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Life Science Research and Sustainable Development ISBN: 978-98-84663-33-9

per cent more by 2025. For rice consuming countries there is need to produce 40 per cent more rice
by 2030 (Khush, 2005 and Zhu et al., 2011). To achieve this ambitious goal various rice varieties with
greatly improved agronomic traits such as high yield potential, stress tolerance etc., should be
developed.
Rice is unique in its ability among cereal crops to grow in a wide range of environments.
Depending on the hydrology, the rice environment can be classified intofour major ecosystems
are generally recognized (Khush, 1984) as follows: (1) irrigated, (2) rainfed lowland, (3) upland,
and (4) floodprone. These environments vary with respect to elevation, rainfall pattern, depth of
flooding and drainage, hydrological status, soil type and by the adaptation of rice to agro
ecological factors (Huke and Huke, 1997 and Maclean et al., 2002). Approximately, 55% of the
world rice area planted to rice is irrigated and is the most productive rice growing system,
perhaps contributes 75% of the world rice production. Large areas of rice are grown under
lowland and upland rainfed conditions.As a complex agronomic trait, grain yield of a rice plant
is multiplicatively determined by three component traits: number of panicles per plant, number
of grains per panicle, and grain weight.Different mappingpopulations have been used to explore
the QTLs controlling yield related traits.
Rice to be grown successfully under a variety of climatic conditions across the globe;
breeders maintain rice at high genetic diversity. Globally, rice is grown on 154 million hectares
(Mha), and approximately 45 % of this area is under rainfed conditions that have very low-yield
potential (Verulkaret al., 2010). About 80 million ha of irrigated lowland provide 75% and about
60 million ha of rainfed lowlands supply about 20% of the rice production. Rice needs more water
compared to other crops, on an average about 2,500 liters of water is needed to produce 1 kg of
rough rice. Irrigated rice receives an estimated 34-43% of the total world’s irrigation water.
Worldwide water for agriculture is becoming increasingly scarce, day by day due to uncertain
and uneven rainfall distribution patterns, shrinking groundwater resources, increasing level of
salts in soil solution and diverting the fresh water resources to competing urban and industrial
uses. In the coming future, water availability may be more affected due to ongoing changes in
global climate. Because of semi-aquatic ancestry, rice is extremely sensitive to water shortage.
Drought is the major constraint to rice production in rainfed areas across Asia. Drought can be
simply defined as reduction in yield due to shortage of water (Bernier et al., 2008). Throughout
the world, about 34 per cent (~54 million hectare) of the total land under rice cultivation is under
rainfed condition (Maclean et al., 2002). Asia occupies 32.1 per cent rainfed low land rice of the
total rice area which currently averages production of 2.3 tonnes per hectare (Tuong and Bouman,
2003). Drought is the most devastating among abiotic stresses and it depresses yield by 15-50 per
cent depending on the vigor and period of stress in rice (Srividyaet al., 2011). The global reduction
in rice production due to drought averages 18 million tonnes annually (O’Toole, 2004). Rice is
sensitive to drought stress during reproductive growth and even moderate stress can result in
drastic reduction in grain yield (Hsiao,1982; O’Toole, 1982 andVenuprasadet al., 2008).
2. Yield and its component traits
As a complex agronomic trait, grain yield of a rice plant is multiplicatively determined
bythree component traits: number of panicles perplant, number of grains per panicle, and
grainweight. Number of panicles is dependent on the ability of the plant to produce tillers
(tilleringability), including primary, secondary, and tertiary tillers. Number of grains per panicle
can also be attributed to two subcomponents: number of spikelets, which is mainly determined
by the numbers of primary and secondary branches, and seed setting rate of the spikelets. Grain
weight is largely determined by grain size, which is specified by its three dimensions(length,

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