**2. Effects of water deficits on soybean: Crop yields and general responses**

Water participates in nearly all physiological and biochemical processes in plants, compris‐ ing approximately 90% of their mass (Farias et al, 2007). It is responsible for the thermal reg‐ ulation of the plant, acting both to maintain the cooling and heat distribution and to promote mechanical support of the plant (Taiz & Zeiger, 2009). It also functions as a solvent, through which gases, minerals and other solutes enter cells and move within plants (Nepo‐ muceno et al, 1994).

The need for water in soybean increases with plant development, peaking during the flow‐ ering-grain filling stages (7-8 mm day-1) and decreasing thereafter. The total water require‐ ment for maximum productivity varies between 450 and 800 mm, depending on weather conditions, crop management practices and cycle timing (Embrapa, 2011, Farias et al, 2007). The loss of productivity under water deficit conditions depends on the soybean phenologi‐ cal stage, duration and intensity of water shortages (Doss & Thurlow, 1974). Kron et al. (2008) evaluated the responses of soybean to water stress induced in different phases in the plants and concluded that plants subjected to water stress during the V4 stage showed an increased tolerance to water shortages in later stages. This stage was considered to represent a "developmental window" in soybean, characterized as a specific period during plant de‐ velopment when environmental disturbances can be embodied, thereby improving subse‐ quent plant resistance to environmental changes (Kron et al., 2008).

Desclaux et al. (2000) evaluated the effects of water stress at various stages of development in soybean plants and found the average length of the internodes to be the most sensitive feature to drought imposed during the vegetative stages (V4) and flowering (R1-R3), and a reduction in plant height was associated with water stress induced in the V4 stage. The number of pods per unit of shoot dry matter was significantly affected by water deficits in the reproductive stages (R3-R5). When stress occurred during grain filling (R5), the charac‐ teristics of the plant that were most affected were the number of grains per pod and the grain weight. Rosolem (2005) notes that the water demand of soybean is highest at the initia‐ tion of flowering, but a water deficit from pod initiation (R3) until 50% yellow leaves (R7) is the most critical stage for productivity. In a study performed by the same author correlating rainfall with grain yields, it was found that when water restriction occurred between flower‐ ing and the emergence of pods, the grain yield of soybean was 1,275 kg ha-1, but under no water limitation at this stage, there may be an increase in productivity of 3.8 kg ha-1 for each mm of rain. When water restriction occurred during grain filling, the yield was 878 kg ha-1, and for each mm of rain, there was an increase in productivity of 13 kg ha-1, indicating the greater susceptibility of the soybean to water stress during grain filling, although the highest water demand in the crop occurred at the beginning of flowering (Desclaux et al., 2000). These results are in agreement with those of Nogueira& Nagai (1988). However, other stud‐ ies observed that seed filling is not the most drought prone period of soybean development. When the water deficit starts during R1 (early flowering) and R4 growth stage, the seed yield may be significantly reduced (Eck et al., 1987, Brown et al., 1985, Hoogenboom et al., 1987) compared to R6-R7 growth stage.

Although the effects of various environmental factors interfere with the performance of crops, water restriction is the main limiting environmental factor that contributes to the fail‐ ure to obtain maximum soybean yields (Casagrande et al, 2001), influencing the use of other environmental resources. According to Confalone & Navarro Dujmovich (1999), the efficien‐ cy of the use of solar radiation by soybean remains relatively constant at different develop‐ ment stages. When there is moderate water stress, soybean tends to maximize the efficiency of radiation utilization and reduce the efficiency of the interception of photosynthetically ac‐ tive radiation, while under severe water deficits, there is a reduction of the efficiency of radi‐

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

Lisar et al. (2012) report that the impacts of water stress in crop plants can reduce pro‐ ductivity by 50% in various parts of the world. Under stress conditions, the plants present a series of changes in their morphology, physiology and biochemistry, negatively affecting their growth and productivity. According to Gerten & Rost (2010), two-thirds of world food production through cultivation occurs under water stress. In this context and because of the prospect of global climate change, most crops will be exposed to negative

