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Analysis of Agronomic Traits of Soybeans Adaptable to Northern Area of the Korean Peninsula

Myoung Ryoul Park1*, Min-Jung Seo1, Hong-Tae Yun1, Yong Hawn Ryu2, Huhn Pal Moon2, Dong Soon Kim3

1Central Area Crop Breeding Research Div., National Institute of Crop Science, Suwon 16429, Korea
2Northern Agriculture Research Institute Inc., Anyang 14034, Korea
3Department of Agriculture, Yanbian University, Jilin 133002, China
Corresponding author: *Myoung Ryoul Park, park5260@korea.kr, Tel: +82-31-695-4047, Fax: +82-31-695-4029
Received July 26, 2019; Revised September 16, 2019; Accepted September 24, 2019.
This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
We have conducted this study to select soybeans adaptable to North Korea using growing degree days (GDD) as a heuristic tool. For analyzing climatic conditions, we assumed that climatic conditions of Longjing, Donggang, and Yeoncheon are similar to the Eastern Coast Northern Zone, Suyangsan Northern Zone, and Suyangsan Southern Zone in North Korea, respectively. The predicted days to flowering of 100 soybeans grown at Longjing, Donggang, and Yeoncheon were in ranges from 7/17 to 7/30, from 7/16 to 7/28, and from 7/30 to 8/10, respectively, while the last date of flowering for completely harvesting were predicted by July 12th, August 2nd, and August 11th, respectively. The predicted days to flowering using GDD were almost similar to the days investigated in the three fields. These results showed that GDD is highly effective in predicting development rates of soybean, such as days to flowering and days to maturity. Although most of the tested soybeans were difficult to adapt to Longjing, we selected 12, 15, and 16 soybean varieties/lines based on the predicted days to flowering and last date of flowering for completely harvesting and yield of the soybeans grown at Longjing, Donggang and Yeoncheon, respectively, for further analyzing effects of the regional characteristics on agricultural traits of the selected soybeans. The 100-seed weight and yield of the selected soybeans grown at Longjing was significantly low compared with those at the other locations. The results implied that the soybeans would be exposed to freezing temperature before R6 stage and the reduced size would contribute to soybean yield loss. Taken together, we concluded that Suwon212, Iksan13 and ‘Shinpaldalkong2’ which were stable in degree of lodging, 100-seed weight, and yield at Donggang and Yeoncheon could be adaptable to Suyangsan Northern Zone, and Suyangsan Southern Zone in North Korea.
Keywords : Soybean, Climatic condition, Growing degree days
INTRODUCTION

The World Food Program (WFP) and FAO in 2006 estimated a requirement of 5.3 to 6.5 million tons of grain in North Korea when domestic production fulfilled only 3.8 million tons (HRW 2006). The yield of crops for food production in 2018 decreased by 3.4 percent from 4.71 million tons in 2017 to 4.55 million tons (RDA 2018). As such, it continues to fall short of the grain requirement for North Korea. Soybean (Glycine max L. Merr.) is one of the main crops for food production in North Korea. Soybean yield, which was around 450,000 tons toward the end of the 1980s in North Korea, was also reduced to 140,000 tons in 2018 (RDA 2018). As of 2013, most households in the country have poor food consumption, and consumption of protein was inadequate (FAO/WFP 2013). Agricultural resources in North Korea limit its agricultural production. Therefore, international aid is needed to overcome famine in the country.

Climatic conditions including mean temperature, accumulated temperature, and precipitation are some of the major factors to estimate development of crops (Thompson 1969; Hofstrand 2011). For this reason, selecting varieties that will be adaptable to the climatic conditions of North Korea is very important. Many researchers have reported that temperature is closely related to agricultural performance of soybean, such as floral development, stem length, 100-seed weight, and yield (Hadley et al. 1984; Downs and Thomas 1990; Hollinger and Changnon 1993). Since most information for selecting varieties of soybean including climatic condition is unknown and difficult to access in North Korea, we have to use heuristic tools or temporary expedient for predicting growth, development, and agricultural performance of crops in the country.

