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a Agric. Res. Stn., Fort Valley State Univ., Fort Valley, GA 31030
b Agric. Res. Stn., Virginia State Univ., Petersburg, VA 23806
* Corresponding author (raom{at}mail.fvsu.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DAP, days after planting • LAI, leaf are index • PAR, photosynthetically active radiation • MG, maturity group
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In the USA, specialty soybean is a niche market commodity that fetch a premium ranging from $18 to $589 per Mg above the market price of commodity soybean (Carter and Wilson, 1998). Limited consumer base and lack of suitable cultivars are some of the factors limiting vegetable soybean production in the USA. There is a need to introduce, evaluate, and characterize vegetable soybean cultivars of domestic and Asian origin for cultivation in the USA. Development of improved soybean cultivars for vegetable purpose offers potential for expanding the domestic and international soybean market.
In the USA, information on agronomic and nutritional characteristics of vegetable soybean is limited, although there is a long history of trying to promote soybean as a vegetable crop (Carter and Shanmugasundaram, 1993). A chronology of vegetable soybean including its history in the USA has recently been published (Shurtleff and Lumpkin, 2001). A number of large-seeded Japanese, Korean, and Chinese vegetable soybean cultivars were reportedly renamed and released in 1930s and 1940s for cultivation in the USA. A few vegetable soybean cultivars were developed for adaptation to the USA by crossing Japanese and Korean vegetable soybean cultivars with high yielding, pest and shatter resistant grain soybean cultivars adapted to the U.S. environments. Carter and Shanmugasundaram (1993) reported limited breeding efforts in the USA. A few large-seeded soybean cultivars particularly suited for vegetable purposes have been released by Iowa State University (Bernard, 2001). Although several cultivars through crossing American cultivars with Japanese food-grade cultivars have been developed for production in the USA, none of these cultivars have become popular because the American consumer has not adapted to the taste and flavor (Carter and Shanmugasundaram, 1993). In the USA, Japanese cultivars belonging to MG I- through III have been reported to be suitable for production in Washington, Oregon (Miles, 2001), and Colorado (Johnson et al., 1999). No such efforts have been reported for identifying edamame cultivars for production in the southeastern USA. While breeding edamame genotypes adapted to the southeastern USA is a long-term possibility, selection of edamame cultivars developed for production in the Asian countries for potential production in the southeastern USA could be a viable option in the short term. The objective of this research was to evaluate 10 Japanese vegetable soybean genotypes, including four plant introductions with desirable vegetable soybean traits, two Chinese vegetable soybean cultivars, and two adapted U.S. soybean cultivars for LAI, light interception, fresh green seed yield, and green seed compositional traits in the southeastern USA.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Flowering
Date of first flower was recorded each year. Thirty days after planting (DAP), the crop was checked at 2-d interval to record flowering.
Interception of Photosynthetically Active Radiation (PAR)
The interception of PAR was recorded with a Sunfleck PAR Ceptometer (Decagon Devices, Inc., Pullman, WA). In each plot, the total PAR incident on crop canopy and that reaching the base of the crop were recorded. The difference between the incident PAR and transmitted PAR was calculated as that intercepted by the crop canopy. Two observations were recorded for each plot and the average of the two expressed as percentage is reported as PAR intercepted by the crop canopy.
Leaf Area Index (LAI) and above Ground Biomass
Plants from 0.5-m row were harvested from each plot for estimating LAI and above ground biomass at the R1 stage (Fehr et al., 1971), when the crop was at peak vegetative to early flowering phase. The leaf area was determined with leaf area meter (LI-COR 3100, LI-COR Inc., Lincoln, NE) and LAI was calculated as the ratio of leaf area to land area.
Fresh Green Seed Yield and Yield Components at the R6 Stage
Each year, plants from 0.5-m rows were sampled from each plot when the crop was at the R6 stage [when the pods are fully developed but still green and immature with seeds still green and about 80% matured (Fehr et al., 1971)] to determine the green seed yield and yield components. The harvested plants were separated into stems, leaves, and pods. Fresh weight of the pods with seeds were recorded. The pods were then shelled to determine shell and green seed fresh weights. The fresh weight of 100 green seeds was recorded and number of seeds per pod and per per square meter were computed. All seed yield data were expressed on fresh weight basis.
