Biology Reference
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benefits or possible negative trade-offs of any
given trait/mechanism and thereby estimate its
potential value for region-specific breeding pro-
grams.
As an example, the genotype-specific stom-
atal closure at high evaporative demand, one of
the “water-saving” traits reviewed above, is a
feasible proposition. The idea of intra-specific
variability in VPD(vapor pressure deficit)-driven
stomatal closure was proposed long ago (e.g.,
Squire 1979), although it was not until recently
that this mechanism was explored by model-
ing (Sinclair et al. 2005; 2010). In the mean-
time, variability for VPD response was found
across other crop species (e.g., groundnut: Devi
et al. 2010; pearl millet: Kholova et al. 2010;
and chickpea: Zamman-Allah et al. 2011). There
is evidence that similar variability exists in
sorghum (Gholipoor et al. 2010), and, indeed the
model suggests that this mechanism will lead
to desired improvement of the
post-rainy sea-
son
sorghum cultivation in terms of absolute
grain yield as well as yield stability (our work
in progress).
likely reorient the breeding of stay-green towards
the breeding for its most important compo-
nents. This will require a “re-mapping” of these
explanatory traits and possibly the identification
of new/better donor sources for these traits than
the donors for stay-green that are currently in
use. The recent dramatic progress achieved in
terms of density of marker coverage across the
full nuclear genome will be extremely useful for
precisely mapping these explanatory traits. Sev-
eral of the putative traits leading to stay-green
expression closely interact with the environment.
Therefore their manipulation will also require
a thorough understanding of these interaction
effects, and the use of crop simulation modeling
will then become increasingly important to help
the breeding program navigate the complexity
of plant-trait interactions with the environment
(including crop management practices), in order
to better target the type of trait combinations
needed for each specific target environment.
Acknowledgement
The authors are grateful for a grant from ACIAR
(CIM/2007/120) (Improving postrainy sorghum
varieties to meet the growing grain and fodder
demand in India), which has supported a sub-
stantial portion of the work described here.
Conclusions
Much progress has been achieved in the deci-
phering of genomic regions responsible for
the expression of the stay-green phenotype in
sorghum. This information has been used in a
number of breeding programs worldwide, mostly
through marker-assisted backcrossing to move
donor parent alleles for this trait into other-
wise locally-adapted agronomically elite open-
pollinated varieties and/or hybrid parental lines.
However, the physiological mechanisms under-
lying the expression of stay-green are less clear.
It appears now that the availability of water dur-
ing the grain-filling period, when stay-green is
scored, is the most likely candidate, and may
likely be additive to N absorption after anthe-
sis. The sources of water availability could be
several, including water-saving traits but also
possibly deeper rooting capacity. Clearly, the
more recent progress pointing at clear mecha-
nisms explaining the stay-green phenotype will
References
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