**5. Morphological and physiological adjustments of soybean under drought stress**

For ease of discussion, we define the term drought tolerance loosely to include all mecha‐ nisms that allow soybean to survive better under drought. Soybean cultivars of different drought tolerance exhibit a spectrum of differential morphological and physiological changes under drought stress, presumably due to the differences in their genotypes.

#### **5.1. Morphological and growth adjustments**

Morphological adjustments are sometimes effective means to avoid drought stress. A num‐ ber of root-related traits have been proposed as indicators of drought tolerance in soybean [30-34]. Root distribution, which is measured in terms of horizontal and vertical root length density or dry matter in soil of different depth [34, 35], will change in drought tolerant soy‐ bean cultivars under drought stress [36]. It was reported that under seasonal drought, there is a low root density in the dry surface soil but a high root density in the deeper region of the soil where the water content is higher [34]. Moreover, using data from drought tolerant soybean cultivars, it was found that there is a positive correlation between drought toler‐ ance and dry root weight/ plant weight; total root length/ plant weight, and root volume/ plant weight [30].

Root to shoot ratio increases under water deficit conditions [37]. It has been proposed that the cessation of shoot but not root growth can be explained by the higher sensitivity to wa‐ ter deficit of shoot than root [37]. The differential growth is closely related to the differential change in cell wall composition, which involves the thickening of shoot cell wall and relax‐ ing of the expansion of root cell wall by certain catalytic enzymes and stiffening agents [37]. There are only limited reports on related studies in soybean. The study on GmRD22 from soybean suggested a relationship between osmotic stress and cell wall metabolism. GmRD22 is a BURP-domain containing protein localized in the apoplast, which may play a role in stress tolerance by regulating lignin content of cell wall under stress, presumably through interacting with cell wall peroxidases [38].

The adjustments of leaf morphology may play a role in drought tolerance. Some cultivars take advantage from the maintenance of leaf area which provides a possible benefit for the growth of soybean plant after the stress is relieved [39]. Under stress, drought tolerant soy‐ bean cultivars exhibited a larger leaf area when compared with less tolerant cultivars [23, 35]. This phenomenon was associated with the larger extent of reduction in stomatal con‐ ductance and yet a smaller extent of reduction in photosynthetic rate in the tolerant cultivar [23]. In this case, the drought tolerant cultivar may benefit from the reduction of water loss while minimizing the cost of reduction of photosynthesis.

### **5.2. Physiological and biochemical adjustments**

**Index Description Refs**

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

5 levels.

**Table 3.** Common parameters for assessing drought tolerance of soybean cultivars.

**5.1. Morphological and growth adjustments**

Level 1: RI > 0.6500

Level 4: RI <0.3500

**5. Morphological and physiological adjustments of soybean under**

changes under drought stress, presumably due to the differences in their genotypes.

Level 2: RI between 0.5000-0.6500 Level 3: RI between 0.3500-0.5000

Level 5: Plant died or cannot reproduce

For ease of discussion, we define the term drought tolerance loosely to include all mecha‐ nisms that allow soybean to survive better under drought. Soybean cultivars of different drought tolerance exhibit a spectrum of differential morphological and physiological

Morphological adjustments are sometimes effective means to avoid drought stress. A num‐ ber of root-related traits have been proposed as indicators of drought tolerance in soybean [30-34]. Root distribution, which is measured in terms of horizontal and vertical root length density or dry matter in soil of different depth [34, 35], will change in drought tolerant soy‐ bean cultivars under drought stress [36]. It was reported that under seasonal drought, there is a low root density in the dry surface soil but a high root density in the deeper region of the soil where the water content is higher [34]. Moreover, using data from drought tolerant soybean cultivars, it was found that there is a positive correlation between drought toler‐ ance and dry root weight/ plant weight; total root length/ plant weight, and root volume/

Root to shoot ratio increases under water deficit conditions [37]. It has been proposed that the cessation of shoot but not root growth can be explained by the higher sensitivity to wa‐ ter deficit of shoot than root [37]. The differential growth is closely related to the differential

Test should be carried out in field with precipitation less than 50mm. Seeds of each germplasm in each treatment are sown in single row of 1.5 m. The control field is irrigated (7 times) to maintain the field soil moisture. In the treatment field, irrigation is only applied before sowing to ensure the germination of seeds. Plant height, number of branching, number of pods per plant and yield per plant of 10 plants are determined upon harvest. Drought tolerance coefficients of each trait are calculated. Average of coefficients (RI) of all traits will be used to rank the germplasm into

[29]

2. Whole-growth-stage

tolerance

Relationships

216

**drought stress**

plant weight [30].