Growing degree days (GDD) is a heuristic tool in phenology and a measure of heat accumulation to predict plant development rates such as flowering and maturity dates. Farmers and agriculturists did not have a model to predict when their soybean crop will mature. Soybean grown in northern (high latitude) regions are limited in growth duration due to the short growing season. Soybean plants need to be mature before the first fall frost date. Therefore, the beginning of the frost period will be considered when calculating growth duration of soybean in northern regions. Akyuz et al. (2017) have estimated needed accumulated growing degree days (AGDD) for adapted soybean maturity groups to reach maturity (R8). The predicted values based on AGDDs were used to create a model that allows producers to select the maturity groups of their cultivar, date of planting, and the nearest station. An equivalent formulation for approximately GDD uses the average of the daily maximum and minimum temperatures compared to a Tbase. In addition, Kumar et al. (2008) reported that a base temperature (Tbase) of 50oF (10℃) should be utilized for soybeans. As an equation:

$GDD=Tmax+Tmin2-Tbase$

It is already known that the GDD for days to flowering of all the soybeans was in a range from 811 to 1,097℃ in South Korea and the lowest GDD from R1 to R6 for completely harvesting of the soybeans was 823℃ (NICS 2017).

We have conducted this study to predict development rates of the tested soybeans and to select soybeans adaptable to North Korea using GDD. In company with these goals, we analyzed agricultural performance of the soybean grown at three stations as a temporary expedient, which are the nearest regions to North Korea, to evaluate the effect of the regional climatic condition on agricultural performance of the soybeans, and to use the results as a basic data for developing new soybean varieties adaptable to North Korea.

MATERIALS AND METHODS

### Analysis of climatic condition

We used climatic data of each filed in normal years (1981-2010) to predict days to flowering of the soybeans and the last date of flowering for completely harvesting, which were obtained from the Chinese Meteorological Administration and the Korean Meteorological Administration. We calculated AGDD using an equation which Kumar et al. (2008) have reported: the known GDD for days to flowering is in a range from 811 to 1,097℃ and the lowest GDD is 823℃ in the Republic of Korea (NICS 2017). Climatic data including mean temperature, precipitation, and duration of sunlight during the growth period of the soybeans at the three experimental fields in 2017 and 2018 were investigated to analyze correlations with agronomic traits of the tested soybeans.

### Experimental field tests

We conducted experimental field tests using 100 soybean varieties/lines at three experimental fields of China [Donggang (39°52′N 124°09′E), Longjing (42°46′N 129° 26′E)] and the Republic of Korea [Yeoncheon (38°06ʹN 127°05′E)] (Fig. 1) in 2017 and 2018 to select soybean varieties adaptable to North Korea. The experimental plots were completely randomized with four replicates by each field for this study. We sowed soybean seeds at Longjing, Donggang, and Yeoncheon on May 3rd, May 18th, and June 10th of 2017, and April 29th, May 12th, and June 20th of 2018, respectively, and harvested according to days to harvest of each variety/line in all three fields. The three fields were treated with a basic granular fertilizer of N: P2O5:K2O = 40:70:60 kg/ha before sowing.

### Investigation of agricultural performances

Agricultural traits related to soybean growth and yield were measured according to the agricultural science technology standards for investigation of research of the Rural Development Administration (RDA), Republic of Korea (RDA 2012). These include: 1) days to flowering (R1), defined as the days to the first bloom on the main stem after sowing; 2) stem length, defined as the length from the cotyledonary node to the top of the main stem; 3) degree of lodging, measured as degree of lodging from 1 to 9 (1: 5%, 3: 6-10%, 5: 11-50%, 7: 51-75%, 9: over 76%); 4) number of nodes on the main stem; 5) number of branches per plant; 6) number of pods per plant; 7) full seed stage (R6), defined as seed is fully filled in pod; 8) days to maturity, defined as the days from R1 to full maturity (95% of the pods reach a mature pod color; stage R8); 9) 100-seed weight, defined as the weight of 100 seeds measured under 13% moisture content; 10) yield (kg/10 a), calculated as the total weight (kg) of seeds harvested in 1 m2 × 1000.

### Statistical analysis

Analysis of variance was conducted with SAS 9.2 (SAS Institute Inc., Cary, NC, USA). Multiple comparisons of the soybeans within the experimental fields were performed using the least significant difference (LSD; P < 0.05).

RESULTS

### Climatic conditions of the experimental fields

In July and August of 2017 and 2018, the months which growth and flowering of the soybeans are highly vigorous, precipitations of all fields drastically increased compared to the other months although precipitation of Yeoncheon in all months was not statistically different. The field in Longjing had the lowest mean temperature by months (Fig. 2).

