**2. Soybean seed yield**

### **2.1. Potential of soybean yield**

The world average soybean yield has been increasing by about 60% for 30 years from 1980 (1.6 t ha-1) to 2010 (2.6 t ha-1). The highest yield of soybean in Japan was recorded at 7.8 t ha-1, and soybean seed yield can reach 4-6 t ha-1 with well-managed fields under good climatic and soil conditions [4]. Recently, an amazing high soybean seed yield over 10 t ha-1 was re‐ corded in 2008 and 2010 by a farmer, Mr. Kip Cullers in Missouri, USA [5]. Therefore, the potential productivity of soybean should be much higher than we have thought.

Figure 1 shows the yield components of soybean. Soybean "seed yield" is calculated by mul‐ tiplying the "seed number" per area and one average "seed weight". Seed number per area is calculated by the "pod number" and the average "seed number per pod". The pod num‐ ber is decided by "flower number" and the "pod formation rate". The flower number de‐ pends on the "node number". The node number per area is decided by "stem number per plant" and "planting density".

Farmers can control the planting density. Planting density is an important factor for soybean growth and seed yield, although the planting density is not directly proportional to the dry matter production and seed yield. When planting density is high, the branching of each plant is depressed and the number of the lateral stems decrease. In addition, under excess planting density the competition for photosynthesis and nutrient absorption among plants become severe and the stems grow tall and thin and plants are proneto lodge.

**Figure 1.** Yield components in soybean cultivation.

bean seeds is very high about 4-5 times higher than that of rice, wheat and corn, but

A Comprehensive Survey of International Soybean Research - Genetics, Physiology, Agronomy and Nitrogen

The storage protein of soybean seeds mainly consists of glycinin and β-conglycinin. The βconglycinin is comprised of three subunits, designated as α', α, and β-subunits. The β-subu‐ nit of β-conglycinin is especially low in sulfur amino acids, containing only one cysteine and no methionine residue in its mature form. Soybean seeds contain a large amount of lipids (20%), and about 90% is unsaturated fatty acid (linoleic acid 51-57%, oleic acid 32-36% and linolenic acid 2-10%) and 10% is saturated fatty acid (palmitic acid 4-7%, stearic acid 4-7%) [3]. Linoleic acid and linolenic acid are essential lipids, which cannot be synthesized by our‐ selves. Although soybean seeds contain about 28% carbohydrates, most of them are struc‐ tural carbon like cell walls and oligosaccharides (sucrose 5%, stachyose 4%, raffinose 1%). Starch is tentatively accumulated in young immature soybean seed, however, it decreases and converts to lipid and protein at maturity. Soybean seeds contain relatively a high amount of minerals (5%) compared with cereal seeds (about 1%). Soybean seeds contain abundant potassium (1,900 mg), calcium (240 mg), magnesium (220 mg), phosphorous (220 mg), iron (9.4 mg), zinc (3.2 mg) per 100g seeds. Soybean seeds contain vitamins, both lipid soluble vitamins (Vitamin E (1.8 mg)) and water soluble vitamins (V B1 (0.83 mg) and V B2 (0.30 mg)). Soybean seeds contain isoflavonoids, daidzein and genistein. These isoflavonoids

are expected to play a role as a female hormone or to decrease fat in blood.

The world average soybean yield has been increasing by about 60% for 30 years from 1980 (1.6 t ha-1) to 2010 (2.6 t ha-1). The highest yield of soybean in Japan was recorded at 7.8 t ha-1, and soybean seed yield can reach 4-6 t ha-1 with well-managed fields under good climatic and soil conditions [4]. Recently, an amazing high soybean seed yield over 10 t ha-1 was re‐ corded in 2008 and 2010 by a farmer, Mr. Kip Cullers in Missouri, USA [5]. Therefore, the

Figure 1 shows the yield components of soybean. Soybean "seed yield" is calculated by mul‐ tiplying the "seed number" per area and one average "seed weight". Seed number per area is calculated by the "pod number" and the average "seed number per pod". The pod num‐ ber is decided by "flower number" and the "pod formation rate". The flower number de‐ pends on the "node number". The node number per area is decided by "stem number per

Farmers can control the planting density. Planting density is an important factor for soybean growth and seed yield, although the planting density is not directly proportional to the dry matter production and seed yield. When planting density is high, the branching of each plant is depressed and the number of the lateral stems decrease. In addition, under excess planting density the competition for photosynthesis and nutrient absorption among plants

potential productivity of soybean should be much higher than we have thought.

become severe and the stems grow tall and thin and plants are proneto lodge.

carbohydrate concentration is lower.

