Biology Reference
In-Depth Information
flooded ecosystems, as we can see in today's
lowland rice (Pampolino et al. 2008); however,
too much water at any stage of development
could lead to serious injury or total crop loss.
Recently weather patterns have become increas-
ingly irregular as a result of climate change
(Wassmann et al. 2009; Jagadish et al. 2012), and
unexpected heavy rains can inundate rice fields
along riverbanks and in low-lying areas and dam-
age crop production. In recent years, destruc-
tive typhoons and heavy rains have caused huge
damage in Bangladesh, Cambodia, Myanmar,
the Philippines, Laos, Vietnam, Pakistan, and,
most recently, Thailand, causing these countries
to lose millions of tons of rice.
The most common flooding stress is the “flash
flooding” that occurs during the monsoon season
with different intensities and durations. During
these episodes, water completely submerges the
established rice crop for a short period from sev-
eral days up to two weeks. If rice plants remain
submerged for more than five days, they start to
die and do not recover after the water recedes.
The severity of the stress and the number of days
that rice plants can survive underwater depends
on environmental conditions, such as tempera-
ture, water turbidity, solar radiation, and soil fer-
tility (Setter et al. 1997; Das et al. 2009). In
flash flood-prone regions, farmers usually culti-
vate landraces that are tall and moderately tol-
erant of submergence but that have low yield.
In some other areas where high-yielding but
submergence-intolerant rice varieties have been
cultivated, farmers often suffer from crop losses
or significant yield reduction caused by flash-
flood episodes.
It is not uncommon for floodwaters to stay in
the field from two weeks to several months in
some low-lying rice areas. In this situation, the
water level is generally 20 to 50 cm in depth,
which is referred to as stagnant flooding (Septin-
ingsih et al. 2009; Mackill et al. 2010; Singh et al.
2011). Even though plants are not completely
submerged under these conditions, grain produc-
tion is greatly reduced due to poor tillering and
greater susceptibility to lodging (Tuong et al.
2000; Singh et al. 2011). In some areas, stagnant
flooding can also immediately follow a flash-
flood period. In this situation, farmers usually
rely on landraces that can cope with both stresses,
although these tend to be very low yielding. In
areas where progressive flood waters reach a
depth of several meters for several months, farm-
ers usually cultivate “floating rice” or “deepwater
rice” that shows rapid internode elongation with
the rising floodwater, maintaining the uppermost
leaves and panicles above the surface (Catling
1992). Deepwater rice generally has poor yield
due to excessive vegetative growth.
Flash floods can also cause excessive damage
during germination and early seedling growth,
resulting in poor crop establishment in direct-
seeded rice areas. This can occur in both irri-
gated areas when the land is not level and in
flood-prone rain-fed ecosystems when rainfall
occurs within a few days following seeding
(Ismail et al. 2009; Angaji et al. 2010). Most
rice varieties are unable to vigorously germi-
nate, elongate, and survive under complete sub-
mergence and, as a consequence, seedling estab-
lishment is very poor or completely absent in
fields that are flooded. Poor seedling germina-
tion is also a problem if fields are not properly
leveled, resulting in the formation of puddles in
which seedlings are submerged, and in fields
with drainage problems, if heavy rains occur
directly after seeding. The hazards of flooding
during germination can prevent farmers from
adopting direct-seeded rice technology or force
them to discontinue this practice (Konchan and
Kono 1996). Farmers usually wait for the “right”
time to sow or broadcast their seeds; however,
if an unexpected flood destroys their crops, they
reseed or transplant their fields. Apart from being
costly and labor demanding, the delay caused by
re-planting and thereby late harvest of the first
crop can delay the planting of the following sea-
son's crop.
Efforts to identify submergence-tolerant rice
genotypes were initiated at the International Rice
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