### Analysis of growing degree days (GDD)

We predicted days to flowering of the soybeans at each field from the known GDD (811-1,097℃) of the soybeans and the normal daily mean temperature of each field. The predicted days to flowering of the soybeans grown at Longjing, Donggang, and Yeoncheon were in ranges from 7/17 to 7/30 (75-88 days), from 7/16 to 7/28 (59-71 days), and from 7/30 to 8/10 (49-61 days), respectively (Table 1). Accumulated growing degree days (AGDD) from the sowing date to the first frost date in the three fields, Longjing, Donggang, and Yeoncheon, were 1,393, 1,730, and 1,790℃. The last date of flowering for complete harvesting was calculated from the lowest GDD (823℃) from R1 to R6 of the soybeans and the normal beginning date of the frost period in each field (Table 1). When we investigated a date which its AGDD reaches the lowest GDD inversely from the beginning date of the frost period in each field as the last date of flowering for harvesting, the date of the three fields, Longjing, Donggang, and Yeoncheon, were July 12th, August 2nd, and August 11th in 2017, respectively (Table 1).

### Agricultural performances of all soybean varieties/lines

All the tested soybeans grown in all three fields in 2017 were classified into a wide range of days to flowering. Most of the soybeans grown in Longjing were classified into days to flowering of over 81 days, while the 65 soybeans grown in Yeoncheon were classified into a range from 46 to 50 days (Fig. 3A, Supplementary Table S1). Stem length of the soybeans at each field also varied by variety/line, and the stem lengths were different for the test fields even though they were the same soybeans. On average, stem length of the soybeans grown at Longjing was longer than the others (Fig. 3B, Table 2 and Supplementary Table S1). Distribution of soybeans grown in Donggang and Yeoncheon were similar in terms of 100-seed weight with a range of 11 to 25 g, but Longjing-grown soybeans were classified into the range of 6 to 10 g, about 3-fold higher than that of the Yeoncheon-grown soybeans (Fig. 3C, Supplementary Table S1). Among the Donggang and Yeoncheon-grown soybeans, number of soybeans which were classified into a range from 2.01 to 2.50 t/ha in yield was the greatest although number of the Longjing-grown which were classified into a range from 1.51 to 2.00 t/ha was the biggest (

### Agricultural performances of the soybeans selected at each field

Among the soybeans which the days to flowering investigated at each field were in ranges of the expected days to flowering and the predicted last date of flowering for complete harvesting using GDD (Table 1) while showing relatively high yield, we selected 15 and 16 soybean varieties/lines grown in Donggang and Yeoncheon, respect ively, as soybeans adaptable to North Korea (Supplementary Table S2). However, all the soybeans grown in Longjing were not in the range of the predicted last date of flowering for complete harvesting (Table 1, Supplementary Table S1). For further analyzing the effect of the regional characteristics on agricultural traits of each soybean variety/line, we additionally selected 12 soybean varieties/lines grown in Longjing for analyzing yield of the soybeans without reference to days to flowering (Supplementary Table S2), then analyzed agronomic traits of the soybeans, Suwon212, Iksan13, ‘Seonnogkong’, ‘Gangil’, ‘Shinpaldalkong2’, and ‘Joyang 1’, which were simultaneously selected at the three fields (Supplementary Table S2).

Days to flowering and stem length of the 6 selected soybeans grown at Longjing among the fields were the highest (Fig. 4A and 4B, Supplementary Table S2). All of the 6 soybeans selected from Longjing were lodged but a part of those at Donggang or Yeoncheon were not at all (Fig. 4C). Numbers of nodes on the main stem and branches per plant of the soybeans selected at Yeoncheon except ‘Shinpaldalkong2’ were lower (Fig. 4D and 4E, Supplementary Table S2) than those of the Longjing-selected, but number of pods per plant of each soybean was statistically not different by the fields (Fig. 4F, Table 3 and Supplementary Table S2). The soybeans selected at Yeoncheon among the fields were the highest in 100-seed weight (Fig. 4G, Table 3 and Supplementary Table S2), but in yield, except for ‘Seonnogkong’ and ‘Joyang 1’ (Fig. 4H).

DISCUSSION

### Correlation between the experimental fields and agricultural zones of North Korea

As we have taken together the report of NAS (2012) and analysis of climatic condition, average temperature and AGDD, of the three fields, we assumed that climatic characteristics of Longjing, Donggang, and Yeoncheon are similar to those of the Eastern Coast Northern Zone, Suyangsan Northern Zone, and Suyangsan Southern Zone among the agricultural zones of North Korea (Fig. 1), respectively.

