**2. Effects of drought on soybean production**

Soybean is among the top 10 of the most widely grown crops, with a total production of over 260 million tonnes in 2010 (FAO data). The cultivated area of soybean occupies more than 100 million hectares worldwide, with about half in the U.S.A. and Brazil (FAO data). Greenhouse and field studies showed that drought stress led to significant reduction in seed yield (24~50%) from distinct locations and time [4, 5].

Numerous efforts have been put to examine the effects of drought and irrigation at various vegetative stages on soybean production. A 2-year field experiment by Brown et al (1985) on 4 determinate cultivars Davis, Lee 74, Sohoma and Centennial demonstrated that mositure stress initiated at R2 or R4 reduced yield significantly [6].

**Types of indices Examples Advantages**

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

Simple; required data can be easily accessed

Straightforward monitoring of trends in the

Better understanding about the interaction between crops and the environment during drought; help determine the influence of drought

Fully consider both crop and soil water supply and

Required spectral data from instruments are easily available for most parts of the world; can monitor

Comprehensive and flexible for monitoring different types of drought in different places

frequency or intensity of events

on the crop growth and yield

vegetation conditions

demand

Simple to apply; more comprehensive

Days without Rainfall

Relative Soil Moisture; Accumulated Drought Index

Aridity Anomaly Index;

Soil Moisture Index

Remote sensing-based Normalized Difference Vegetation Index;

Composite Utilize and incorporate a consolidation

solved problems and possible strategies to tackle them.

**2. Effects of drought on soybean production**

yield (24~50%) from distinct locations and time [4, 5].

**Table 1.** Types of commonly used drought indices.

applied for soybean cultivation.

Soil Moisture Anomaly and Relative

Temperature Condition Index

of indices into a comprehensive one

In addition to the genetic improvement programs for soybean, agronomic practices aimed at minimizing water input, reducing water loss, and increasing plant water usage efficiency have also been developed to cope with the problem of water scarcity. Some of these can be

In this chapter, we will summarize the understandings of drought stress and drought toler‐ ance in soybean from available literatures. We have integrated information from traditional breeding and agronomic measures to molecular aspects of this subject, and highlighted un‐

Soybean is among the top 10 of the most widely grown crops, with a total production of over 260 million tonnes in 2010 (FAO data). The cultivated area of soybean occupies more than 100 million hectares worldwide, with about half in the U.S.A. and Brazil (FAO data). Greenhouse and field studies showed that drought stress led to significant reduction in seed

Warm Spell Duration Index

Palmer Drought Severity Index; Standardized Precipitation Evapotranspiration Index

Precipitation-based Standardized Precipitation Index;

Temperature-based Cold Spell Duration Index;

Precipitation- and temperature-based

Relationships

210

Precipitation, temperature, and soil moisture/ soil characteristics based

Temperature, relative humidity, solar radiation, wind speed, and soil moisture/ soil characteristics

based

An in-depth analysis of the effects of drought at various growth stages on seed yield of soy‐ bean cultivar Douglas was reported by Eck et al (1987) [7]. In their study, yield loss was the most severe when drought stress was applied throughout the seed development period (R5- R7), resulting in a reduction of 45% and 88% respectively in two consecutive years [7]. Besides, Desclaux et al (2000) conducted a comprehensive analysis of yield components when drought stress was applied to soybean cultivar Weber at different developmental stages [8]. In this ex‐ periment, the stress condition was attained by temporally withholding irrigation for 4 to 5 days until the plant available water reduced to 50% or 30% of the normal conditions. The major findings showing various adverse effects of drought were summarized in Table 2. The most se‐ vere effect of this treatment was observed during the seed filling period [8].

On the other hand, Korte et al (1983) conducted a 3-year study on 8 soybean cultivars to as‐ sess yield enhancement by irrigation, using non-irrigated soybean plants as the control group [9]. The experimental groups were irrigated at different developmental stages (one stage or different stages in combination), including the flowering stage (R1-R2), the pod elongation stage (R3-R5), and the seed enlargement stage (R5-R6) [9]. Results of factorial analysis indicated that the yield was sensitive to the enhancement by irrigation, at pod elon‐ gation stage (R3-R4) and the seed enlargement stage (R5-R6) [9]. For 5 cultivars, the en‐ hancement effect by irrigation followed the order: seed enlargement stage (R5-R6) > pod elongation stage (R3-R4) > flowering (R1-R2) [9]. A separate experiment by Kadhem et al (1985) supported the sensitivity toward irrigation at the pod elongation stage in determinate cultivars (R3.7 and R4.7) [10].