Determination of Oil, Protein, Glucose, and Phytate
Fresh green seeds harvested at the R6 stage were packed in plastic bags and shipped overnight in frozen condition to Virginia State University for the determination of green seed compositional traits. Moisture content was determined by drying the seeds at 105 to 110°C in an air oven (Fisher Isothermal Oven model 350) to a constant weight and then percent moisture content was calculated. Five grams of fresh green seeds were homogenized in a mixture of hexane and isopropanol (3:2 v/v) to extract oils at room temperature as described previously (Mohamed et al., 1995). After removal of the solvent under nitrogen, the oil was weighed. The remaining defatted meal was placed in the oven at 60°C overnight to remove excess solvents, then the dried meal was used to determine total concentrations of protein, glucose, and phytate. A BÜCHI 430 digestor was used for the digestion process using H2O2/H2SO4 (4:1 v/v). Total N2 content was determined by means of the Nessler reagent (AOAC, 1984). The protein content was calculated by multiplying the total nitrogen by a factor of 6.25. Total phytate was extracted according to the procedures reported by Mohamed et al. (1986). The phytate was then purified on an anion exchange resin (Dowex - 1x 8 Cl-) to remove inorganic phosphorus, di-, tri-, and tetraphosphate inositol. Total soluble sugar from the meal was extracted with deionized H2O. Protein was eliminated from the extract by the addition of acetonitrile to a final concentration of 50%, followed by centrifugation. Then, acetonitrile was eliminated and concentrated samples were diluted and used for sugar analysis by the phenol sulfuric acid assay (Dubois et al., 1956) and absorbance measured at 490 nm. Glucose is the second major carbohydrate (Tsou and Hong, 1991) and is a good indicator of sweetness of vegetable soybean. Therefore, glucose was used as a standard. All seed compositional traits have been expressed on dry weight basis.
Statistical Analysis
The data were subjected to statistical analysis with SAS software [SAS, 2000]. PROC MIXED analysis was carried out using Year and Year x Cultivar as random effects. Year x Cultivar was used to test differences between cultivars. The important yield contributing components were determined by stepwise regression analysis.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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150 mg seed-1). Among control cultivars, Ware is a large-seeded cultivar adapted to southeastern USA and Hutcheson is an elite U.S. cultivar belonging to MG V. It produced consistently high yields across four locations in the southeastern USA in a concurrent study (Rao et al., 2002).
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Interception of Photosynthetically Active Radiation
The interception of PAR by the 14 genotypes varied between 82 and 90% of that incident upon the crop at R1 stage (Fehr et al., 1971) (Table 2). All genotypes intercepted similar levels of PAR. Although the genotypes produced an average LAI of 4.8, the interception of PAR was less than 90% of that incident upon the canopy. The Japanese cultivars, were observed to have planophyll leaf architecture, which usually results in poor interception as the larger horizontally disposed leaves restrict light penetration to lower layers of the crop canopy.
Biomass
The mean biomass at R1 stage varied between 287 g m-2 for PI 416981 and 385 g m-2 for Akiyoshi on dry weight basis (Table 2). The PI 416981, which had a lower LAI and percent intercepted PAR also produced smaller amount of biomass than Akiyoshi, Tambagura, and Shangrao Wan Qingsi. The Japanese genotypes, which produced higher LAI, also produced more biomass than the U.S. cultivars. Generally, later flowering cultivars also produce greater biomass than early flowering cultivars, but in this study, such a relationship was not clear.
Fresh Green Seed Yield and Yield Components at R6 Stage
Number of Days to R6 Stage
There were significant genotypic differences for days to achieve R6 stage, when the green pods could be harvested (Table 3)
. The average number of days from planting to R6 stage ranged from 99 (Mian Yan) to 134 (Tambagura). The two early flowering cultivars, Mian Yan and Ware, also achieved R6 stage significantly earlier than all other genotypes, except Tousan-122, Guanyun Da Hei Dun, and Hutcheson. The genotypes that achieved the R6 stage within 120 DAP may be categorized as early. PI 417427, Tomahomare, and Houjaku achieved the R6 stage significantly earlier than Tambagura but significantly later than Mian Yan. These genotypes which took about 120 to 121 d to reach R6 stage could be categorized as medium range, whereas remaining genotypes, Akiyoshi, Tambagura, Shangrao Wan Qingsi, PI 181565, and PI 200506 which attained the R6 stage after 124 DAP may be classified as late under Georgia conditions. Genotypes Mian Yan and Ware may be better suited for planting early in the season as a short season cash crop. Planting early and late maturing genotypes in a sequence will enable the farmer to market fresh vegetable soybean over a longer duration.