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116

**2. Soybean seed yield**

**2.1. Potential of soybean yield**

plant" and "planting density".

Generally, soybean seed yield depends mostly on pod number per area. Generally over 50% of soybean flowers and young pods abort and they don't make mature pods and seeds. Un‐ der bad weather and growing conditions, the percentage of flower and pod abortion in‐ creases. The average number of seeds in a pod is relatively constant, although the seed number per pod differs from 1 to 4 in soybean. Average seed weight is affected by growing conditions in late growth stages.

**Figure 2.** Pod number per a plant grown in the same row in Nagakura field in 2011. A: The number shows the original position in the row. B: The number of the plant is sorted from low to high pod number.

The low average yield compared with potential productivity (10 t ha-1) may be due to sever‐ al factors that interfere with maximum growth. First, soybean plants are very susceptible to physical, chemical and biological conditions of the soil as well as climatic conditions. Figure 2 shows the pod number per plant of soybean plants grown in a row. Figure 2A shows the data of the plants at the original position of the row, and Figure 2B shows data are sorted

from low to high pod number. In this row, the highest pod number was about 180 and the lowest was only 18, and the average pod number was about 100 pods per a plant. As shown in Figure 2A, a plant with many pods tended to neighbor to a plant with low pod number. This may be mainly due to competition for solar radiation, and the plant growth is easily depressed by shading of neighbor bigger plant.

Another example of adaptation of soybean plants to environmental conditions is shown in Fig‐ ure 3 and 4. When soybean plants are planted in a small pot, the growth was inferior (Figure 3) and the "dwarf" soybean formed only 3-5 pods with normal seeds. Figure 4 shows an example of "giant" soybean cultivated with a low planting density at 2 plants m-2 [6]. This plant had a very thick basal stem (25 mm diameter), and the dry weight per a plant was 572g (leaves 100g, stems 204g, pods 220g, roots 41g, and nodules 7g). It had 1,874 nodules on the roots. The plant had 17 lateral branches, 178 stem nodes, 600 pods and 1,687 seeds as shown in Figure 5.

**Figure 3.** Soybean plants grown in a small flask.

**Figure 4.** Soybean plants (cv. Williams) cultivated at the density of 2 plants m-2 in Ikarashi sandy dune field in 2000. [6]

**Figure 5.** Characteristics of soybean plant (cv. Williams) cultivated at the density of 2 plants m-2 in Ikarashi sandy dune field in 2000. [6]

### **2.2. Characteristics of soybean growth**

from low to high pod number. In this row, the highest pod number was about 180 and the lowest was only 18, and the average pod number was about 100 pods per a plant. As shown in Figure 2A, a plant with many pods tended to neighbor to a plant with low pod number. This may be mainly due to competition for solar radiation, and the plant growth is easily

A Comprehensive Survey of International Soybean Research - Genetics, Physiology, Agronomy and Nitrogen

Another example of adaptation of soybean plants to environmental conditions is shown in Fig‐ ure 3 and 4. When soybean plants are planted in a small pot, the growth was inferior (Figure 3) and the "dwarf" soybean formed only 3-5 pods with normal seeds. Figure 4 shows an example of "giant" soybean cultivated with a low planting density at 2 plants m-2 [6]. This plant had a very thick basal stem (25 mm diameter), and the dry weight per a plant was 572g (leaves 100g, stems 204g, pods 220g, roots 41g, and nodules 7g). It had 1,874 nodules on the roots. The plant

had 17 lateral branches, 178 stem nodes, 600 pods and 1,687 seeds as shown in Figure 5.

**Figure 4.** Soybean plants (cv. Williams) cultivated at the density of 2 plants m-2 in Ikarashi sandy dune field in 2000. [6]

depressed by shading of neighbor bigger plant.

Relationships

118

**Figure 3.** Soybean plants grown in a small flask.

Optimal planting date for a cultivar in the growing area is very important to get good growth and seed yield. When planting is delayed only a few weeks, the stem length and plant dry matter accumulation at R1 stage may decrease by half. Optimum planting density is also important. In Niigata, the planting density for cultivar "Enrei" is 89,000 plants ha-1 by a single seed planting, which means one seed is planted in each seeding spot, with 75 cm row spacing and 15 cm planting distance in a row. When the germination rate of the seeds is not good, the planting density will decrease. Therefore, multiple seeding is sometimes car‐ ried out to avoid subnormal plant population. However, the total number of nodes, pods and seeds per area are almost the same between single-seeded and multi-seeded planting.

