Biology Reference
In-Depth Information
produce a harvestable yield. Drought tolerance
is a quantitative trait long considered a complex
trait because of its phenotypic and genetic con-
trol by multiple components (McWilliam 1989).
The mechanisms behind drought tolerance in the
context of grain yield have been considered com-
plex by many physiologists as well as breeders,
since it is combinations of different plant traits
that are thought to determine the overall ability
of plants to produce good yield under drought.
Recently, drought tolerance has been described
to be as simple a trait as any quantitative trait -
provided the severity and intensities of drought
stress are clearly classified and the ecosystem is
well defined. The label of 'complex' has been
attributed to the large number (1000
insufficient or sporadic rainfall. Decline in total
rainfall as well as uneven distribution of rainfall
during the growing season reduce rice yields in
rain-fed areas. Furthermore, the diverse topogra-
phy of the rain-fed ecosystem that ranges from
the upper topo-sequence to lower topo-sequence,
and includes upland and lowland ecosystems,
exposing plants to variable severity of drought
stress. Improving rice yields under drought must
therefore overcome a range of different types
of
drought
stress
in
terms
of
both
severity
and timing.
The CurrentDrought-Tolerance
Improvement Strategy atthe
International Rice Research
Institute (IRRI)
) of genes
differentially regulated under dehydration stress
in gene expression studies (Blum 2011). When
drought tolerance is measured in terms of yield
under drought stress, grain yield is found to
be associated with many large- and small-effect
QTLs, leading to the conclusion that drought tol-
erance is complex (Blum 2011). On the other
hand, there are very few reports of successful
improvement to grain yield under drought by
improving a single physiological trait. There-
fore, although the concept of drought tolerance
may be considered either simple or complex, the
prospects of improving drought tolerance have
clearly been regarded as challenging.
+
Direct Selection for Grain Yield is
Achieved with Proper Drought
Treatments
Many of the initial efforts to improve grain yield
under drought focused on improving secondary
traits such as root architecture, leaf water poten-
tial, panicle water potential, osmotic adjustment,
and relative water content (Fukai et al. 1999;
Price and Courtois 1999; Jongdee et al. 2002;
Pantuwan et al. 2002; Toorchi et al. 2003). Ear-
lier attempts to improve grain yield through
improvement of physiological traits were based
on the findings of several studies in which low
heritability for grain yield under drought stress
was reported (Rosielle and Hamblin 1981; Blum
1988; Edmeades et al. 1989). With the increased
ability to appropriately classify drought stress,
the ability to do better standardized phenotyp-
ing, and the availability of advanced statistical
designs, many recent studies (Atlin and Lafitte
2002; Bernier et al. 2007; Kumar et al. 2008;
Vikram et al. 2011) have shown that broad-
sense heritability (H) for grain yield can be
as high as that of its component traits, which
are often not highly correlated with grain yield.
Recently, in many of the mapping studies follow-
ing direct selection for grain yield, the heritabil-
ity of grain yield under drought was comparable
with the heritability of grain yield under irrigated
Rice and Drought
Rice is the staple food for more than half of the
world's population and is grown in more than
110 countries around the world. However, more
than 90% of global rice is grown and consumed
in Asia (Asia Rice Foundation 2005). In 2011,
estimates showed a production of around 690
million tons of rice from a planted area of about
160 million hectares (FAO 2011). About 40% of
the cultivated rice area is classified as 'rain-fed,'
that is, totally dependent on rainfall as a source
of water supply, with no or very little irrigation
infrastructure available. Around 23 million ha of
rice area in Asia is considered to be drought-
prone (Pandey and Bhandari 2008) because of
Search WWH ::
Custom Search