Maclean et al. - 2002 - Rice almanac source book for the most important e

year, was actually less than 19 million t, about
4% of the estimated global total. Without this
research, international protocols would have required
India to halve its rice cultivation in the
short term! Emissions in other countries are also
much less than estimated.
The research demonstrated ways to reduce
emissions without sacrificing yields, such as
• using intermittent drainage in irrigated
systems, at the same time saving water;
• improving crop residue management
through composting, mulching, etc.; and
• using direct seeding, at the same time
reducing water and labor requirements.
The project explored trade-offs between
mitigation technologies and socioeconomic
aspects and found that
• intermittent drainage in irrigated systems
reduces emissions and can also save water;
• improved crop residue management
through composting, mulching, and early
incorporation of organic manure can also
reduce emissions; and
• direct seeding results in less labor and
water input and at the same time reduces
methane emissions.
However, management practices that make
conditions less anaerobic favor production of
another greenhouse gas, nitrous oxide (N 2
O),
which is 10 times more powerful in its
atmospheric warming potential than methane.
Agriculture is the main source of N 2
O emissions;
it is produced in the soil as an intermediate
product during microbial nitrification and
denitrification. Potential N 2
O emissions increases
when available N increases through, for
example, fertilization. Although N 2
O is not a
problem in continuously flooded systems, using
more aerobic rice-growing conditions or growing
aerobic rice could result in significant N 2
O
emissions. This is a new area for research.
Biotechnology issues
Some desired features of future improved rice
varieties are superior grain quality, higher yield
potential, enhanced resistance to pests and
diseases, and greater tolerance for stresses such
as drought, cold, and nutrient deficiencies.
Biotechnology is seen as perhaps the most important
new resource for achieving varietal
improvement (Table 1).
Table 1. Applications of biotechnology techniques to rice
improvement.
Technique
Embryo rescue
Anther culture
Molecular marker-aided
selection
DNA fingerprinting
Transformation
(Agrobacterium,
protoplast, and
biolistic methods)
Application
Transfer of genes from wild
rice to cultivated rice
Rapid stabilization of new
lines
Acceleration of breeding
programs by use of
genetic markers rather
than phenotypic selection
Identification of genetic
variation in pests and
pathogens
Introduction of novel genes
into rice
Rice biotechnology techniques encompass
plant tissue culture and molecular biology. Tissue
culture techniques such as embryo rescue and
anther culture have made important
contributions. Embryo rescue enables breeders to
attempt wide crosses between varieties that could
not be hybridized before; anther culture allows
faster stabilization of breeding lines. Molecular
techniques help to accelerate traditional breeding
programs through gene tagging, streamlined
germplasm management, and assessment of
population structures in insect pests and
pathogens through DNA fingerprinting.
Marker-assisted selection, for example, has
been used recently at IRRI to improve crops in
saline areas. Scientists developed a large number
of plants whose genetic tolerance for salinity was
proven by marker-assisted selection. The plants
are being designed specifically for use in
Bangladeshi coastal wetlands and are being
tested and further bred by farmers there under
supervision in their own fields.
After extensive study, a marker has been
identified linked to recessive major-gene
resistance to the devastating rice yellow mottle
virus (RYMV). RYMV is a major threat to
irrigated and lowland rice in West Africa, and
has also been found in East Africa. The way is
now open for marker-assisted selection of
resistant varieties.
40 Rice almanac