Figure 6 shows the stage description for the vegetative and reproductive growth of soybean proposed by Fehr and Caviness [8]. Soybean seeds germinate in a few days and emergence of seedling occurs about 7-10 days after sowing (PE stage), if soil moisture and temperature are optimum. After cotyledon leaves appear, a pair of unifoliolate leaves unroll (VC stage). During this stage, the storage compounds in the cotyledons support the nutrition for root and shoot growth. Then trifoliolate leaves appear one by one at V1 (the first trifoliolate leaf), V2 (the second trifoliolate leaf) and so on (Vn stage), and the shoot and roots grow during the vegetative stage.

The reproductive growth starts from beginning bloom (R1 stage). Soybean is short-day plant, and they bloom when the day length become shorter than 14 hrs, although it depends on the varieties and planting date. Bloom period lasts for 15-50 days from the beginning to the end of bloom. Full bloom is described as R2 stage. Pod initiation starts (R3 stage) about one month after the R1 stage. Then about one month after R3 stage, the seed begins to en‐ large (R5 stage) to full seed (R6 stage). By one month after R5, the soybean plant starts to become mature (R7 stage). At 2-3 weeks after the R7 stage, the plant dries down to harvest

maturity (R8 stage). Vegetative growth continues after R1 in both determinate and indeter‐ minate soybeans. Vegetative growth of stems and leaves does not stop until about the R5 stage.



**Figure 6.** Stage description for the vegetative and reproductive growth of soybean. (from Clemson Cooperative Exten‐ sion, Home Page)

Root growth starts with a seminal root, which becomes a primary root. The secondary roots are formed from the primary root. The first root nodules are formed on the basal part of the primary roots, and they become visible at about 10 days after planting. They start to fix ni‐ trogen (N2) at about 15-20 days after planting when the diameter reaches about 2 mm [9]. In the later stage, the nodules formed at the basal part of primary roots degrade, and a large number of new nodules form on the lateral roots near the soil surface, and they play an im‐ portant role for supplying N during the pod filling stage.

#### **2.3. Factors affecting soybean yield**

Soybean plants are very susceptible to environmental conditions, such as climatic conditions (solar radiation, day length, temperature, rain fall etc), soil conditions (drought, excess water, pH, soil fertility, mineral nutrition, etc). Secondly, soybean seed yield often severely declines with pests, such as insects, weeds, diseases, and nematodes. Third, nitrogen fixation by the root nodules (Figure 7) with the soil microorganism bradyrhizobia is very important for soybean production [10,11], however, it is difficult to obtain the optimum condition of nodulation and nitrogen fixation. The nodule formation and nitrogen fixation is sensitive to the external factors such as climate, soil properties, pests etc, and internal factors such as competition among plants or competition among organs, pods, leaves, roots and nodules. Therefore, many stress condi‐ tions, such as drought stress, decrease in oxygen supply, a high or low pH, nutrient imbalance etc., may depress nodule formation and nitrogen fixation activity. In addition, low population of compatible bradyrhizobia or the dominance of inefficient strains of indigenous bradyrhizo‐ bia in the field may decrease nitrogen fixation activity. The inoculation of efficient strains of bra‐ dyrhizobia may promote soybean growth and seed yield.

**Figure 7.** A photograph of the nodulated roots of soybean plant (cv. Williams) inoculated with Bradyrhizobium japoni‐ *cum* USDA110 and cultivated in a glass bottle with culture solution.

In Japan, over 80% of soybean cultivation is carried out in rotated paddy fields by block ro‐ tation with rice, because rice has been over produced in Japan. For example, one block of rice field is drained and soybean cultivation continued there for 3 years. Then this field is returned to rice cultivation for the next 3 years. When drainage of water is good and the ground-water level is maintained at lower than 30cm, soybean cultivation will be successful. However, bad drainage of water like in heavy clay soil near Niigata will depress root and nodule development. Therefore, soybean growth and seed yield is very poor. Hosokawa de‐ veloped a new method of a raised planting bed cultivation by changing the blades of a re‐ verse rotary tiller. Soybean plants grow very well in a raised bed due to efficient drainage of water especially after heavy rain fall. Nagumo et al. reported that a higher seed yield has been obtained by raised bed tillage with sigmoidal releasing-type coated urea fertilizer in the rotated paddy field under poor drainage conditions. Respiration of nodules and roots is severely depressed by excess water in soil due to oxygen deficiency. Soybean plant can be cultivated in water culture (Figure 7) when aeration is good. However, under water logging conditions, oxygen deficiency in soil occurs, because diffusion of oxygen through water is very slow compared with gas diffusion through soil, and soil microorganisms respire O2 ac‐ tively under high temperature conditions. It is known that soybean nodule respiration is about 4 times higher than that in roots in order to support nitrogen fixation activity and ni‐ trogen assimilation to ureides. Therefore, low oxygen supply is fatal for nodules.

