Wassmann und Aulakh - 2000 - The role of rice plants in regulating mechanisms o

Biol Fertil Soils (2000) 31:20–29 Q Springer-Verlag 2000
REVIEW ARTICLE
Reiner Wassmann 7 Milkha S. Aulakh
The role of rice plants in regulating mechanisms of methane missions
Received: 7 April 1999
Abstract Rice plants play a pivotal role in different
levels of the methane (CH 4 ) budget of rice fields. CH 4
production in rice fields largely depends on plant-borne
material that can be either decaying tissue or root exudates.
The quantity and quality of root exudates is affected
by mechanical impedance, presence of toxic elements,
nutrient deficiencies, water status of growing
medium, and nitrogenase activity in the rhizosphere.
CH 4 oxidation in rice fields is localized in the rhizosphere
where the concentration gradients of CH 4 and
oxygen overlap. CH 4 oxidation capacity is a function of
the downward transport of oxygen through the aerenchyma,
which, in turn, also acts as a conduit for CH 4
from the soil to the atmosphere. The decisive step in
the passage of CH 4 through rice plant is the transition
from root to stem. However, rice plants show an enormous
variety of morphological and physiological properties,
including differences in root exudation and gas
transfer capacity. Comparative studies on different cultivars
are deemed crucial for accomplishing a better understanding
of the mechanisms of CH 4 consumption in
the rhizosphere and CH 4 transport through the rice
plant as well as the interaction of these processes. The
results of such studies are considered tools for devising
mitigation options.
Key words Methane production 7 Methane oxidation 7
Methane emission 7 Rice fields 7 Plant-mediated gas
transfer
R. Wassmann (Y) 7 M.S. Aulakh
International Rice Research Institute, P.O. Box 3127,
1271 Makati City, The Philippines
e-mail: R.Wassmann6cgiar.org
Fax: c63-2-891292
R. Wassmann
Institute for Atmospheric Environmental Research,
Garmisch-Partenkirchen, Germany
M.S. Aulakh
Department of Soils, Punjab Agricultural University, Ludhiana,
Punjab, India
Introduction
The atmospheric concentration of the radiative active
methane (CH 4 ) gas has increased at rate of F0.8% per
year during previous decades (IPCC 1994). However,
the concentration increase since 1992 has shown large
interannual variation between 0 and 0.5% (Dlugokencky
et al. 1998). Rice cultivation is one of the most
important anthropogenic sources of atmospheric CH 4
(IPCC 1994). The development of methods and strategies
to reduce the emission of CH 4 from paddy fields is
a central component of the ongoing efforts to protect
the earth’s atmosphere and to avert a possible climatic
change (Rennenberg et al. 1992). Rice plants act in
three key functions regulating the CH 4 budget: (1) as a
source of methanogenic substrate, (2) as a conduit for
CH 4 through a well-developed system of intercellular
air spaces (aerenchyma), and (3) as an active CH 4 oxidizing-site
in rhizosphere by supporting O 2 countertransport
through aerenchyma system (Fig. 1). However,
an assessment of the role of plants has to consider
the enormous genotypic and phenotypic variations. The
genus Oryza comprises approximately 80,000 known
cultivars with large variations in morphology and physiology.
Furthermore, the traits of rice plants are modified
by the growth conditions such as nutrient supply,
climate, local management practices etc., which makes
a generic assessment even more difficult.
Global estimates of CH 4 emission from rice cultivation
vary widely ranging from 20 to 150 Tg CH 4 year –1
(IPCC 1992) while more recent estimates are in the
range of 30–50 Tg CH 4 year –1 (Neue and Sass 1998).
The complexity of the role of rice plants for regulating
CH 4 fluxes to the atmosphere is one of the main reasons
for the uncertainties in the global estimates of this
CH 4 source. Field and laboratory experiments clearly
indicate that there is a cultivar impact (Watanabe et al.
