gene editing can be used to develop soybean varieties with improved drought
tolerance (Deshmukh et al. 2014; Phang et al. 2008; Manavalan et al. 2009).
Climate models have indicated that drought episodes will become more frequent
because of the long-term effects of global warming (Salinger et al. 2005; Cook et al.
2007) and may significantly affect soybean yield in many countries (Long et al.
2005; Easterling et al. 2007). While increase in CO2 under climate change might be
expected to increase soybean productivity, vicious effects of frequent droughts and
associated diseases and herbivores infestation may counteract such benefits. Drought
is considered one of the most devastating among abiotic stress factors (Manavalan
et al. 2009; Tran and Mochida 2010) reducing about 40% soybean yield annually
(Specht et al. 1999) and depending upon the intensity of its occurrence at critical
growth stage, the losses could be as high as 80% (Oya et al. 2004; Dias et al.
2012). Efficient resource acquisition and remobilization are challenges for soybean
productivity under water-limited scenario. The hydrostatic pressure created by
transpiration from the shoot is transmitted to the xylem vessels of the shoot and
the roots, which drives water in the root cylinder toward the xylem vessels (Steudle
1995; Tyree 1997) and are governed by hydraulic mechanisms such as leaf conductance (Sinclair et al. 2010), leaf canopy size (Ratnakumar and Vadez 2011; Vadez
et al. 2011), control of leaf expansion (Simonneau et al. 2009), and transpiration
response in soybean to high vapor pressure deficit (VPD) (Sinclair et al. 2008;
Ocheltree et al. 2014). In soybean, vegetative growth is sensitive to water deficits.
Besides, usual inhibitory effects on leaf expansion, transpiration and photosynthesis,
water deficit also inhibit nitrogen fixation in soybean. Water stress, occurring during
the beginning of pod setting and full seed-filling, has a greater negative irreversible
impact on yield through reducing seed size considerably (Doss and Thurlow 1974),
as compared to other stages, and thus invite designing of breeding strategy targeting
the final expression of yield under drought, i.e. seed size by integrating constitutive
plant traits and stress-responsive processes (Blum 2011). A drought resistance index
in terms of yield can be developed by comparing yield between stress and non-stress
conditions, which is gaining popularity as a useful criterion in selection for drought
resistance (Fukai et al. 1999). Drought resistance is measured by phenotyping the
specific and relevant attributes of dehydration tolerance and dehydration avoidance,
where a concrete breeding program integrates robust, reliable, relatively fast, and
economical phenotyping facilities. Dehydration tolerance, phenotyped only on the
basis of similar plant water status in all the genotypes, includes the most prominent
feature of whole plant dehydration tolerance assessing the capacity for stem reserve
utilization for seed filling by chemical desiccation method. Protocols for dehydration
avoidance include measuring plant water status, in terms of visual symptoms of leaf
senescence, relative water content, and constitutive traits without exposure to
drought stress such as root system architecture traits. (Blum 2011).
An important component of reproductive success of the crop under drought stress
is the capacity for seed filling from stem reserve, when transient photosynthesis is
inhibited by stress. This is a dehydration tolerance mechanism since the transport of
reserves from stem to seed takes place in dehydrated plants, in the case of severe
drought in the field. It can be phenotyped in large populations by the chemical
4 Breeding and Molecular Approaches for Evolving Drought-Tolerant Soybeans
85
tolerance (Deshmukh et al. 2014; Phang et al. 2008; Manavalan et al. 2009).
Climate models have indicated that drought episodes will become more frequent
because of the long-term effects of global warming (Salinger et al. 2005; Cook et al.
2007) and may significantly affect soybean yield in many countries (Long et al.
2005; Easterling et al. 2007). While increase in CO2 under climate change might be
expected to increase soybean productivity, vicious effects of frequent droughts and
associated diseases and herbivores infestation may counteract such benefits. Drought
is considered one of the most devastating among abiotic stress factors (Manavalan
et al. 2009; Tran and Mochida 2010) reducing about 40% soybean yield annually
(Specht et al. 1999) and depending upon the intensity of its occurrence at critical
growth stage, the losses could be as high as 80% (Oya et al. 2004; Dias et al.
2012). Efficient resource acquisition and remobilization are challenges for soybean
productivity under water-limited scenario. The hydrostatic pressure created by
transpiration from the shoot is transmitted to the xylem vessels of the shoot and
the roots, which drives water in the root cylinder toward the xylem vessels (Steudle
1995; Tyree 1997) and are governed by hydraulic mechanisms such as leaf conductance (Sinclair et al. 2010), leaf canopy size (Ratnakumar and Vadez 2011; Vadez
et al. 2011), control of leaf expansion (Simonneau et al. 2009), and transpiration
response in soybean to high vapor pressure deficit (VPD) (Sinclair et al. 2008;
Ocheltree et al. 2014). In soybean, vegetative growth is sensitive to water deficits.
Besides, usual inhibitory effects on leaf expansion, transpiration and photosynthesis,
water deficit also inhibit nitrogen fixation in soybean. Water stress, occurring during
the beginning of pod setting and full seed-filling, has a greater negative irreversible
impact on yield through reducing seed size considerably (Doss and Thurlow 1974),
as compared to other stages, and thus invite designing of breeding strategy targeting
the final expression of yield under drought, i.e. seed size by integrating constitutive
plant traits and stress-responsive processes (Blum 2011). A drought resistance index
in terms of yield can be developed by comparing yield between stress and non-stress
conditions, which is gaining popularity as a useful criterion in selection for drought
resistance (Fukai et al. 1999). Drought resistance is measured by phenotyping the
specific and relevant attributes of dehydration tolerance and dehydration avoidance,
where a concrete breeding program integrates robust, reliable, relatively fast, and
economical phenotyping facilities. Dehydration tolerance, phenotyped only on the
basis of similar plant water status in all the genotypes, includes the most prominent
feature of whole plant dehydration tolerance assessing the capacity for stem reserve
utilization for seed filling by chemical desiccation method. Protocols for dehydration
avoidance include measuring plant water status, in terms of visual symptoms of leaf
senescence, relative water content, and constitutive traits without exposure to
drought stress such as root system architecture traits. (Blum 2011).
An important component of reproductive success of the crop under drought stress
is the capacity for seed filling from stem reserve, when transient photosynthesis is
inhibited by stress. This is a dehydration tolerance mechanism since the transport of
reserves from stem to seed takes place in dehydrated plants, in the case of severe
drought in the field. It can be phenotyped in large populations by the chemical
4 Breeding and Molecular Approaches for Evolving Drought-Tolerant Soybeans
85