**2. Effects of water deficits on soybean: Crop yields and general responses**

Water participates in nearly all physiological and biochemical processes in plants, compris‐ ing approximately 90% of their mass (Farias et al, 2007). It is responsible for the thermal reg‐ ulation of the plant, acting both to maintain the cooling and heat distribution and to promote mechanical support of the plant (Taiz & Zeiger, 2009). It also functions as a solvent, through which gases, minerals and other solutes enter cells and move within plants (Nepo‐

The need for water in soybean increases with plant development, peaking during the flow‐ ering-grain filling stages (7-8 mm day-1) and decreasing thereafter. The total water require‐ ment for maximum productivity varies between 450 and 800 mm, depending on weather conditions, crop management practices and cycle timing (Embrapa, 2011, Farias et al, 2007). The loss of productivity under water deficit conditions depends on the soybean phenologi‐ cal stage, duration and intensity of water shortages (Doss & Thurlow, 1974). Kron et al. (2008) evaluated the responses of soybean to water stress induced in different phases in the plants and concluded that plants subjected to water stress during the V4 stage showed an increased tolerance to water shortages in later stages. This stage was considered to represent a "developmental window" in soybean, characterized as a specific period during plant de‐ velopment when environmental disturbances can be embodied, thereby improving subse‐

Desclaux et al. (2000) evaluated the effects of water stress at various stages of development in soybean plants and found the average length of the internodes to be the most sensitive feature to drought imposed during the vegetative stages (V4) and flowering (R1-R3), and a reduction in plant height was associated with water stress induced in the V4 stage. The

quent plant resistance to environmental changes (Kron et al., 2008).

ation utilization.

Relationships

274

impacts caused by drought.

muceno et al, 1994).

According to Santos & Carlesso (1998), the most prominent responses of plants to water deficits in terms of morphological processes are decreases in leaf area and acceleration of the senescence and abscission of leaves. Catuchi et al. (2011) studied the conventional cultivar CD 220 and the transgenic cultivar CD 226RR under water deficits induced at the V4 stage, and they observed a reduction of approximately 40% of leaf area per plant compared to control plants and decreasing shoot dry mass of approximately 50% for both cultivars. Akinci & Losel (2012) reported that water stress drastically decreased root elongation and the expansion of leaf area in soybean, though these two processes were not equally affected because leaf expansion is usually reduced by a greater proportion than root growth, and carbon partitioning shifts to increase the root/shoot ratio. Catuchi et al. (2012) studied the responses of biomass and leaf area in plants of two soybean cul‐ tivars, BR 48 and EMBRAPA 16, which are considered tolerant and susceptible to water restriction, respectively, grown under water deficits. The authors observed a reduction of all traits in both cultivars due to a water deficit imposed in the reproductive stage of culture, with the exception of root dry mass in Embrapa 48, which remained unchanged, even under conditions of water restriction (Table 1). This phenomenon may occur be‐ cause drought can promote the expansion of the root system to reach additional deeper moisture zones in the soil profile, a process that begins gradually after drying of the soil surface (Santos & Carlesso, 1998). The reduction of the other biomass parameters under conditions of water deficits is related to decreased photosynthetic rates, and biomass ac‐ cumulation and translocation to grain are consequently impaired (Neumaier et al., 2000).

One of the most important processes of nitrogen nutrition of soybean, which results in im‐ provements of productivity and profitability of the crop, is the symbiotic nitrogen fixation. Nevertheless, this process is negatively influenced by low moisture (Purcell et al, 2004; Pur‐

cell & Specht, 2004). Decreased nitrogen fixation starts when water potential of root nodules starts falling below -0.2 to -0.4 MPa (Pankhurst & Sprent, 1975). According to the Purcell et al. (2000), the water deficit promotes the accumulation of products of N2 fixation (ureides) in the shoot of soybean plant, causing a feedback reduction in fixation of N2. Thus, the authors report that proper nutrition with manganese (Mn+2) promotes the breaking of ureides and extends N2 fixation in plants under water deficit. Furthermore, soybean plants that produce larger nodules are less susceptible to reduction of nitrogen fixation in water deficit condi‐ tions (King & Purcell, 2001).