1995b; Butterbach-Bahl et al. 1997; Sigren et al. 1997;
Wang et al. 1997a), but the net-effect of substituting
cultivars, e.g. traditional by modern lines, is still uncer-

21
Fig. 1 Schematic view of the
methane budget of rice fields
with emphasis on plant impacts;
insets depict CH 4 diffusion
through aerenchyma in a
root, b root-stem transition
zone, and c a stem-leaf section
zone (modified from Nouchi
and Mariko 1993; Wassmann
et al. 1998)
tain. This is contrasted by the impact of different management
practices, such as organic versus mineral fertilizer,
continuous flooding versus intermittent drainage,
that can be predicted with reasonable accuracy (Wassmann
et al. 1993a, b; Neue et al. 1997). In view of the
future rice demand for feeding the increasing world
population, the traits of high-yielding rice varieties will
affect the CH 4 source strength of rice cultivation. A
thorough understanding of the mechanisms involved is
required to direct the endeavours of selection – and
possibly breeding – toward high-yielding rice plants
with a limited emission potential. This paper reviews
and compiles the current state of knowledge on mechanisms
and factors that control plant-mediated and
plant-borne CH 4 emissions as well as the significance of
this process for the overall CH 4 flux from rice fields.
Gaps in our knowledge and future research needs are
indicated.
CH 4 production in soil
Strictly anaerobic conditions and availability of readily
decomposable organic substances are essential for the
process of CH 4 production in soil. Rice plants can influence
the CH 4 production by enhancing the anaerobiosis
in soil as well as by supplying organic compounds
through their root exudates, which have the ability to
readily supply C and energy source to the microorganisms
present in the rhizosphere.
Soil redox potential and ecophysiological sequence
of reduction processes
Soil anaerobiosis, measured in terms of redox potential
(Eh), ranging from about –100 to –200 mV, is needed
for the initiation of CH 4 production in paddy soils (Takai
1961, 1966; Cicerone et al. 1983; Yagi and Minami
1990; Lindau et al. 1991; Wang et al. 1993). The intensity
and capacity of soil reduction are controlled by the
nature and extent of organic substances (electron donors),
temperature, degree of waterlogging, and the nature
and quantity of electron acceptors (Ponnamperuma
1972). In laboratory experiments (Takai 1961; Watanabe
1984), it has been demonstrated that the ratio of
CO 2 to CH 4 production in several paddy soils varied by
more than a factor of 10 and correlated with the relative
ratio of total oxidizing (electron-accepting) capacity
to reducing (electron-donating) capacity of the soils.
Soils containing high amounts of readily decomposable
organic substrates (e.g. acetate, formate, methanol, methylated
amines etc.) and low amounts of electron acceptors
(NO 3– , Mn 4c , Fe 3c , SO 4
2–
) are likely to show a
high production of CH 4 (Parashar et al. 1991; Yagi et
al. 1994).
As soon as a soil becomes flooded, the trapped O 2 is
respired and in the following steps, NO 3– , Mn 4c , Fe 3c ,
compounds are reduced successively (Takai and Kamura
1966; Ottow 1969; Yoshida 1975). During these reductive
processes, facultative and obligate anaerobic
bacteria use NO 3– , Mn 4c , Fe 3c , oxides and hydroxides
as alternative electron acceptors to continue their energy-conserving
metabolic reactions at the expense of
easily decomposable organic material (Takai and Kamura
1966; Munch and Ottow 1977; Ottow and Munch
1978). For example, Munch and Ottow (1983) showed
that NO 3– , Mn 4c and Fe 3c are reduced directly by bacterial
reductases and not through chemical processes.
The microbial population may even use non-crystalline
Fe 3c oxides in preference to crystalline forms as external
electron acceptors (Munch and Ottow 1980).
Rice plants can influence the soil Eh by consuming
O 2 from the rhizosphere (root respiration) and by enhancing
the supply of electron donors, i.e. readily decomposable
organic substrates through root exudates,
sloughed-off tissues and debris, and microbially reduced
Mn 2c and Fe 2c ions and chelates. The variability
in content and composition of exudates of different
rice plants could influence soil Eh and CH 4 production

22
differently. For instance, Wang (1995) observed lower
Eh values in soil with Dular, a traditional variety from
the Philippines, than with the modern varieties IR 72
and IR 65538 at heading and ripening growth stages.
Dular showed highest dry matter production resulting
in higher root weight and, as a consequence, larger root
exudation of organic C.
Root exudates
Root exudates contain both high-molecular-weight substances,
mainly mucilage and ecto-enzymes, as well as
low-molecular-weight substances consisting of organic
acids, phenols and amino acids (Andal et al. 1956;
MacRae and Castro 1966; Trolldenier 1977, 1981;
Marschner 1995). The amount of knowledge available
on exudation from rice plants is minute, so that a discussion
on the factors affecting exudation has to rely on
findings obtained with other crops. Total amounts as
well as the proportion of different compounds in root
exudates vary considerably due to various endogenous
and exogenous factors such as mechanical impedance,
presence of toxic elements, nutrient deficiencies, water
status of growing medium, and nitrogenase activity.
