A strong negative trade-off between seed number and 100-seed weight stalls genetic yield gains in northern Japanese soybean cultivars in comparison with Midwestern US cultivars
Introduction
Soybeans (Glycine max [L.] Merr.) are major sources of animal feed and vegetable protein and oil, and contribute significantly to the world’s food supply. In the United States (US), on-farm soybean yield has increased steadily. From the 1960 s to the 2010 s, mean soybean yield has increased from 1.68 t ha–1 to 3.12 t ha–1 (Fig. S1; FAO, 2021). The genetic gain of cultivars during breeding accounts for about half of the on-farm yield increase in the US. The other half is believed to be the result of improved agronomic management practices and synergistic interactions between genetics and agronomics (Rowntree et al., 2013, Rowntree et al., 2014, Suhre et al., 2014). In contrast, on-farm yields in Japan are much lower than in the US and have increased much more slowly (Fig. S1; FAO, 2021). The average yield in the 2010 s was only 1.65 t ha–1 and the average increase since the 1960 s has been less than 0.3 t ha–1. There are several possible reasons for the low soybean yield in Japan. First, low yields in Japan can be attributed to unfavorable conditions, such as a rainy season during early vegetative growth and drought stress after the rainy season (Kumagai et al., 2018, Matsuo et al., 2016). In fields converted from paddy rice, excessive soil moisture due to poor drainage and a high-water table often inhibit soybean germination and vegetative growth, which is a major constraint in Japanese soybean production because more than 80% of soybeans are grown in fields converted from paddy rice (Matsuo et al., 2013). Second, cultivation conditions have not been optimized, even though researchers have shown that yields improved with changes in sowing dates and planting densities (Kumagai, 2018, Kumagai, 2021, Matsuo et al., 2015). Finally, a delay in the introduction of modern high-yielding soybean cultivars may be one reason for the low yield in Japan. Although Japanese cultivars show significant genetic yield variation (e.g., Kumagai, 2021), we are unaware of any systematic comparisons of how breeding has impacted seed yield in Japan.
Seed number and seed mass (100-seed weight) are two fundamental yield components of soybean. US soybean breeding programs have focused on optimizing these yield components and the seed oil and protein contents. Increases of 100-seed weight due to breeding have been inconsistent (Boehm et al., 2019, Rincker et al., 2014, Rowntree et al., 2013, Tamagno et al., 2020), suggesting that the increase in seed number contributed more to the genetic yield gain. In recent years, soybeans have accounted for more than 80% of the edible vegetable oils and fats consumed in the US (Shurtleff and Aoyagi, 2016). With the breeding of US cultivars, seed oil content has increased, and seed protein content has decreased (Rincker et al., 2014, Rowntree et al., 2013). Previous studies in the US have found relationships between seed protein and yield and protein and oil content in a set of historical cultivars, and concluded that breeding for improved yield has reduced seed protein content and increased oil content (Rincker et al., 2014, Rogers et al., 2015, Ustun et al., 2001, Voldeng et al., 1997).
On the other hand, most domestic soybean cultivars in Japan are used for traditional food products such as bean curd (tofu), boiled beans (nimame), fermented steamed beans (natto), and fermented steamed bean paste (miso). The quality required for food -grade soybeans depends on the type of food. Seeds with a high protein content (>45%), a low oil content, and a high sucrose content are most suitable for making tofu (Jegadeesan and Yu, 2020). Seed mass is one of the most important traits affecting the processing quality of food-grade soybeans. Cultivars with large seeds (more than 30 g per 100 seeds) have been used for tofu and nimame. Seed mass is positively correlated with the fresh tofu yield because the ratio of insoluble seed coat to seed volume is lower in cultivars with large seeds than in cultivars with small seeds (Cui et al., 2004). Therefore, the Japanese soybean breeding program for increasing farmer productivity and profitability needed to focus on improving these qualities, in addition to improving yields. In the US, many researchers have examined changes in yield and the seed oil and protein contents associated with past breeding, but no studies have examined these changes in Japan. The genetic diversity of Japanese cultivars is narrow, and the use of genetic resources from other countries is recognized as an effective strategy to improve Japanese cultivars (Kaga et al., 2012). Several Japanese studies have reported clear differences in seed yield, yield components, and seed oil and protein contents between Japanese and US soybean cultivars (Kawasaki et al., 2016, Matsuo et al., 2016). From a broad perspective, comparing traits of cultivars from the US and Japan, which have different genetic backgrounds, yield components, and seed oil and protein contents, will be useful in identifying the bottleneck in Japanese cultivars and indicating future directions for breeding in Japan.
