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cross between QL33 and B35. Their work clearly
showed that QTLs varied across environments
and years, and that three stay-green QTLs were
each detected in more than two environments.
Xu and colleagues (2000) identified four stay-
green QTLs (
Stg1
,
Stg2
,
Stg3
, and
Stg4
) in a map-
ping population based on a cross between B35
and Tx7000. Two trials were conducted in two
locations and two years, and stress was imposed
by stopping irrigation at anthesis. Stay-green was
assessed with a plot score of 1 to 5 at physio-
logical maturity.
Stg1
and
Stg2
, found on link-
age group A (SBI-03), were consistently identi-
fied across locations and in both years, whereas
Stg3
and
Stg4
were on linkage group D (SBI-
02) and J (SBI-05) and were found in specific
seasons only. Using the same mapping popula-
tion in other field trials, Subhudi and colleagues
(2000) compared their QTL results to those
of Crasta and colleagues (1999) and Tuinstra
and colleagues (1997), and showed consistency
of QTLs in different genetic backgrounds. Sub-
udhi and colleagues (2000) showed that
Stg2
,
Stg3
and
Stg4
of the current populations corre-
sponded to
StgA
,
StgD
, and
StgJ
of Crasta and
colleagues (1999) and asserted that
Stg2
was
likely the most important for the expression of
the stay-green phenotype. Although
Stg1
of Sub-
hudi and colleagues (2000) found no equivalent
in Crasta and colleagues (1999), it was likely
very closely related to
StgB
and
Stg I.1
of Crasta
and colleagues (1999). Sanchez and colleagues
(2002) reported on the development of near-
isogenic introgression lines for four stay-green
QTLs (
Stg1
,
Stg2
,
Stg3
and
Stg4
of Subudhi
et al., 2000) in a marker-assisted backcrossing
program involving B35 as donor and Tx7000 as
recurrent parent.
Kebede and colleagues (2001) have mapped
QTLs for stay-green with another donor parent,
SC56, a conversion line (3-dwarf plant height
and reduced photoperiod sensitivity) derived
from a Sudanese caudatum-nigricans landrace.
Another donor parent for stay-green, E 36-1,
a cultivar of Ethiopian origin, has also been
used to map QTLs for the stay-green trait in
two RIL mapping populations from which a
total of seven QTLs were identified (Haussmann
et al., 2002), with three of them being common
to both populations. Kassahun and colleagues
(2010) reported results validating
StgB
, which
appears to be identical to a stay-green QTL on
the long arm of SBI-02 that has been incorpo-
rated into elite breeding material in Australia by
conventional pedigree selection (D. Jordan and
A. Borrell, pers. comm.). More recently, a third
stay-green QTL on SBI-02, in addition to the
previously reported
Stg3
and
StgB
, was reported
by Haryarimana and colleagues (2010) as map-
ping to the interval flanked by SSR markers
Xtxp19
and
Xtxp298
on the short arm of SBI-
02. Finally, Sabadin and colleagues (2012) have
mapped two stay-green QTLs on SBI-02 (
St2-1
and
St2-2
) as well as single stay-green QTLs
on SBI-03 (
St3
), SBI-04 (
St4
), SBI-05 (
St5
),
SBI-06 (
St6
), SBI-08 (
St8
), and SBI-09 (
St9
),
together explaining 65 to 69% of the genetic
variance for stay-green in two water-stressed
environments, using an SSR-anchored, DArT-
saturated (Diversity Arrays Technology) linkage
map for a modest-sized RIL population based
on the cross of BR007, a breeding line from the
Embrapa Maize and Sorghum program in Brazil,
and SC283, a USDA sorghum conversion line
based on guinea landrace IS7173C, from Tanza-
nia. When flowering time and plant height were
used as cofactors in the QTL analysis, many of
the originally detected stay-green QTLs in this
population were no longer statistically signifi-
cant; on the other hand,
St3
,
St4
,
St8
and newly
detected
St10
were statistically significant and
together explained 30 to 35% of genetic varia-
tion for stay-green in the water-stressed environ-
ments in the two years of testing.
Overall, six sources of the stay-green trait
(B35
BTx642, E 36-1, QL41, SC56, SC283,
and SDS 1948-3) have so far been used for
the identification of QTLs for this pheno-
type in sorghum. Several of the stay-green
QTLs identified have been validated in dif-
ferent backgrounds. However, there is to date
only scant understanding and knowledge of the
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