Maize grown in a nutrient culture solution exuded a
three times lower amount of sugars and vitamins than
exudation by plants grown in a solid substrate (Schönwitz
and Ziegler 1982). Plants increase their root exudation
to improve their ability to tolerate toxic elements
such as Pb, Cd and Al (Horst et al. 1990). High
exudation restricts the uptake of these elements by preferential
binding and immobilization with mucilage
(Horst et al. 1982). Exudation can also help to cope
with nutrient deficiency through mobilization of nutrients
at the root-soil interface, e.g. phosphate desorption
from clay surfaces by the polygalacturonic acid
component of mucilage (Nagarajah et al. 1970). Under
dry soil condition, high amounts of mucilage facilitate
transport of nutrients from soil particles to the plasma
membrane of root cells (Nambiar 1976).
Plants do not only adjust the quantity but also the
quality of root exudates. In general, nutrient deficiencies
are responded to by higher amounts of low-molecular-weight
organic acids in root exudates (Marschner
1993). If there is a deficiency of Mn and Fe, these acids
increase the solubility of MnO 2 and Fe-oxides by chelation
facilitating root uptake (Godo and Reisenauer
1980; Jauregui and Reisenauer 1982). High excretions
of citric and malic acid result in local acidification of
the rhizosphere which – in turn – mobilizes the sparingly
soluble inorganic compounds such as phosphates, Fe,
Mn and Zn (Hoffland et al. 1989; Marschner 1995).
These few examples illustrate that the total amount and
proportion of different compounds in root exudates
that constitute a dynamic rather than a static plant parameter.
In a study with rice, Lin and You (1989) reported
that root exudates from rice contained varying amounts
of organic acids, carbohydrates and amino acids.
Among organic acids, citric acid had the highest concentration
followed by malic, succinic and lactic acid in
decreasing order. These findings illustrate a large variation
in composition and content of root exudates of different
rice varieties (Fig. 2). They further observed that
inoculation with Alcaligenes faecalis stimulated the excretion
from rice roots which was related to the nitrogenase
activity in the rhizosphere. Nagarajan and Prasad
(1983) also indicated that significant differences
could exist among rice cultivars in supplying different
amounts as well as in the chemical composition of their
exudates. More recently, Wang (1995) compared the
quantities of organic C released by roots of different
cultivars and found that the total amount of C exuded
was closely related to root dry weight (r 2 p0.919) as
well as to above-ground dry matter production
(r 2 p0.954).
These studies help to explain the seasonal variation
in CH 4 emission rates that often follow plant development.
High emission rates at the panicle differentiation/
flowering stage can clearly be attributed to recent
plant-borne material, either root exudates or decaying
root tissue (Watanabe et al. 1997). In laboratory incubations
of soil cores, CH 4 production by soil bacteria
was found to be highest near the soil surface in the rice
row and decreased with depth and distance from the
plant (Sass et al. 1991b). As the season progressed and
the root system expanded to deeper soil and further
away from the plant base, CH 4 production also increased
in these soil regions. Similarly a positive correlation
between rice yield and CH 4 emissions often observed
under field conditions (Neue et al. 1997) could
be due to a close relationship between plant size and C
Fig. 2 Content of different organic acids and carbohydrates in
root exudates of four rice cultivars from China (drawn from data
of Lin and You 1989)

23
excreted from roots which – in turn – influence CH 4
production.
From these studies, it is obvious that rice plants play
a key role in the production of CH 4 in soils. However,
unfortunately most of these studies are based on the
assumption that changes in CH 4 efflux rates reflect
changes in CH 4 production. This conclusion may not be
valid since CH 4 efflux is the net result of two opposite
mechanisms – production and oxidation of CH 4 .
Effects on CH 4 oxidation in soil
CH 4 is consumed in soil by bacteria that are strictly
obligate aerobes (Papen and Rennenberg 1990). These
bacteria can use CH 4 , but also a few other C compounds
such as methanol as a substrate. As they require
molecular O 2 (Bedard and Knowles 1989; King
1992; Knowles 1993), methanotrophs occur and are active
in oxic-anoxic interfaces where concentration gradients
of CH 4 and O 2 overlap. In flooded rice fields,
these conditions are generally confined to the rhizosphere
and a thin layer of the topsoil interfacing with the
water. However, depending on water regime, O 2 -rich
water may intrude into the lower soil layers and temporarily
stimulate CH 4 oxidation.