### Analysis of adaptability of the soybeans using growing degree days

Growing degree days (GDD), also called growing degree units (GDU), are dependent upon the minimum (Tmin) and maximum (Tmax) temperatures which affect the plant’s growth. The effect of GDD to predict soybean development has been compared to other potential prediction models with mixed results (Ramesh and Gopalaswamy 1991; Kumar et al. 2008). Ramesh and Gopalaswamy (1991) have found that GDD is superior to other methods. Our results also confirmed the GDD accuracy that the predicted days to flowering (Table 1) was almost similar to the days investigated in the three fields (

As we analyzed AGDD for growth duration of the soybeans, we could confirm that AGDD of Longjing was very low than those of Yeoncheon and Donggang (Table 1), meaning which soybean varieties/lines adaptable to Longjing will be highly different with those of two fields. Moreover, the result using GDD to predict the last date of flowering for harvesting showed that most of the soybeans are difficult to be adapted to Longjing.

### Effect of regional conditions on the agricultural performances of simultaneously selected soybeans at three fields

Many researchers have reported that agricultural performance of soybean is highly affected by climatic condition (Thompson 1969; Frederick et al. 2001; Kumar et al. 2008). Previous studies proposed an interactive effect of day-length and temperature on soybean floral development, indicating that days to flowering of soybean is controlled by these climatic conditions (Garner and Allard 1930). Despite being the same soybean variety, it has been observed that flowering time varies with environmental condition such as day-length and latitude (Kumar et al. 2008; Park et al. 2016), similarly seen in our results (Fig. 3A and 4A).

In case of stem length, the result was similar to that of the previous study (Park et al. 2016), whereas another study reported that stem length of soybean is positively affected by mean temperature (Downs and Thomas 1990). Especially, our finding was coincident with the previous report that the plant height is positively correlated with lodging in field trial (Xiang et al. 2013).

Although moisture in soybean fields affects the efficiency of photosynthesis (Berlato 1981), leaf area, crop growth rate, and shoot dry matter (Pandey et al. 1984), our results showed that the relatively high precipitation of Yeoncheon in July and August did not result in higher biomass of the soybeans, such as stem length, number of branches per plant. In addition, our result differed from previous results that increased temperature resulted in increased number of soybean nodes (Van Schaik and Probst 1958). Therefore, further research should evaluate effect of each climatic factor by developmental stages of soybean.

Meckel et al. (1984) has reported that number of soybean pods among the yield components is the most sensitive to moisture while seed weight is least affected, although effect of moisture on the two components are different by reproductive stages (Foroud et al. 1993). For our results, we could not assume effects of the regional precipitation on the components because in spite of the same period, reproductive stages were very different according to varieties/ lines. Many researchers have reported that 100-seed weight and yield of soybeans have a similar response to latitude, precipitation, mean temperature, and day-length (Korte et al. 1983; Hollinger and Changnon 1993; Kantolic and Slafer 2007; Saryoko et al. 2017). Our results also showed that the 100-seed weight and yield except for ‘Seonnogkong’ and ‘Joyang 1’ showed a similar aspect according to the experimental fields but Park et al. (2016) reported that yield of soybean grown at the higher latitude was relatively higher than that at the lower latitude. It is well known that seed number and size contributing to yield were significantly reduced by freezing temperatures before the R6 stage of soybean (Saliba et al. 1982). Therefore, as shown in Table 1 and Fig. 4G, the result that the 100-seed weight of the soybeans grown at Longjing being significantly low compared with the others can be closely linked to the fact that most of the soybeans were exposed to freezing temperatures (frost) before the R6 stage. However, we assumed that the reduced size of the soybeans would contribute to yield losses.

Taken together, we indirectly estimated characteristics of some agricultural zones of North Korea, Suyangsan Northern Zone, Eastern Coast Northern Zone, and Suyangsan Southern Zone, and productivity of the selected soybeans in the country. We confirmed that GDD is highly useful as a heuristic tool to select the soybeans that would be adaptable to the country, and most of the tested soybeans were unadaptable at the Eastern Coast Northern Zone of North Korea but adaptable to the Suyangsan Northern Zone and Suyangsan Southern Zone. Especially, we inferred that freezing temperatures before the R6 stage would result in the decreased size and yield loss of the Longjing-grown soybeans. However, we confirmed that Suwon212, Iksan13 and ‘Shinpaldalkong2’ were stable in degree of lodging, 100-seed weight, and yield at Donggang and Yeoncheon and concluded that the soybeans could be adaptable to Suyangsan Northern Zone, and Suyangsan Southern Zone in North Korea.