(1)Means followed by the same letters between the levels of water reposition in each cultivar not differ by Tukey test (P = 5%).

**Table 1.** Seed mass per plant (SM), shoot dry mass (SDm), root dry mass (RDm), overall dry mass (ODm) and overall area leaf (Al )of BR-16 and Embrapa 48 soybean cultivars with 100% and 40% water reposition (adapted from Catuchi et al., 2012)

Water restriction may be caused by several factors in plants, with the principal cause being an absence or an irregular distribution of rainfall during the crop cycle (Gopefert et al., 1993). In recent years, due to global climate change, climate stability, which allows the culti‐ vation of crops to be planned, has been more limited. For plants to withstand periods of wa‐ ter restriction, they should be able to maintain their water status at normal turgor pressure during the hottest hours of the day, when the water vapor atmospheric demand is greater. This requires that the plant have a well-developed root system allowing it to reach water in deeper layers in the soil profile (Farias et al, 2007). In some cases, the limited extent of the root system reduces the water supply to plants. These responses are typical of soils contain‐ ing toxic aluminum (Al3+) combined with low rainfall during the crop cycle. The presence of Al3+ could limit the development of the root system due to inhibition of DNA synthesis and cell division, limiting the elongation of the roots and thus, the absorption of water from the deeper layers of soil. It also causes changes in nutrient uptake and in the overall nutritional balance of plants (Machado, 1997). Mascarenhas et al. (1984) observed a reduction of pri‐ mary roots of plants of two cultivars of soybeans due to increased levels of Al3+ in a nutrient solution. Nolla et al. (2007) assessed the root development of soybean seedlings grown in solution with various concentrations of Al3+ (0.0, 0.30, 0.60, and 1.20 mmol L-1 Al) and ob‐ served a significant reduction of the root dry mass due to the increased concentration of Al3+ at pH = 4 (Figure 1).

cell & Specht, 2004). Decreased nitrogen fixation starts when water potential of root nodules starts falling below -0.2 to -0.4 MPa (Pankhurst & Sprent, 1975). According to the Purcell et al. (2000), the water deficit promotes the accumulation of products of N2 fixation (ureides) in the shoot of soybean plant, causing a feedback reduction in fixation of N2. Thus, the authors report that proper nutrition with manganese (Mn+2) promotes the breaking of ureides and extends N2 fixation in plants under water deficit. Furthermore, soybean plants that produce larger nodules are less susceptible to reduction of nitrogen fixation in water deficit condi‐

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

SM (g) 26.3A(1) 13.9B 23.9A 11.4B SDm (g) 57.1A 29.1B 51.8A 25.2B RDm (g) 8.3A 5.1B 5.6A 4.6A ODm (g) 65.4A 34.3B 57.4A 29.8B

(cm2) 1637.7A 756.7B 1356.1A 724.3B

)of BR-16 and Embrapa 48 soybean cultivars with 100% and 40% water reposition (adapted from Catuchi et al.,

(1)Means followed by the same letters between the levels of water reposition in each cultivar not differ by Tukey

**Table 1.** Seed mass per plant (SM), shoot dry mass (SDm), root dry mass (RDm), overall dry mass (ODm) and overall area

Water restriction may be caused by several factors in plants, with the principal cause being an absence or an irregular distribution of rainfall during the crop cycle (Gopefert et al., 1993). In recent years, due to global climate change, climate stability, which allows the culti‐ vation of crops to be planned, has been more limited. For plants to withstand periods of wa‐ ter restriction, they should be able to maintain their water status at normal turgor pressure during the hottest hours of the day, when the water vapor atmospheric demand is greater. This requires that the plant have a well-developed root system allowing it to reach water in deeper layers in the soil profile (Farias et al, 2007). In some cases, the limited extent of the root system reduces the water supply to plants. These responses are typical of soils contain‐ ing toxic aluminum (Al3+) combined with low rainfall during the crop cycle. The presence of Al3+ could limit the development of the root system due to inhibition of DNA synthesis and cell division, limiting the elongation of the roots and thus, the absorption of water from the deeper layers of soil. It also causes changes in nutrient uptake and in the overall nutritional balance of plants (Machado, 1997). Mascarenhas et al. (1984) observed a reduction of pri‐ mary roots of plants of two cultivars of soybeans due to increased levels of Al3+ in a nutrient solution. Nolla et al. (2007) assessed the root development of soybean seedlings grown in solution with various concentrations of Al3+ (0.0, 0.30, 0.60, and 1.20 mmol L-1 Al) and ob‐