To survive over an extended drought period, it is important for the soybean leaves to adjust its stomatal conductance to prevent excessive water loss. For example, after 30 days of water stress, the drought tolerant soybean variety MG/BR46 exhibited a higher degree of reduction in stomatal conductance when compared to the drought sensitive cultivar BR16 (65% versus 50% reduction) [23]. After 45 days of stress, the reduction in stomatal conductance was no longer detectable in the sensitive cultivar while it had reached 79% in the tolerant cultivar [23].

Another important adjustment under drought stress is to maintain cell turgidity. In a field test conducted using the drought tolerance soybean cultivar PI 416937 and the sensitive cul‐ tivar Forrest, it was found that PI 416937 maintained a lower solute potential yet a higher water potential and water use efficiency. As a result, PI 416937 gave a higher seed weight and yield than Forrest under drought. This report provided evidence on the positive correla‐ tion between turgor maintenance of leaves and drought tolerance [40].

To maintain cell turgidity under stress, osmotic adjustment is a common mechanism which involves active accumulation of solutes in cells [39]. In soybean, drought stress up-regulates the expression of the soybean *P5CS* gene which encodes the enzymeΔ1-pyrroline-5-carboxy‐ late synthase, a key enzyme in proline biosynthesis [41]. When the expression of the soybean *P5CS* gene was knocked-down, survival under drought stress was hampered [42]. However, a recent study comparing a drought tolerant and a drought sensitive soybean did not reveal an increase in proline level under stress, although the proline level of the tolerant cultivar was higher than that in the sensitive cultivar [43]. The involvement of proline accumulation in drought stress adjustment in soybean awaits further confirmation.

The cellular biochemical adjustment under drought stress involves the scavenging of reac‐ tive oxygen species (ROS). Under normal situation, ROS including singlet oxygen, superox‐ ide radical, hydrogen peroxide, and hydroxyl radical are continuously synthesized and

eliminated in plant cells as "by-products" of photosynthesis, photorespiration, and respira‐ tion in chloroplast and mitochondria [44]. Under drought stress, ROS accumulates when the production outweighs the removal [45]. The over-produced ROS will attack cellular compo‐ nents including nucleic acids, protein, and lipid and eventually leads to cell death [46].

ROS scavenging enzymatic activities of superoxide dismutase, catalase, and glutathione per‐ oxidase increased in 5 soybean germplasms under drought stress [24]. The tested germ‐ plasms displayed different basal and treatment-induced level of ROS scavenging enzymatic activities, which were correlated positively to the final seed yield [24]. The study on GmPAP3 from soybean provides another example for the correlation between enhanced ROS scavenging activity and the adaptation to osmotic stress. GmPAP3 is a mitochondria localized purple acid phosphatase [47]. Ectopic expression of the *GmPAP3* gene significantly reduces ROS accumulation and thereby alleviates osmotic stress [48].

Adverse environmental conditions can bring forth the misfolding of proteins that will accu‐ mulate in endoplasmic reticulum (ER) [49]. The resulting ER stress will activate unfolded protein response [49]. By global expression-profiling analyses on soybean leaves exposed to ER stress inducers and polyethylene glycol, a number of genes were identified as candidate regulatory components integrating ER stress signaling and osmotic stress responses [50]. Moreover, overexpression of soybean BiP (binding protein), an ER-resident molecular chap‐ erone, can enhance drought tolerance in soybean [51]. This evidence tightens the link be‐ tween ER stress and drought response through the activity of chaperones.