The estimates of the proportion of locally produced
CH 4 that is oxidized in the soil ranges from 58% (Sass
et al. 1991a, b) to 80% and higher (Holzapfel-Pschorn
et al. 1985; Conrad and Rothfuss 1991; Inubushi et al.
1992). Low CH 4 concentrations in the floodwater in
contrast to high concentrations in the soils (Holzapfel-
Pschorn and Seiler 1986; Nouchi and Mariko 1993) corroborate
the concept of a very efficient removal of CH 4
during the passage through oxidized layers. The rate of
CH 4 oxidation is often higher in rice-planted soil than
in unplanted soil (Inubushi et al. 1992). Rice plants influence
CH 4 oxidation in two ways, i.e. by diffusion of
atmospheric O 2 via aerenchyma into the rhizosphere,
and by enzymatic oxidation as measured by N flush inhibition
technique (Epp and Chanton 1993) and a-
naphthylamine oxidation method (Wang 1995).
Subsequent to flooding, plant roots encounter
anoxic conditions as a result of depletion of soil O 2 by
microbial and plant root respiration (Gambrell and Patrick
1978). In order to adapt to the flooded anoxic conditions,
most hydrophytes, including rice, develop an
aerenchyma system in both root and stem for maintaining
aerobic respiration (van Raalte 1941; Armstrong
1978, 1979; Keeley 1979; Grosse and Schroeder 1985).
O 2 diffuses through the aerenchyma to the roots (Barber
et al. 1962) while CO 2 and CH 4 move in the inverse
direction to the atmosphere. In addition to diffusion,
mass flow could also be an important mechanism of air
movement to the submerged parts of the plant through
the aerenchyma (Raskin and Kende 1985). However,
the pore size of the aerenchyma is the main plant parameter
that controls O 2 transport through the plant to
the rhizosphere and often shows a positive correlation
with O 2 concentration in the rhizosphere (Ueckert et
al. 1990).
Several factors have been reported to affect the O 2
release from the rice roots. For instance, metabolic inhibitors
such as DNP, NaN 3 and KCN could increase
the O 2 release rate (Ando et al. 1983). The soil Eh
could also influence this process (Kludze et al. 1993).
The release rates of O 2 were negatively correlated with
temperature but were unaffected by light conditions,
photosynthesis and respiration rates (Ando et al. 1983;
Nouchi et al. 1990).
The development of the aerenchyma is determined
by the intensity of anaerobiosis. For example, Kludze et
al. (1994) studied the development of aerenchyma in
plants at a soil Eh of –250B10 mV as compared to
plants under well-aerated conditions (515B25 mV). As
a result of enlarged aerenchyma, the root porosity was
increased to 41.4% in the flooded plants as compared
to only 13.3% in non-flooded or drained plants. Increased
porosity enhanced the transport of O 2 from the
atmosphere to the roots; O 2 loss from the roots increased
to 4.6 mmol O 2 g –1 day –1 in the flooded plants
as compared to only 1.4 mmol O 2 g –1 day –1 in drained
plants. Frenzel et al. (1992) detected O 2 down to the
depth of 40 mm in a flooded soil planted with rice
whereas it was confined to a thin surface layer (3.5 mm)
in an unplanted soil. This result illustrates the transport
of O 2 by rice plants to the flooded anaerobic soil. The
supply of O 2 by the plant to the rhizosphere often stimulates
high activities of CH 4 -oxidizing bacteria in the
vicinity of rice roots (Watanabe et al. 1997). De Bont et
al. (1978) counted 10 times more CH 4 -oxidizing bacteria
in the rhizosphere of rice at the tillering stage than
in the bulk of the anaerobic soil and one third more
than in the oxidized surface soil-water interface.
Different rice cultivars can support different rates of
CH 4 oxidation by developing variable root porosity and
oxidation powers. Wang et al. (1997a) found that at the
tillering stage, the root air spaces were small and did
not vary among three rice cultivars. Root air space continued
to increase up to the ripening stage in some cultivars
whereas it decreased in the commonly grown cultivar
IR 72. In the study of Wang et al. (1997a), dry
matter (Fig. 3a) had a direct effect on the CH 4 emission
rate (Fig. 3b) whereas root porosity (Fig. 3c) is not correlated
to the oxidation power of the plant (Fig. 3d).