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Number of Fresh Green Pods
The mean number of green pods across years ranged from 890 for Tambagura to 1552 for PI 181565 (Table 3). PI 181565, Hutcheson, and Shangrao Wan Qingsi produced significantly greater number of pods than all other genotypes. Tambagura, Tousan - 122, and Guanyun Da Hei Dun produced fewer pods than most other genotypes.
Fresh Green Pod Yield
The fresh green pod yield varied between 14.6 Mg ha-1 and 21.7 Mg ha-1 and the differences among the genotypes were significant (Table 3). Tambagura had the highest mean pod yield among the genotypes whereas Ware produced the lowest. The average green pod yield across genotypes was 18.5 Mg ha-1. The green pod yield of Tambagura, PI 181565, Tomahomare, and Shangrao Wan Qingsi was in excess of 20 Mg ha-1 and differed significantly from that of Ware. The fresh green pod yield of the rest of the genotypes did not differ significantly. PI 181565, Shangrao Wan Qingsi, and Tomahomare which produced more pods than most other genotypes also had high pod yields. Tambagura produced significantly higher pod yield than PI 416981, PI 417427, and Ware but similar to all other genotypes. Although Hutcheson also produced a similar number of pods as the higher yielding genotypes, its pod yield was considerably lower perhaps because of its smaller seeds and earlier maturity. On the other hand, Tambagura produced high pod yield, although it had significantly fewer pods than PI 181565, Shangrao Wan Qingsi, Hutcheson, and Tomahomare because of its heavier seeds.
The pod yields of genotypes tested in this study were considerably higher than those reported for vegetable soybean breeding lines in Taiwan (Shanmugasundaram et al., 1991). In this study, the mean pod yield ranged from 15 to 22 Mg ha-1 compared with 10 to 13, 6 to 9, and 6 to 10 Mg ha-1 during spring, summer, and autumn seasons, respectively, in Taiwan. This could be due to the planting of Group V cultivars in the tropics (Taiwan), which normally results in low plant height and low yields. For example, edamame cultivar Blueside grown in Taiwan is of MG V (T.E. Carter, Jr. 2001. Personal communication). The vegetable soybean improvement program at AVRDC has reportedly increased potential pod yields of some Taiwanese edamame varieties to about 24 Mg ha-1 (Shanmugasundaram and Yan, 1999). Konovsky et al. (1996) evaluated 36 edamame genotypes composed of 32 Japanese, three U.S., and 1 Taiwanese genotypes for heritability of yield and quality traits in Washington and reported gross yields ranging from 11.2 to 13.6 Mg ha-1 and net yields of around 7.2 to 8.4 Mg ha-1.
Number of Fresh Green Seeds
Tambagura had the lowest number of green seeds per pod, whereas Hutcheson had significantly more seeds per pod than all other genotypes (Table 3). Tambagura retained significantly fewer seeds per pod than did Ware, Tousan 122, PI 181565, PI 416981, and Hutcheson. The number of seeds per pod is one of the important quality characteristics that determine the marketability and profitability of edamame. Pods with more than two seeds are generally preferred and fetch premium prices in the Asian markets (Shanmugasundaram et al., 1991). In this study, only Hutcheson retained more than two seeds per pod. The PI 181565 had significantly greater number of seeds per pod than Tomahomare, Guanyun Da Hei Dun, and Tambagura but was similar to the rest. A similar number of seeds per pod was reported for several vegetable soybean genotypes grown in Virginia (Mebrahtu et al., 1997) and Washington (Konovsky et al., 1996). The number of seeds per pod and seed weight are generally negatively related as they compete for the same resources. A compensatory mechanism between number of seeds per pod and seed weight may have been operative in the present study since Hutcheson with smaller, lighter seeds could retain more seeds per pod than vegetable soybean genotypes which produced heavier seeds.