\* indicates significant effect of drought on the trait. Growth stages were characterized by the Fehr and Caviness scale [11]. Experiments were carried on the indeterminate cultivar, Weber.

**Table 2.** Effects of drought at different developmental stages on different agronomic traits (Modified from [8])

It has been clearly demonstrated that water availability will affect seed yield, though the growth stages that are most sensitive to drought stress vary among reports on different cul‐ tivars. In contrast, there are controversial reports on the effects of drought on soybean seed quality. Germination rate is a crucial criterion for assessing seed quality. A 2-year field study conducted on 3 soybean cultivars of Maturity Group (MG) IV, V and VI respectively in the southern U.S.A. reported a reduction of seed germination to less than 80% of the con‐ trol, when drought stress was applied at any of the tested reproductive stages [12]. This ob‐ servation is supported by a greenhouse experiment reporting that the germination rate was reduced in medium seeds from plants subjected to drought during seed filling period [13].

On the contrary, in a greenhouse experiment using the cultivar Gnome [14], drought stress led to a reduction of seed yield mainly due to the reduction of seed number. Nevertheless, there were only slight reductions in standard germination percentage and seedling axis dry weight of the harvested seeds. The authors suggested that drought stress affects the seed yield to a larger extend than seed quality. This result is supported by a separate experiment using other determinate and indeterminate cultivars (Essex, Union, Harper and McCall), in which drought did not result in production of seeds with reduced germination rate or vigor, except for those shriveled, flat, and underdeveloped seeds [15].

The study by Dornbos and Mullen (1991) further showed that the effect of drought on the germination rate of seeds from stressed plants was more significant when the air tempera‐ ture reached 35°C. The authors also reported an increase in the percentage of hard seeds with increased duration of drought stress, and a negative relationship between seed weight and the percentage of hard seeds [16]. Hard seeds possess impermeable seed coats that will impede germination. In conclusion, drought clearly affects seed quality on some soybean cultivars. However, the discrepancies among different reports suggest that such effects are not universal to all cultivars under different stress conditions.

The contents of seed protein and oil are major parameters determining the nutritional value of soybean. Soybean seed protein content in general is negatively correlated with the amount of seed oil [17]. A differential irrigation experiment performed on soybean cultivars Gnome and Hodgson 78 in a greenhouse setting reported a 4.4% increase in protein content and 2.6% decrease in oil content under severe drought [18]. Furthermore, a 6-year field ex‐ periment was conducted using 60 soybean cultivars and breeding lines (Figure 1). The re‐ sults confirmed both the negative correlation between seed protein and seed oil contents as well as the effect of drought on seed protein and seed oil contents [19]. The variations in contents of seed protein and oil were attributed largely to the differential rainfall during the seed filling stage [19].

Soybean seeds are also rich in isoflavones, a group of secondary metabolites exhibiting es‐ trogenic, antifungal, and antibacterial activities [20]. The level of isoflavones is affected by drought during seed development [21]. While drought stress reduced the total content of isoflavones in soybean seeds under 28°C and 700 ppm CO2, an increase was observed when the drought stress was applied at 23°C and 700 ppm CO2 [21]. The results implied that the isoflavone content in soybean seeds is responsive to drought but also to other environmen‐ tal factors including temperature and CO2 level.

**Figure 1.** Variations of the average seed protein and oil contents of 60 soybean germplasms and breeding lines, and the amount of rainfall at the experimental field during the reporting year (based on data from [19]).

## **3. Parameters for measuring the degree of drought stress in soybean**

### **3.1. Parameters related to seed**

It has been clearly demonstrated that water availability will affect seed yield, though the growth stages that are most sensitive to drought stress vary among reports on different cul‐ tivars. In contrast, there are controversial reports on the effects of drought on soybean seed quality. Germination rate is a crucial criterion for assessing seed quality. A 2-year field study conducted on 3 soybean cultivars of Maturity Group (MG) IV, V and VI respectively in the southern U.S.A. reported a reduction of seed germination to less than 80% of the con‐ trol, when drought stress was applied at any of the tested reproductive stages [12]. This ob‐ servation is supported by a greenhouse experiment reporting that the germination rate was reduced in medium seeds from plants subjected to drought during seed filling period [13]. On the contrary, in a greenhouse experiment using the cultivar Gnome [14], drought stress led to a reduction of seed yield mainly due to the reduction of seed number. Nevertheless, there were only slight reductions in standard germination percentage and seedling axis dry weight of the harvested seeds. The authors suggested that drought stress affects the seed yield to a larger extend than seed quality. This result is supported by a separate experiment using other determinate and indeterminate cultivars (Essex, Union, Harper and McCall), in which drought did not result in production of seeds with reduced germination rate or vigor,

