stress, as compared to controls where selection was performed without chemical
desiccation (Blum 2011). After two cycles of selection, intensive selection of
candidate genotypes for multiple drought tolerance related traits is practiced in
advance generations using three-tier selection scheme (Satpute et al. 2018) followed
by multi-traits indexing (Satpute et al. 2020) for analyzing traits function.
4.3.1 Three-Tier Selection Scheme
Selection for yield per se under water-limited environment confounding the complexity of breeding implores identifying less complex traits related to the drought
tolerance and separating them into major components which may ease further
understanding of the genetic basis. A large-scale three-tier sequence of selection
scheme, which utilizes a set of drought related traits, has previously been employed
by a research group (Sinclair 2011) in recognizing drought-tolerant soybean lines.
From a screen of about 3500 well-watered soybean plant introduction (PI) lines,
about 250 lines (slightly <10%) with low petiole ureide concentrations were
selected. High leaf ureide concentrations had been shown to be associated with
sensitivity to nitrogen fixation to drought. The lines in the second-tier screen were
grown for a field test of nitrogen accumulation under dry conditions on a sandy soil,
which had both a low nitrogen content and low water holding capacity. The lines
were lightly irrigated approximately every other day to maintain them for about
3 weeks at or slightly above the soil water content resulting in slight leaf wilting.
Selected 10% of these field-tested lines (24 lines) were screened in the third tier for
intensive measurement of nitrogen fixation response to soil drying in the greenhouse
test. Ultimately, 11 PI lines were identified that had substantial superiority in
nitrogen fixation tolerance to soil drying. The group identified candidate parental
soybean lines for use in breeding efforts to sustain nitrogen fixation activity during
soil water deficit.
On the similar line, a three-tier selection scheme has been reported in order to
evaluate large number of germplasm accessions (Fig. 4.1) and breeding populations
(Satpute et al. 2019a, 2020). Unlike previous approach, with choice of traits, viz. low
petiole ureide concentrations under well-watered condition, field test of nitrogen
accumulation and greenhouse test of intensive nitrogen fixation response under dry
conditions, 1000 soybean germplasm accessions, procured from Medium Term
Storage (MTS) facility at ICAR-Indian Institute of Soybean Research, Indore,
India were evaluated for delayed leaf senescence trait, stem reserve mobilization,
drought tolerance related above-ground traits, viz. drought tolerance index, canopy
temperature depression, relative water content, which are reliable and speedy in
measurement in large breeding populations, and root system architecture traits
(Fig. 4.1).
In the first tier of three-tier selection scheme, about 86 accessions (slightly less
than 10%) with 4-5 scores were selected for delayed leaf senescence, in a field trial
during summer season. Accessions were sown in early, medium and late flowering
groups and irrigations were withheld at pod initiation stage for imposing 21 days
90
G. K. Satpute et al.
desiccation (Blum 2011). After two cycles of selection, intensive selection of
candidate genotypes for multiple drought tolerance related traits is practiced in
advance generations using three-tier selection scheme (Satpute et al. 2018) followed
by multi-traits indexing (Satpute et al. 2020) for analyzing traits function.
4.3.1 Three-Tier Selection Scheme
Selection for yield per se under water-limited environment confounding the complexity of breeding implores identifying less complex traits related to the drought
tolerance and separating them into major components which may ease further
understanding of the genetic basis. A large-scale three-tier sequence of selection
scheme, which utilizes a set of drought related traits, has previously been employed
by a research group (Sinclair 2011) in recognizing drought-tolerant soybean lines.
From a screen of about 3500 well-watered soybean plant introduction (PI) lines,
about 250 lines (slightly <10%) with low petiole ureide concentrations were
selected. High leaf ureide concentrations had been shown to be associated with
sensitivity to nitrogen fixation to drought. The lines in the second-tier screen were
grown for a field test of nitrogen accumulation under dry conditions on a sandy soil,
which had both a low nitrogen content and low water holding capacity. The lines
were lightly irrigated approximately every other day to maintain them for about
3 weeks at or slightly above the soil water content resulting in slight leaf wilting.
Selected 10% of these field-tested lines (24 lines) were screened in the third tier for
intensive measurement of nitrogen fixation response to soil drying in the greenhouse
test. Ultimately, 11 PI lines were identified that had substantial superiority in
nitrogen fixation tolerance to soil drying. The group identified candidate parental
soybean lines for use in breeding efforts to sustain nitrogen fixation activity during
soil water deficit.
On the similar line, a three-tier selection scheme has been reported in order to
evaluate large number of germplasm accessions (Fig. 4.1) and breeding populations
(Satpute et al. 2019a, 2020). Unlike previous approach, with choice of traits, viz. low
petiole ureide concentrations under well-watered condition, field test of nitrogen
accumulation and greenhouse test of intensive nitrogen fixation response under dry
conditions, 1000 soybean germplasm accessions, procured from Medium Term
Storage (MTS) facility at ICAR-Indian Institute of Soybean Research, Indore,
India were evaluated for delayed leaf senescence trait, stem reserve mobilization,
drought tolerance related above-ground traits, viz. drought tolerance index, canopy
temperature depression, relative water content, which are reliable and speedy in
measurement in large breeding populations, and root system architecture traits
(Fig. 4.1).
In the first tier of three-tier selection scheme, about 86 accessions (slightly less
than 10%) with 4-5 scores were selected for delayed leaf senescence, in a field trial
during summer season. Accessions were sown in early, medium and late flowering
groups and irrigations were withheld at pod initiation stage for imposing 21 days
90
G. K. Satpute et al.