The number of seeds per pod is one of the yield determinant of soybean. Shanmugasundaram et al. (1991) reported a significant linear relationship between seed fresh weight and pod length (r2 = 0.674) and pod width (r2 = 0.689) suggesting that these two physical characteristics could be useful selection criteria for breeding vegetable soybeans with more seeds per pod.
The number of seeds per unit area is a function of number of pods per unit area and is an important yield determinant. Hutcheson produced a significantly greater number of seeds than all other genotypes except PI 181565 (Table 3). Tambagura which produced fewer pods and fewer number of seeds per pod also had fewer number of seeds per square meter than half of the tested genotypes. PI 181565 produced number of seeds per square meter similar to Shangrao Wan Qingsi, PI 416981, and Tomahomare, but significantly more than the rest of the genotypes.
Fresh Green Seed Weight
The mean green seed fresh weight ranged from 315 mg seed-1 for Hutcheson to 948 mg seed-1 for Tambagura (Table 3). Tambagura, Tomahomare, Akiyoshi, Shangrao Wan Qingsi, Guanyun Da Hei Dun, PI 200506, and Houjaku had a mean seed fresh weight above 500 mg seed-1. The green seed fresh weight of Tambagura was three times greater than that of Hutcheson and it was twice that of most other genotypes.
Although, Tambagura produced fewer pods and fewer seeds per pod, it had higher pod yield because of heavier seeds. On the other hand, Hutcheson had a greater number of pods and seeds per pod, but its pod yield was not as high as many other genotypes because of smaller seeds. Seed fresh weight is an important yield determinant and quality parameter that determines consumer acceptability (Shanmugasundaram et al., 1991; Mbuvi and Litchfield, 1995). Generally, seed quality characteristics achieve their peak levels when the seed size is also at its maximum. The mean fresh seed weight of the genotypes tested in this study with the exception of Hutcheson and Ware, was higher than that reported for vegetable soybean cultivars in Taiwan (Chen et al., 1991). However, Shanmugasundaram et al. (1991) reported higher seed fresh weights for some vegetable soybean breeding lines in Taiwan.
Green Seed Yield
All genotypes produced high fresh green seed yield which ranged from 7.3 for Ware to 11.6 Mg ha-1 for Tomahomare (Table 3). Ware produced a significantly lower green seed yield than Tomahomare, Shangrao Wan Qingsi, PI 181565, and Tambagura, but was similar to the rest of the genotypes. Tomahomare, PI 181565, Tambagura, Shangrao Wan Qingsi, and Hutcheson had mean green seed yield of above 10 Mg ha-1. The overall mean seed yield across genotypes was 9.6 Mg ha-1. Variations in the mean green seed yield between genotypes could be attributed to variation among yield components. Hutcheson produced greater number of fresh green pods and seeds m-2 which contributed to its high fresh green seed yield. In Tambagura it was mainly the seed fresh weight that resulted in higher fresh pod weight and green seed yields. In Guanyun Da Hei Dun, Tomahomare, and Shangrao Wan Qingsi it was a combination of number of green pods and seeds per square meter and green seed fresh weight that was responsible for higher green seed yield than many of the genotypes. Ware, Tousan-122, and Mian Yan, which had relatively fewer pods and seeds, and lighter seeds also had lower green seed yields.
Seed Compositional Traits
The oil content ranged from 130.7 to 155.8 g kg-1 on dry weight basis (Table 4)
. The PI 416981, Akiyoshi, and Hutcheson had a significantly greater oil content than Tomahomare, Houjaku, Tambagura, and Shangrao Wan Qingsi. The mean oil content was about 29% lower than that (20 g kg-1) of mature commodity soybean seed (Burton, 1990). The oil contents of the genotypes in this study were lower than those reported for selected Japanese soyfood cultivars (Brar and Carter, 1993).
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The glucose content ranged from 60.3 to 74.0 g kg-1 (Table 4). Tsou and Hong (1991) reported that the sugar content of vegetable soybean in Taiwan ranged from 7.35 to13.71 mg g-1. Hutcheson had a significantly higher glucose content than Guanyun Da Hei Dun, PI 181565, Akiyoshi, PI 417427, Tomahomare, and Mian Yan. Whereas, Guanyun Da Hei Dun had a significantly lower glucose content than Tousan - 122, Ware, PI 416981, Hutcheson, Tambagura, and Shangrao Wan Qingsi. Although Hutcheson had a higher seed glucose content than many genotypes, it does not qualify as a vegetable soybean since it has small seeds and does not possess pod and seed physical traits that characterize vegetable soybean (Shanmugasundaram, 1991). The total soluble sugar content of the fresh green beans is an important nutritional trait that directly influences the organoleptic properties of edamame and determines consumer acceptability. The sugar content of soybean is considered to be at its peak level at the R6 stage (Masuda, 1991).