The process of CH 4 oxidation is generally considered
to be an important in situ sink for CH 4 produced
in paddy soils (King 1992). The ratio of plant-mediated
versus floodwater-mediated oxidation, however, remains
unclear. Several experiments indicated that up to
40% of the potential CH 4 flux could be oxidized in the
rhizosphere (Epp and Chanton 1993; Denier van der
Gon and Neue 1996). Indirect assessments suggested
that 50–90% of the CH 4 transported to the rhizosphere
of the rice plant was oxidized (Frenzel et al. 1992).
However, the impact of rice variety characteristics on
CH 4 oxidation is not well understood. Varietal differences
may also become important when discussing CH 4

24
As mentioned earlier, the amount of CH 4 emitted from
the rice field to the atmosphere is the balance of the
two opposite processes, i.e. CH 4 production and oxidation.
CH 4 escapes from the rice field to the atmosphere
via three processes, viz. ebullition, diffusion, and transport
through the rice plant. The impact of plants on
these processes is discussed below.
Indirect impact of plants on ebullition and diffusion
Fig. 3 Variability among three rice cultivars: IR72 (high-yielding
dwarf), NPT (new plant type IR65598) and Dular (traditional
tall) in a dry matter, b CH 4 emission rate, c root oxidation power
and d root porosity at three growth stages. (Redrawn and modified
from data of Wang et al. 1997a,c)
oxidation within the aerenchyma. Even though recent
results indicate the presence of methanotrophic activity
associated with roots and, to a lesser extent, lower parts
of the stem (Watanabe et al. 1997), the significance of
CH 4 oxidation during passage through the rice plants is
still unknown.
As noted earlier, most of the findings on CH 4 production
and oxidation are based on indirect estimates
from CH 4 efflux rates. This is essentially due to the lack
of appropriate methods. Now techniques for inhibiting
oxidation and monitoring CH 4 production rates are
available, e.g. acetylene, dimethyl ether, nitrapyrin, methyl
fluoride, propylene oxide formation (Oremland
and Culbertson 1992a,b; Epp and Chanton 1993;
Kludze et al. 1993; Oremland and Taylor 1995; Watanabe
et al. 1995a). Therefore, future studies should be
able to address the effects of rice plant traits on both
CH 4 production and oxidation directly. The measurement
of 13 C abundance and 13 C fractionation could improve
knowledge of the function of the rhizosphere in
CH 4 oxidation.
CH 4 emission to the atmosphere
Ebullition and diffusion are purely physical processes
and, thus, are only indirectly affected by rice plants. A
well-developed root system acts as a sink and a barrier
for diffusive upward transport of CH 4 . Ebullition dominates
the overall flux only during the initial plant
growth stage (Wassmann et al. 1996) and upon disturbance
of soil due to weeding, harrowing etc. (Denier
van der Gon et al. 1992). Ebullition rates generally follow
a bimodal pattern over one season with high rates
in the early and late seasons (Wassmann et al. 1996).
The decrease in ebullition rate in mid-season is attributed
to the increasing CH 4 transfer through the aerenchyma,
reducing the pool size of entrapped CH 4 (Wassmann
et al. 1996). During this period, emerging gas
bubbles contain less CH 4 and high amounts of N 2
(Chidthaisong and Watanabe 1997a, b). During the later
growth stage, however, CH 4 production is very intense
due to the release of organic material from the
plant (Chidthaisong and Watanabe 1997a) which results
in increases in the CH 4 pool and the ebullition
rates observed during this period (Wassmann et al.
1996).
The mechanism of CH 4 transport through the rice
plant
The primary function of aerenchyma formation in hydrophilic
plants, including rice, is the delivery of O 2 to
the roots, but several gases are transferred in the inverse
direction. Aerenchyma transport of CO 2 (Higudchi
1982; Higudchi et al. 1984), N 2 and N 2 O (Mosier et
al. 1990; Prade and Trolldenier 1990; Aulakh et al.
1992; Bhadrachalam et al. 1992) and CH 4 (Raimbault et
al. 1977; Cicerone and Shetter 1981; Seiler et al. 1984;
Sebacher et al. 1985; Nouchi et al. 1990; Schütz et al.
1991; Nouchi and Mariko 1993) by rice and other plants
is well documented.
The phenomenon of CH 4 transport through the rice
plant from roots to above-ground portions and release
to the atmosphere has been elucidated by Nouchi et al.
