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

UV radiation effects
UV-B radiation damages leaf tissues in rice
seedlings. Leaves become stunted, stomata
collapse, and photosynthesis decreases. Some
rice varieties appear to be better able than others
to withstand the adverse effects of UV radiation.
Leaves of tolerant varieties contain phenolic
compounds, which are natural chemicals that
filter out harmful UV-B radiation before it can
damage sensitive tissues. Research is now in
progress to predict possible regional losses in
rice productivity if UV-B radiation continues to
increase, and whether plant breeders can prevent
those yield losses by developing new varieties
that tolerate UV radiation.
In addition to its adverse direct effects on
rice plants, UV-B may change the susceptibility
to and/or tolerance for disease. Although there is
no evidence yet that UV-B affects susceptibility
to blast, it appears that the tolerance for blast
decreases. In other words, UV-B does not
increase disease frequency, but enhances the
effects of disease on plant growth.
Global warming
Although increasing atmospheric CO 2
stimulates
plant growth, the beneficial effects on rice
growth have been observed for levels up to only
500 ppm. Some plant species respond positively
to CO 2
levels up to 1,000 ppm. Experts predict
that atmospheric CO 2
will surpass 650 ppm
before the end of the 21st century. Furthermore,
the benefits of increased CO 2
would be lost if
temperatures also rise. That is because increased
temperature shortens the period over which rice
grows. Research is being conducted to identify
means by which rice plants may better benefit
from increases in atmospheric CO 2
while
minimizing the adverse effects of warmer
temperatures (Fig. 1).
Emissions of greenhouse gases
from rice fields
Methane (CH 4
) is second in importance to CO 2
as a greenhouse gas. CH 4
concentration in the
atmosphere has more than doubled during the
last 200 years. Some of this CH 4
is produced by
rice fields (Fig. 2). To reduce the burden and
harmful effects of CH 4
in the atmosphere, emissions
from all anthropogenic sources have to be
mitigated.
Methane is produced in the anaerobic
conditions associated with submerged soils.
Much of it escapes from the soil to the
atmosphere via gas spaces in rice roots. The
remainder bubbles up from the soil or diffuses
slowly through the soil and overlying floodwater.
The potential for CH 4
emissions from rice
fields has long been noted, but comprehensive
measurements of CH 4
fluxes in rice fields have
been reported only since the early 1990s. Water
regime, organic matter management,
temperature, and soil properties as well as rice
plants are the major factors determining the
production and flux of CH 4
in rice fields.
Irrigated rice areas are the major source of CH 4
emissions from rice fields. The assured water
supply and control, intensive soil preparation,
and resultant improved growth of rice favor CH 4
production and emissions.
With financial support from the U.S.
Environmental Protection Agency, IRRI
undertook baseline research on CH 4
fluxes in rice
fields in collaboration with the Fraunhofer
Institute for Atmospheric Environmental Research,
Germany, and the Wetland
Biogeochemistry Institute of Louisiana State
University, USA. Other collaborating institutes
were the Wageningen Agricultural University,
The Netherlands; the Laboratory for Microbiology,
French Institute of Scientific Research for
Development Cooperation; the Université de
Provence, France; and the University of Georgia,
USA. IRRI also coordinated an interregional
research program on CH 4
emissions from rice
fields funded by the Global Environmental Facility
of the United Nations Development
Programme. This activity comprised
collaborative CH 4
research on irrigated, rainfed,
and deepwater rice in China, India, Indonesia,
the Philippines, and Thailand.
The studies, which took almost a decade of
work, concluded that CH 4
emissions from rice
fields are much smaller than originally thought,
contributing on the order of only 10% of total
global CH 4
emissions. High methane emissions
are associated with specific rice management
practices, and management practices can be
modified to reduce emissions without reducing
yields.
One result was the finding that CH 4
production from Indian rice production,
originally estimated to be some 38 million t per
38 Rice almanac