtures of selected families was 219 to 276% superior compared with source material.
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Table 4 Number of early fresh pod harvest 53 and 60 d after planting (DAP) and total fresh pod yield 73 DAP of the fourth-generation selected progenies and of the source material, at dense stand, in the open field (no. and weight of pods/plant)
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Discussion
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The maintenance of a cultivar encloses some obligations for the breeder. During multiplication, there are four requirements placed upon the breeder: purity, quality, health, and uniformity (Simmonds, 1979). The first three are basically the requirements of certification, while the fourth, uniformity, is a special requirement placed upon the breeder by usage. In meeting the three first requirements, the breeder is simply doing what the commercial multiplier will do later, but he is doing it at an extremely high level. The effects of a mistake or bad luck will inevitably be increased during subsequent multiplication of the cultivar. In the present paper, this problem was confronted. It consisted of late anthesis and segregation of seed uniformity. A strategy, that would take under control the following points, ought to be planned: (i) determination of end-target, (ii) place of maintenance, (iii) methods to be applied, and (iv) utilization of end-product (Simmonds, 1979). In our case, the place of maintenance was predetermined to remain at the Agricultural Research Center of Macedonia-Thrace, taking into account administrative considerations, scientific experience, communications, local climate, diseases, and quarantine arrangements. Also, the maintenance procedure was enriched with genetic improvement. Thus, the above-mentioned points were adjusted as follows.
First Stage
The end-target was to upgrade the local cultivar by improving early maturity, stabilizing high yield, and ensuring product uniformity. Although common bean is a self-pollinated crop, extremely high rates of natural outcrossing in some environments and genotypes have been recorded (Wells et al., 1988). Thus, all the experiments were established in isolated environments. Breeding progress for common bean cultivars is slow because several desirable traits need to be combined in specific seed and plant types for a particular growing region and cropping system. For simultaneous improvement of two or more traits that could not be screened in the same nursery, selection is practiced in alternate generations (Singh, 1992). For this purpose, estimating existing genetic variability in the traditional cultivar was considered as the beginning. Examining the single-plant frequency distributions of source material for number of pods per plant, in three subsequent dates, the data showed that: (i) for early maturity, with either strong positive skewness (Fig. 2) or positive skewness (Fig. 3), the frequency of unfavorable alleles was high; and (ii) for total pod yield per plant, with a normal distribution (Fig. 4), the genetic variability was mainly consisting of additive genes and remained in equilibrium.
Large differences (50–250 d) in days to maturity are found in cultivated common bean (Singh, 1992). These differences are associated with differences in growth habit, degree of sensitivity to photoperiod and temperatures, and growing environments (Singh, 1992). Genetic control of earliness vs. lateness depends on prevailing day and night temperatures, photoperiod, and genotypes utilized in the study (Singh, 1993). Coyne and Mattson (1964) indicated that the photoperiodic response under long days was controlled primarily by qualitative genes. Later, Coyne (1966) observed polygenic control of this response under long days in two crosses, while some qualitative gene effects were observed in another cross. Under short days it appears that qualitative genes are not expressed and the variation is mainly due to the action of polygenes and environment. Coyne and Schuster (1974) found that early flowering and determinate growth habit were each determined by a single recessive gene with coupling linkage. Al-Mukhtar (1981) and Mohan (Singh, 1993) report monogenic control with complete dominance, whereas Leyna et al. (1982) report incomplete dominance in the F1 generation for early flowering/maturity. It was found that, within bush determinate upright germplasm (type 1 growth habit), an early maturing high-yielding cultivar can be easily possessed. Yield was found to be the best selection criterion in common bean (Singh, 1992). The results obtained on the evaluation of source material, showed that the end-target should be selection for early maturity, keeping and stabilizing high yield.
Second Stage
The selection method was determined by (i) the number of traits to be combined together and their heritability (Singh, 1992), and (ii) the effectiveness of honeycomb selection, a method proposed by Fasoulas (1973, 1977) for the evaluation of quantitative traits of widely spaced single plants (Bos, 1983; Robertson and Frey, 1987; Jensen, 1988; Borojevic, 1990; Sotiriou et al., 1996). Honeycomb selection aims to eliminate deleterious genes and to exploit additive genetic effects prior to the exploitation of nonadditive effects. This approach should permit the accumulation of all the favorable genes in one genomic variant (Fasoulas, 1988). Experimental designs, which are well adapted, when field plots as units of evaluation and selection are replaced by widely spaced single plants, are the honeycomb field designs (Fasoulas, 1988, 1993). Honeycomb designs maximize efficiency in plant breeding by (i) preventing competition, thus identifying heritable superiority, and (ii) maximizing phenotypic expression and differentiation.
The second stage lasted three growing seasons, until the fourth generation. The evaluation and selection of first-generation progenies for early maturity and productivity indicate that high selection pressure for each trait could not be effective for simultaneous improvement of the two. Thus, selection was practiced mainly for early maturity and for stabilizing high yield. The third criterion of selection, that of seed type, was also a factor restricting the simultaneous improvement for multiple traits. The evaluation of the second-generation progenies resulted in their differentiation according to earliness and productivity, while the evaluation of the third-generation improved selections revealed eight families with high-yielding capacity, early maturity, and performance stability.
Third Stage
The end-product of the program applied was to restore or even improve the cultivar. The evaluation of eight mixtures of improved selections of the fourth-generation families, tracing back to five first-generation ones and of the source material, at dense stand, revealed that all families were the only ones producing high and stable early fresh pod harvest, even 53 DAP, while the control was still at the vegetative phase. The total pod yield of source material was 5.06 Mg/ha, while the one of all selected families ranged between 16.16 and 19.03 Mg/ha, which is a superiority of 219 to 276%.
These results indicate that the applied combined pedigree intraselection for early maturity and pod yield, and additionally, for seed shape uniformity, based on single-plant behavior, resulted in developing selections valuable according to update demands. The derived selections have more potential to fit into multiple-cropping systems. Conclusively, the maintenance of local cultivars encloses an essential role of the plant breeder, the saving of the seed nowadays. This role should not permit the downgrade of local cultivars in their main characters that render them competitive in the market. A methodology of widely spaced single-plant combined pedigree intraselection is proposed, which can be applied in a repetitive way, in time defined by the breeder.Coyne Mattson 1964
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NOTES
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This research was funded by National Agricultural Research Foundation of Greece, Vegetables Seed Production program.
Received for publication September 28, 1999.
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ABSTRACT
INTRODUCTION