Phytate, calcium–magnesium–potassium salt of inositol hexaphosphoric acid, commonly known as phytic acid occurs in certain cereal and legume seeds (Reddy et al., 1982), including soybean (Mebrahtu et al., 1997). Phytate is the main source of phosphorus in soybean seed and is known to form complexes with phosphorus, proteins, and minerals such as Ca, Mg, Zn, and Fe (Reddy et al., 1982). This reduces the bioavailability of these minerals, affect seed germination and seedling growth, and cause deficiencies in nonruminant animals. In this study, the mean phytate content was 12.6 g kg-1 and ranged from 10.8 to 13.9 g kg-1 (Table 4). Tambagura and Haujaku had relatively lower phytate content than Akiyoshi, PI 200506, Guanyun Da Hei Dun, Tomahomare, and Mian Yan. The rest of the genotypes did not differ significantly. Phyic acid content of certain cereals and legumes has been reported to vary between 1.4 and 20.5 g kg-1 (Reddy et al., 1982). The phytate content of the genotypes studied here were considerably lower than those reported for several vegetable soybean genotypes harvested at R6 stage in Virginia (Mebrahtu et al., 1997).
The moisture content of fresh green seeds ranged from 539 to 561 g kg-1, but the differences between genotypes were not significant (Table 4). Seed moisture content is another critical factor that affects time of harvest and is an integral part of organoleptic characteristics of vegetable soybean (Mbuvi and Litchfield, 1995).
Relationships among Yield Contributing Parameters and Seed Yield
R1 Stage
The intercepted PAR was significantly correlated with leaf area index (Table 5)
. Biomass at the R1 stage was significantly correlated with LAI. Thus, LAI was critical for both interception of PAR and biomass production. Energy absorption into the plant system is in part dependant upon the total LAI and in part on the interception of PAR by the leaf canopy. In soybean, it has been shown that the maximum photosynthetic rate is partly dependant upon the distribution of radiation within the canopy. Thus, canopy structure is critical for both interception of PAR and its absorption by the plant for photosynthesis. Most of the interception of PAR by soybean canopy occurs at the top leaf surface (Sakamoto and Shaw, 1967) because of planophyll orientation of leaf architecture of soybean. Intercepted PAR, at least, during the pre-anthesis phase has been shown to be linearly related to LAI and dry matter accumulation in soybean (Shibles and Weber, 1965). In soybean, maximal interception of PAR has been reported to occur at a LAI of 3.0 and maximal dry matter production as the LAI approaches 4.0. In the present study, most of the genotypes achieved a mean LAI of 4.0 or greater which may have been adequate for maximal interception of PAR and higher levels of photosynthesis. This may have been the reason for high biomass at final harvest.
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Among seed compositional traits, oil and protein contents were negatively correlated (Table 5). Vegetable soybean seed oil and protein content have been shown to be negatively correlated. Oil and protein were both negatively correlated with glucose and phytate contents. Shanmugasundaram et al. (2001) reported similar negative correlations based on data from 20 trials carried out during different seasons between 1996 and 1999 in Taiwan. At R6 stage the seed sucrose contents are high and oil content low (Masuda and Harada, 2000).
Fresh Green Seed Yield Determining Components
The differences in maturity groups may have been an important factor influencing fresh green pod and seed yields. Stepwise regression analysis of the yield components, excluding maturity group, indicated that at the R6 stage, fresh pod weight was the major determinant of yield with an R2 value of 0.88 followed by number of seeds per square meter, 100-seed fresh weight, and seeds per pod in that order of importance (Table 6)
. Path analysis of data from several experiments comprising a range of agronomic treatments showed seed number as the most important yield determinant of soybean grown in southeastern USA (Board et al., 1997).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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Received for publication October 11, 2001.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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, country of origin, pedigree, seed dry weight, and maturity group of soybean genotypes used in this study.