#### **2.4. Nitrogen assimilation and seed yield**

maturity (R8 stage). Vegetative growth continues after R1 in both determinate and indeter‐ minate soybeans. Vegetative growth of stems and leaves does not stop until about the R5

A Comprehensive Survey of International Soybean Research - Genetics, Physiology, Agronomy and Nitrogen

**Figure 6.** Stage description for the vegetative and reproductive growth of soybean. (from Clemson Cooperative Exten‐

Root growth starts with a seminal root, which becomes a primary root. The secondary roots are formed from the primary root. The first root nodules are formed on the basal part of the primary roots, and they become visible at about 10 days after planting. They start to fix ni‐ trogen (N2) at about 15-20 days after planting when the diameter reaches about 2 mm [9]. In the later stage, the nodules formed at the basal part of primary roots degrade, and a large number of new nodules form on the lateral roots near the soil surface, and they play an im‐

Soybean plants are very susceptible to environmental conditions, such as climatic conditions (solar radiation, day length, temperature, rain fall etc), soil conditions (drought, excess water, pH, soil fertility, mineral nutrition, etc). Secondly, soybean seed yield often severely declines with pests, such as insects, weeds, diseases, and nematodes. Third, nitrogen fixation by the root nodules (Figure 7) with the soil microorganism bradyrhizobia is very important for soybean production [10,11], however, it is difficult to obtain the optimum condition of nodulation and nitrogen fixation. The nodule formation and nitrogen fixation is sensitive to the external factors such as climate, soil properties, pests etc, and internal factors such as competition among plants or competition among organs, pods, leaves, roots and nodules. Therefore, many stress condi‐ tions, such as drought stress, decrease in oxygen supply, a high or low pH, nutrient imbalance etc., may depress nodule formation and nitrogen fixation activity. In addition, low population of compatible bradyrhizobia or the dominance of inefficient strains of indigenous bradyrhizo‐

portant role for supplying N during the pod filling stage.

**2.3. Factors affecting soybean yield**

stage.

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sion, Home Page)

Soybeanplants assimilate a large amount of nitrogen during both vegetative and reproduc‐ tive stages, and the total amount of N assimilated in a plant is highly correlated with the soybean seed yield. One t of soybean seed requires about 70-90 kg N, which is about four

times more than in the case of rice [13]. Soybean plants assimilate the N from three sources, 1) N derived from symbiotic N2 fixation by root nodules (Ndfa), 2) absorbed N from soil mineralized N (Ndfs), and 3) N derived from fertilizer when applied (Ndff) (Figure 8). For the maximum seed yield of soybean, it is necessary to use both N2 fixation and absorbed N from roots [14-15]. When only N2 fixation is available to the plant vigorous vegetative growth does not occur, which results in reduced seed yield. On the other hand, a heavy sup‐ ply of N often depresses nodule development and N2 fixation activity and induces nodule senescence, which also results in reduced seed yield. In addition, a heavy supply of N from fertilizer or from the soil causes luxuriant shoot growth, which result in lodging and poor pod formation. Therefore, for soybean cultivation no nitrogen fertilizer is applied or only a small amount of N fertilizer is applied as a "starter N" to promote the initial growth.

**Figure 8.** Three sources of nitrogen for soybean plants, nitrogen fixed in root nodules, nitrogen absorbed by roots from fertilizer and from soil nitrogen.

In Niigata fields, about 60-75% of N assimilation in soybean was estimated to derive from N2 fixation [16,17]. Figure 9 shows the growth of cultivar "Enrei" and the non-nodulated iso‐ genic line "En1282" planted in the same Nagakura field. It is obvious that non-nodulated soybean grew very poor with pale leaf color due to N deficiency by the lack of nitrogen fixa‐ tion. It is said that the legume nitrogen fixation is variable, but it is a valuable process in world agriculture [18-20].
