Diversity in shape and size of soybean seeds
The results of the analysis of variance showed that there were significant differences in the characteristics of seeds, including length, width, thickness, length-width ratio, length-thickness ratio, width-thickness ratio and weight of 100 seeds (Table 2). This shows that in our study soybean lines were significantly affected in all observed characters. All characters observed have a relatively low coefficient of variation, indicating that the data from each replication is relatively consistent.
The difference in seed length, seed width and seed thickness affects the ratio of the three seed characteristics. As a result, there were variations in the shape and size of seeds of 91 soybean lines tested (Table 3). Based on the ratio between length and width of the seeds, there were two observed seed shapes; namely round (69 lines) and ellipse (22 lines). While based on the weight of 100 seeds, there are two groups of seed sizes, namely 64 large seed lines (≥ 14.00 g) and 27 medium seed lines (≤ 13.99 g).
The form of soybean seeds is one of the important characters for the food industry in several countries, including in the USA and China
(Salas et al., 2006; Xu et al., 2011). Consequently, selection activities in soybean breeding program abroad are more focused on the seed shape compared to the seed size. Selection of seed shapes using individual selection methods can be done in early generation populations
(Cober et al., 1997). The form of soybean seeds is a polygenic characteristic that is controlled by many genes and shows stability in various environments
(Song et al., 2004). This characteristic is also inherited because it has a high heritability, ranging from 0.42-0.88
(Salas et al., 2006; Xu et al., 2011).
In Indonesia, soybean farmers are more concerned with seed size than seed shape. Seed size is one of the characters used by farmers and soy-based food industries in determining which soybean varieties are to be planted. The soybean processing industry in Indonesia for tofu and tempeh prefers imported soybeans because the seed size is bigger than local soybeans
(Ginting et al., 2009).
The soybean breeding program to obtain large seeded soybean varieties in Indonesia has a good chance of success. Our study found 64 of the 91 soybean lines tested (70.33%) belong to the large-seeded soybean group (Table 3). More of the large seeded lines than medium seeded lines were found among the soybean populations tested. This is because the three populations were obtained from crosses involving large seeded soybeans. These large varieties are Grobogan (18 g/100 seeds) and Argomulyo (16 g/100 seeds). Other studies using different populations have also found similar results. The majority of early-generation lines (98%) obtained from a cross between a Madagascar soybean variety and one from Indonesia produced large seed sizes (Sulistyo and Purwantoro, 2018).
The seed size illustrated by the weight of 100 seeds is an important factor in efforts to increase soybean production
(Johnson et al., 2001). Seed size has been known to be genetically inherited
(Brian et al., 2002; Kumar et al., 2020) and has a positive direct effect on soybean yield (El-Badway and Mehasen, 2012).
Genetic parameters
The genotypic coefficient of variation (GCV) of soybean seed characteristics is shown in Table 4. The results showed that almost all the seed characters had a low GCV (<0.10) and only one seed character was classified as moderate GCV (0.10<GCV<0.20), namely weight of 100 seeds. Previous studies gave different results. Some studies report that the weight of 100 seeds has a low GCV
(Dhillon et al., 2005; Ghodrati, 2013) and some studies show that the weight of 100 seeds has a moderate GCV (Aditya
et al. 2011;
Chandrawat et al., 2017).
Although all of the seed characteristics observed had a low GCV, but all of the seed characters have a wide genetic diversity (Table 4). The wide genetic diversity of length, width, thickness, length-width ratio, length-thickness ratio, width-thickness ratio and weight of 100 seeds are caused by different genetic backgrounds of the three populations tested.
An estimate of the values of heritability for each seed characteristic is shown in Table 5. The results show that the weight of 100 seeds had the highest heritability (0.84), while the ratio of seed length to width had the lowest heritability (0.42). Based on the classification of heritability proposed by McWhirter (1979), then almost all the characteristics of soybean seeds in our study were classified as high heritability (h
2 ≥ 0.50). The moderate heritability (0.20 ≤ h
2 ≤ 0.50) is only found in the ratio of seed length to width.
The estimated value of heritability obtained in our study is similar to previous studies. The heritability of soybean seed shape (width-length ratio and thickness-length ratio) is relatively high, respectively with values of 0.49 and 0.76 (offspring-parent method) and 0.59 and 0.79 (narrow sense heritability)
(Cober et al., 1997). Higher heritability of seed characteristics, which is between 0.92 and 0.98, was reported by
Hu et al., (2013). The results of our study indicate that soybean seed characteristics are inherited, because genetic factors play a role more than environmental factors which are reflected in high heritability. Inheritance of the length, width and thickness of the seeds is reportedly controlled through the cytoplasmic gene
(Liang et al., 2005).
The heritability values of soybean seed characteristics obtained in our study showed that the seed size reflected from weight of 100 seeds has the highest heritability (Table 5). This is likely influenced by the length, width and thickness of the seeds which also have high heritability. These three seed characters reportedly have a positive correlation with the weight of 100 seeds
(Liang et al., 2016). However, the shape of seed does not determine the seed size. The results of correlation analysis showed that there was no significant correlation between the ratio (length-width, length-thickness and width-thickness) and the weight of 100 seeds. This is because the shape and size of soybean seeds are controlled by different genes
(Hu et al., 2013).