Supplementary Information
PBB-7-386_Supple.pdf
ACKNOWLEDGEMENTS

This study was supported by Rural Development Administration (RDA) grant PJ012657032019.

Figures
Fig. 1.

The seven agro-climatic zones (I-VII) in North Korea and three experimental fields (●) in China and South Korea; the source for the seven agro-climatic zones (I-VII) in North Korea is retrieved on http://www.gsnj.re.kr/symposium2012/file/symposium2012_ppt_3_3_02.pdf.

Fig. 2.

Mean temperature and precipitation of three experimental fields, Longjing, Donggang, and Yeoncheon, in 2017 and 2018.

Fig. 3.

Distribution of varieties by days to flowering (A), stem length (B), 100-seed weight (C), and yield (D) of 100 soybean varieties/lines grown at three experimental fields, Longjing, Donggang, and Yeoncheon, in 2017.

Fig. 4.

Days to flowering (A), degree of lodging (B), stem length (C), number of nodes on the main stem (D), number of branches per plant (E), number of pods per plant (F), 100-seed weight (G), and yield (H) of the simultaneously selected soybeans grown at three experimental fields, Longjing, Donggang, and Yeoncheon, in 2017 and 2018. The Total is the data from the soybean selected at each field, Longjing (12 varieties/lines), Donggang (15 varieties/ lines), and Yeoncheon (12 varieties/lines), in 2017 and 2018.

Tables

Prediction of days to flowering of soybeans and the last date of flowering for harvesting of soybeans in the three experimental fields, Longjing, Donggang, and Yeoncheon, for growing degree days (GDD).

Experimental field (sowing date)

Longjing (5/03) Donggang (5/18) Yeoncheon (6/10)
Expected days to floweringz) (sowing to flowering) 7/17-7/30 7/16-7/28 7/30-8/10
(75-88 days) (59-71 days) (49-61 days)
The beginning date of frost period 10.03 10.16 10.26
Accumulated growing degree days (AGDD, ℃) 1,393 1,730 1,790
The predicted last date of flowering for completely harvestingy) ∼7.12 ∼8.02 ∼8.11

z)AGDD for days to flowering of 100 soybeans is in a range from 811 to 1,097℃ in the Republic of Korea (NICS 2017).

y)The lowest GDD from R1 to R6 for completely harvesting of 100 soybeans is 823℃ in the Republic of Korea (NICS 2017).

Statistical analysis of agricultural performances of 100 soybean varieties/lines grown at three experimental fields, Longjing, Donggang, and Yeoncheon, in 2017.

Agricultural trait Longjing Donggang Yeoncheon Avg.z) SDy) LSD*
Days to flowering (days) 88.6 66.9 49.6 68.3 19.5 2.93
Stem length (cm) 94.2 71.4 62.2 75.9 16.4 3.67
100-seed weight (g) 13.6 14.5 17.6 15.2 2.1 0.77
Yield (t/ha) 2.12 2.25 2.48 2.28 0.18 0.08

z)Avg.: average.

y)SD: standard deviation.

*LSD: least significant difference; significant at the 0.05 probability level.

Statistical analysis of agricultural performances of the selected soybeans grown at experimental fields, Longjing, Donggang, and Yeoncheon, during two years, 2017 and 2018.

Agricultural trait Longjing Donggang Yeoncheon Avg.z) SDy) LSD*
Days to flowering (days) 84.0 62.6 50.9 65.9 16.8 2.84
Stem length (cm) 89.2 65.1 69.6 74.6 12.4 3.55
Degree of lodging (0-9) 2.6 1.5 0.7 1.6 0.2 0.04
No. of nodes on the main stem (ea) 16.3 14.7 14.5 15.2 1.9 0.95
No. of branches per plant (ea) 5.8 6.0 4.1 5.3 0.9 0.21
No. of pods per plant (ea) 87.4 72.9 64.8 75.0 18.7 ns
100-seed weight (g) 13.5 18.6 20.0 17.4 3.4 0.78
Yield (t/ha) 2.19 2.35 2.59 7.69 0.20 0.08

z)Avg.: average.

y)SD: standard deviation.

*LSD: least significant difference; significant at the 0.05 probability level.

ns: not significant (LSD at P ≥ 0.05).

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