To increase yield potential, it’s necessary to directly select for seed yield, but also to identify agronomic and physiological traits related to seed yield. From a physiological perspective, yield potential results from the interception and conversion of solar radiation into biomass, which are called the radiation interception efficiency (RIE) and radiation use efficiency (RUE), respectively, and the subsequent efficiency of photosynthate allocation to reproductive organs or harvest index (HI) (Monteith, 1977). Therefore, it is important to understand the genetic differences in these three efficiencies and their contribution to yield (Lopez et al., 2021). Koester et al. (2014) reported that in the Midwestern US, yield gains in modern cultivars compared with older cultivars could be attributed to improved RIE, RUE, and HI. Kawasaki et al. (2016) also reported that in western regions of Japan, US cultivars had better yield than Japanese cultivars due to their higher crop growth rate (CGR) and higher RUE during the seed-filling stage. However, we found no studies of whether the three efficiencies have been improved by soybean breeding in Japan.
The objectives of the present study were (1) to estimate genetic gains in soybean yield and agronomic traits in Japan, and (2) to determine whether Japanese cultivars are inferior in yield and agronomic traits compared to US cultivars, and if so, which traits should be improved in Japanese cultivars. To answer these questions, we conducted a 4-year field trial to compare the yield, yield components, and seed protein and oil contents of soybean cultivars released since the 1950 s in northern Japan and the Midwestern US.
Section snippets
Cultivars and field experiments
Two experiments were conducted at Tohoku Agricultural Research Center in Morioka, Iwate Prefecture, Japan (39°44′N, 141°7′E) in 2016, 2017, 2019 and 2020. Different fields were used in each year. Table S1 summarizes the soil fertilities. The soil in each field was an Andosol. The fields received 3 g m–2 of N, 12.5 g m–2 of P (P2O5 equivalent), and 5 g m–2 of K (K2O equivalent) in the form of a compound fertilizer 1 day before sowing in each year. Fertilizers were applied and incorporated to a
Environmental characteristics
Table 1 summarizes the meteorological conditions during the four seasons. In 2016, the 5-month (June to October) mean temperature was 0.7 °C higher than the 30-year average. The mean temperature was 1.2 °C and 2.1 °C higher in August and September, respectively, than the 30-year average. In 2017, the 5-month temperature was the same as the 30-year average. However, the mean temperature was 2.3 °C higher in July and 1.3 °C lower in August. In 2019, the 5-month mean temperature was 1.5 °C higher
Discussion
Our goal was to estimate the extent to which the past 60 years of breeding has improved yield and agronomic traits in the group of cultivars grown in the Tohoku region of Japan, in comparison with the group of soybean cultivars grown in the Midwestern US, whose yields have been steadily improving. We also set out to identify the factors responsible for any differences. We found that regardless of a cultivar’s YOR, the Japanese cultivars had a greater 100-seed weight than the US cultivars (Fig. 1
Conclusion
The present study revealed that increased seed number contributed strongly to the genetic yield gain in new cultivars in the Midwestern US at the expense of 100-seed weight. Moreover, increased total node number was the most important contributor to genetic improvement in seed number. On the other hand, we found no significant difference in yield between the old and new cultivars in the Tohoku region of Japan. Thus, we found no evidence of significant genetic yield gain by recent breeding in
CRediT authorship contribution statement
Etsushi Kumagai: Writing – original draft, Conceptualization, Visualization, Formal analysis, Investigation. Takayuki Yabiku: Writing – review & editing, Investigation. Toshihiro Hasegawa: Writing – review & editing, Investigation, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was funded by a Japanese Society for the Promotion of Science KAKENHI grant [Number 18H02190]. We thank Takashi Sayama of the National Institute of Crop Science and Shin Kato of the Tohoku Agricultural Research Center for providing the seeds of our cultivars. We are grateful to Kaori Hirata, Hisashi Tamura, Eisaku Kumagai, Fumihiko Saitou, Hisaya Tanaka, Hiroshi Kawamukai, Kafumi Segawa, Akihiko Umihata, Koya Yoshida, Yuko Suzuki, Miyuki Nakajima, Mari Murai-Hanano, Mari Namikawa,
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Present address: Institute for Agro-environmental Sciences, National Agriculture and Food Research Organization, Tsukuba, Ibaraki, 305-8604, Japan.