Biology Reference
In-Depth Information
within rice for further analysis and use in breed-
ing tolerant varieties.
genotypes that are inherently taller. Varieties
combining both tolerance of prolonged SF and
SUB1
are
needed
for
broader
adaptation
in
almost all flood-prone areas.
Under more extreme conditions, water can
reach in excess of 100 cm to as much as a
few meters and stagnate for several months,
with the result referred to as deepwater or float-
ing rice (Catling 1992). Elongation ability of
leaves and internodes under these conditions is
essential to keep up with the rising water lev-
els and escape complete submergence. This will
ensure an oxygen supply to roots via the con-
tinuum of aerenchyma tissue, to avoid anoxia
and gain access to CO
2
and light to maintain the
energy supply for such excessive growth (Set-
ter et al. 1997). Traditional varieties adapted to
these environments can elongate by more than
20 cm per day, but have poor grain quality and
are low yielding due to their low tillering ability,
long droopy leaves, and susceptibility to lodg-
ing. Areas under deepwater rice are now shrink-
ing because of the lower productivity of the sys-
tem and the spread of dry-season rice with the
use of shallow tube wells, as in Bangladesh and
Thailand, or due to better flood control, as in
Vietnam. Internode elongation in deepwater and
floating rice is induced by ethylene and GA, in a
manner contrasting to that of the
SUB1
mode of
action, and, recently two genes (
SNORKEL1
and
SNORKEL2
) that are responsible for this intern-
ode elongation under deepwater conditions were
cloned (Hattori et al. 2009).
Escape Strategy under Longer-term
Partial Flooding
Water of 20 to 50 cm depth can accumulate and
persist for most of the season in flood-affected
rain-fed areas, and is referred to as medium-
deep or stagnant floods (SF), and this sometimes
occurs following short-term complete submer-
gence, when the effects are even more devas-
tating (Mackill et al. 2006; Singh et al. 2011).
Modern rice varieties are not adapted to these
conditions and their yield can decline markedly.
This type of flooding received less attention in
the past despite the enormous yield losses and the
vast areas affected. In recent years, efforts have
been devoted to screening large sets of diverse
rice germplasm to identify genotypes with rea-
sonable tolerance. Out of more than 700 acces-
sions screened based on survival, yield, and yield
components, fewer than one-third showed some
tolerance (unpublished data). SF resulted in poor
survival and low yield. Variation in elongation of
the internodes of the tolerant genotypes was also
observed in a manner dependent on the water
depth (facultative elongation) just to keep pace
with the rising water. Survival and yield under SF
are largely dependent on the extent of this par-
tial elongation, together with lesser carbohydrate
depletion, thicker culms for increased internal
oxygen transport, and high tillering underwa-
ter with more productive panicles with greater
spikelet fertility.
Our recent studies highlighted the need to
combine both
SUB1
and SF tolerance for areas
where both stresses are experienced (Singh et al.
2011). As
SUB1
promotes survival of submerged
plants by hindering shoot elongation to con-
serve energy reserves, its incorporation into
shorter varieties makes them more sensitive to
partial stagnant flooding. However, responses
to SF were independent of
SUB1
introgression
but more dependent on the background of the
recipient genotype, with better performance of
Future Prospects
Flooding during Germination
DevelopmentofVarietiesTolerantofFlooding
duringGermination
We have now introduced the major QTL
AG1
derived from Khao Hlan On (KHO) variety into
IR64 and IR64-Sub1. The introgression of this
QTL has also begun in other genetic backgrounds
such as Ciherang-Sub1, PSB Rc18-Sub1, and
PSB Rc82. We will introduce this QTL into more
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