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

The study by Dornbos and Mullen (1991) further showed that the effect of drought on the germination rate of seeds from stressed plants was more significant when the air tempera‐ ture reached 35°C. The authors also reported an increase in the percentage of hard seeds with increased duration of drought stress, and a negative relationship between seed weight and the percentage of hard seeds [16]. Hard seeds possess impermeable seed coats that will impede germination. In conclusion, drought clearly affects seed quality on some soybean cultivars. However, the discrepancies among different reports suggest that such effects are

The contents of seed protein and oil are major parameters determining the nutritional value of soybean. Soybean seed protein content in general is negatively correlated with the amount of seed oil [17]. A differential irrigation experiment performed on soybean cultivars Gnome and Hodgson 78 in a greenhouse setting reported a 4.4% increase in protein content and 2.6% decrease in oil content under severe drought [18]. Furthermore, a 6-year field ex‐ periment was conducted using 60 soybean cultivars and breeding lines (Figure 1). The re‐ sults confirmed both the negative correlation between seed protein and seed oil contents as well as the effect of drought on seed protein and seed oil contents [19]. The variations in contents of seed protein and oil were attributed largely to the differential rainfall during the

Soybean seeds are also rich in isoflavones, a group of secondary metabolites exhibiting es‐ trogenic, antifungal, and antibacterial activities [20]. The level of isoflavones is affected by drought during seed development [21]. While drought stress reduced the total content of isoflavones in soybean seeds under 28°C and 700 ppm CO2, an increase was observed when the drought stress was applied at 23°C and 700 ppm CO2 [21]. The results implied that the isoflavone content in soybean seeds is responsive to drought but also to other environmen‐

except for those shriveled, flat, and underdeveloped seeds [15].

not universal to all cultivars under different stress conditions.

seed filling stage [19].

Relationships

212

tal factors including temperature and CO2 level.

Seed weight can be evaluated using 100-seed weight or seed weight distribution. To elimi‐ nate the effects of the large measurement errors on the weight of a single seed, the weights of batches of 100 seeds are measured instead. Despite the general decreasing trend of seed weight under drought, the seed weight may not reduce uniformly as a function of drought intensity [16]. Therefore, seed weight distribution has become another parameter employed to evaluate the effect of drought on seed weight, through the assessment of weight of seeds of different sizes. Dornbos and Mullen (1991) reported that under severe drought, the pro‐ portion of seeds of diameter larger than 4.8mm was reduced by 30%-40% while the propor‐ tion of seeds of diameter smaller than 3.2mm was increased by 3%-15%. Under drought, soybean plants continued to produce heavy seeds. However, a greater portion of seeds were of low weight [16].

### **3.2. Parameters related to vegetative tissues**

Drought stress reduced the number of nodes which is a result due to the reduction of main stem height and the decreased node emergence rate [8]. Length of internode is also a param‐ eter for evaluating drought stress. However, the change in internode length is dependent on the timing of drought. In the experiment reported by Desclaux et al (2000), only the interno‐ des which initiated during drought stress showed reduction in length [8].

Reduction in leaf area is a convenient morphological parameter for measuring drought stress experienced by the plant. Commercially available leaf area meters provide a non-de‐ structive means to measure leaf area in the field. Alternately, the area of detached leaves can be measured simply by creating a digital image of the leaf using desk-top scanners followed by image analysis by computer software [22].

Drought stress also leads to a reduction in leaf relative growth rate [23], which can be calcu‐ lated using the following formula:

RGR = ln (FDW) – ln (IDW) / (t2– t1)

where, FDW refers to the final dry weight; IDW refers to the initial dry weight; t2 refers to the time in days at the end of the experiment; t1 refers to the time in days at the beginning of the experiment.

The degree of chlorophyll reduction in soybean leaves was correlated with the strength of drought treatments [24]. Chlorophyll can be simply extracted by immersing the plant tissue in N,N-dimethylformamide (DMF) [25]. After incubation and mixing, the DMF is subjected to OD determination. The total chlorophyll content is calculated as Ct = 8.24 *A*664 + 23.97 *A*647 – 16.64 *A*603, where Ct is the total chlorophyll content in µg/ml of the DMF subjected to measurement [25].
