would bias estimated RWC downward. In order to control the severity and timing in
the field, drought stress is affected by stopping water supply, be it by terminating
irrigation or by activating the rainout shelter. Stress will then develop gradually and
it is crucial to be able to translate the number of days without watering into the
desired level of plant stress. When grown on deep soil of good water holding
capacity soybean may take around 2–3 weeks to reach midday RWC of about
60–70%. In phenotyping and selection work, it is not absolutely necessary to
measure all relevant atmospheric and soil variables, in order to estimate daily crop
water-use, and thus to predict the timing and rate of the planned imposed drought,
where one can access the plant and estimate directly or indirectly its water status with
only a minimal reference to environmental variables. Since drought phenotyping is
usually repeated in the same location during the course of breeding, experience
gained can be an important lead for gauging stress treatments (Blum 2011).
Canopy temperature depression, an indirect measure of plant-water status of the
crop, is the difference between air temperature and plant canopy temperature
(Tuberosa 2012). Genotypes use more available soil moisture to cool the canopy
by transpiration under drought stress (Reynolds et al. 2009). Further, plant leaves
emit long-wave infrared radiation according to their temperature. Low water status
in stressed plants leading to reduced transpiration raises canopy temperature. The
infrared thermometer is designed to sense long-wave infrared radiation emitted from
its target, converting it to an average temperature display which can be related to
transpiration and to the genetic potential of roots in exploring soil moisture (Pinto
and Reynolds 2015) and drought susceptibility index in stressed environments
(Blum 1989).
4.2
Exploring Roots of Drought Tolerance
The food security in the 21st century will rely increasingly on the release of cultivars
with improved resistance to drought conditions and with high yield stability
(Swaminathan 2005; Borlaug 2007; Pennisi 2008) and demands attention of plant
scientists belowground (Bishopp and Lynch 2015). The development of crops with
root systems that can capture water and nutrients efficiently would contribute in
improving the economic development in poor nations and the sustainability of
agriculture in rich nations by reducing reliance on intensive fertilization and irrigation (Bishopp and Lynch 2015). Carbon through CO2 from air is stored in the roots
and leaves. Varieties having root systems extended around one meter limit their
access to available water in deeper layers, particularly during drought stress at pod
filling stage. Doubling root biomass to a nominal two meters would lock away more
carbon in soil down to 2 meters in the croplands which could reduce the annual rise
in global CO2 levels in the atmosphere helping fight global warming (Kell 2011) and
develop drought resilient varieties. Root system architecture traits are measured in
soybean, morphologically, to capture narrow root angle to the soil surface, which
promotes lateral root development in the upper root regions where light penetration
is the greatest and at the same time number of forks and number of crossings could
4 Breeding and Molecular Approaches for Evolving Drought-Tolerant Soybeans
87
the field, drought stress is affected by stopping water supply, be it by terminating
irrigation or by activating the rainout shelter. Stress will then develop gradually and
it is crucial to be able to translate the number of days without watering into the
desired level of plant stress. When grown on deep soil of good water holding
capacity soybean may take around 2–3 weeks to reach midday RWC of about
60–70%. In phenotyping and selection work, it is not absolutely necessary to
measure all relevant atmospheric and soil variables, in order to estimate daily crop
water-use, and thus to predict the timing and rate of the planned imposed drought,
where one can access the plant and estimate directly or indirectly its water status with
only a minimal reference to environmental variables. Since drought phenotyping is
usually repeated in the same location during the course of breeding, experience
gained can be an important lead for gauging stress treatments (Blum 2011).
Canopy temperature depression, an indirect measure of plant-water status of the
crop, is the difference between air temperature and plant canopy temperature
(Tuberosa 2012). Genotypes use more available soil moisture to cool the canopy
by transpiration under drought stress (Reynolds et al. 2009). Further, plant leaves
emit long-wave infrared radiation according to their temperature. Low water status
in stressed plants leading to reduced transpiration raises canopy temperature. The
infrared thermometer is designed to sense long-wave infrared radiation emitted from
its target, converting it to an average temperature display which can be related to
transpiration and to the genetic potential of roots in exploring soil moisture (Pinto
and Reynolds 2015) and drought susceptibility index in stressed environments
(Blum 1989).
4.2
Exploring Roots of Drought Tolerance
The food security in the 21st century will rely increasingly on the release of cultivars
with improved resistance to drought conditions and with high yield stability
(Swaminathan 2005; Borlaug 2007; Pennisi 2008) and demands attention of plant
scientists belowground (Bishopp and Lynch 2015). The development of crops with
root systems that can capture water and nutrients efficiently would contribute in
improving the economic development in poor nations and the sustainability of
agriculture in rich nations by reducing reliance on intensive fertilization and irrigation (Bishopp and Lynch 2015). Carbon through CO2 from air is stored in the roots
and leaves. Varieties having root systems extended around one meter limit their
access to available water in deeper layers, particularly during drought stress at pod
filling stage. Doubling root biomass to a nominal two meters would lock away more
carbon in soil down to 2 meters in the croplands which could reduce the annual rise
in global CO2 levels in the atmosphere helping fight global warming (Kell 2011) and
develop drought resilient varieties. Root system architecture traits are measured in
soybean, morphologically, to capture narrow root angle to the soil surface, which
promotes lateral root development in the upper root regions where light penetration
is the greatest and at the same time number of forks and number of crossings could
4 Breeding and Molecular Approaches for Evolving Drought-Tolerant Soybeans
87