**BR-16 Embrapa 48 Control Water deficits Control Water deficits**

tions (King & Purcell, 2001).

Relationships

276

**Attributes**

Al

test (P = 5%).

leaf (Al

2012)

**Figure 1.** Root dry mass of soybean seedlings grown under different concentration of aluminum. (adapted from Nolla et al, 2007).

Moreover, soil compaction caused by the pressure of agricultural implements on farms is another limiting factor for deep root development. In the work carried out by Cardoso et al. (2006), there was a significant correlation found between the root volume of two soybean cultivars and the resistance to penetration (RP), with the root volume decreasing linearly in both cultivars associated with increases in RP. According to Beutler & Centurion (2003), the growth of the soybean root system is limited when the RP is greater than 2 MPa. Beutler et al. (2006) note that soybean yield decreases in RPvalues from 2.24 to 2.97 MPa.

In this sense, the use of implements that do not cause soil compression and cultivation tech‐ niques such as "no-tillage systems" (NTS) that result in better soil physics, promoting better root development at depth, are extremely important to avoid loss of productivity due to wa‐ ter restriction. Furthermore,some agricultural practices, such as lime and gypsum applica‐ tion may promote the correction of the soil profile (Santos et al, 2010).

According to Franchini et al. (2009), under NTS, the maintenance of the soil covering re‐ duces evaporative water loss due to the formation of a physical barrier and reduces the tem‐ perature of the soil and runoff because of the increased capacity of water infiltration associated with protection of the surface of the soil against the impact of raindrops, thus preventing crusting. Similarly, increasing the percentage of soil organic matter (SOM), which is associated with a reduced intensity of soil cultivation, substantially improves the soil structure, which favors the development of soybean roots and thus increases the size of the water reservoir available. In addition, improvements in the soil structure provided by NTS increase infiltration and the water retention of the soil, thus favoring the upward flow of water from deeper layers to upper layers, where the majority of the soybean root system

develops. The effects of SOM are connected with the hygroscopicity and high specific sur‐ face area of this type of material, which promotes increased water retention (Braida et al, 2011). In a long-term study, Franchini et al. (2009) reported that during the first four seasons after the adoption of aNTS, the soybean yield was similar or slightly lower than that ob‐ tained under conventional tillage (CT). However, from the fifth year onward, when the sys‐ tem had matured and consolidated, the soybean yield was higher under the NTS than the CT (Figure 2).

**Figure 2.** Soybean yield in different management systems: conventional tillage (13 years); new no-tillage (3 years) and no-tillage consolidated (13 years), (adapted from Franchini et al., 2009).

Sowing according to agroclimatic zoning for each agricultural environment is another strat‐ egy to avoid productivity losses due to water restriction. The cultivar must be adapted to the region considering the climate and soil type that determine water retention. In regions with higher occurrences of drought, it is essential to cultivate material that is more tolerant to water restriction. Thus, when the chemical and physical conditions of soil are suitable, al‐ lowing good root development at depth, and the cultivar and sowing time are selected to minimize the effects of water restriction, it is possible to obtain a high productivity soybean grain yield.

Overall, to achieve productivity under any conditions, it is essential that the process of pho‐ tosynthesis, which is responsible for all carbon assimilated for the production of biomass, has a minimum efficiency. Under water deficit conditions during the soybean cycle, photo‐ synthesis is one of the main physiological processes affected.