(1990), Butterbach-Bahl (1992), Nouchi and Mariko
(1993), and Wang et al. (1997b). First, the dissolved
CH 4 in soil water surrounding the roots diffuses into
the surface water of the roots and into the cell-wall water
of the root cortex (Fig. 1). This transfer is driven by
the concentration gradient between the soil water surrounding
the roots and the lysigenous intercellular
spaces in the roots. Butterbach-Bahl (1992) identified
the cracks in the junction point of the main root and
the root hairs as predominant entrance ports for CH 4
from surrounding soil solution to the aerenchyma. CH 4
is then gasified in the root cortex and transported to the

25
shoots via the lysigenous intercellular spaces and aerenchyma.
Eventually, CH 4 is released to the atmosphere
from various parts of the rice plant (Fig. 1). Nouchi et
al. (1990) and Nouchi and Mariko (1993) observed that
CH 4 is released mainly through micropores in the leaf
sheath in the lower leaf but not from stomata. They demonstrated
that the closure of stomata openings by application
of abscisic acid did not affect the CH 4 emission
rate although the transpiration rate was decreased
to one-third and stomatal resistance increased threefold.
However, more recently, with the use of 13 C-
labeled CH 4 , Chanton et al. (1997) demonstrated that
although CH 4 is transported by the rice plant predominantly
via molecular diffusion, a small component is
also due to a transpiration-induced flow.
Most of CH 4 release is channelled through the culm
(Nouchi and Mariko 1993) which is an aggregation of
leaf sheaths. In submerged rice plants, many air bubbles
are released from (1) the abaxial epidermis of the
leaf sheath and (2) near the junction of the nodal plate
and leaf sheath (Nouchi and Mariko 1993). Wang et al.
(1997b) observed a shift in the transport pathway with
plant growth; about 50% of the CH 4 was released from
leaf blades before shoot elongation whereas only a
small amount was emitted through leaves as plants
grew older. In addition to the presence of micropores
on the leaf sheath, Wang et al. (1997b) identified cracks
at the junction of internodes. Although CH 4 can also be
released from panicles, this pathway was negligible as
long as leaves and nodes were not submerged. When
the vegetative parts of the plants are submerged, the
number of panicles determines the rate of CH 4 emission
(Wang et al. 1997b).
Factors controlling CH 4 transfer rates through the rice
plant
The actual flux of CH 4 through a rice plant depends on
several factors such as the concentration of CH 4 in soil
water, plant growth, size and shape, and rice cultivar.
Nouchi and Mariko (1993) observed a linear relationship
between CH 4 concentrations in the culture solutions
and CH 4 emission rate from rice plants. CH 4 concentrations
in rice plants show a clear gradient. The
highest concentrations are often found in the aerenchyma
below the water level and highest CH 4 emissions
occur through openings immediately above the water
level (Wang et al. 1997b). The transport capacity of rice
plants also depends on the size and shape of plants.
Emission rates from rice plants with nine tillers were
much larger than those with three tillers while the gap
between flux rates widened with increasing CH 4 concentration
in the soil (Nouchi and Mariko 1993). A similar
relationship was found for leaf area at the tillering
stage when nodes were not yet well developed (Wang
et al. 1997b).
Cutting off the stems of rice plants above the floodwater
did not influence CH 4 emission, indicating that
the rate-limiting step in plant-mediated CH 4 transport
was not located in the cut-off part of the plants (Ando
et al. 1983; Butterbach-Bahl 1992; Denier van der Gon
and van Breemen 1993). In field experiments with cutoff
stems, the pattern and magnitude of CH 4 emissions
remained unaffected over several days (Wassmann et
al. 1994). Tracer gas experiments (Butterbach-Bahl et
al. 1997) provided direct evidence that the root-shoot
transition zone (Fig. 1b) is the main site of resistance to
plant-mediated gas exchange between the soil and the
atmosphere.
Plant-mediated CH 4 transport does not depend on
photosynthetic rates. Ando et al. (1983) demonstrated
that darkening of the plants or increasing CO 2 concentration
in the atmosphere did not significantly affect the
CH 4 emission rates. Partial submergence of stems and
leaves could temporarily reduce the plant-mediated
CH 4 emission while the flux rates readjust within a few
hours (Wang et al. 1997b). From these studies it appears
that once the CH 4 is diffused into the root aerenchyma
and passes through the stem root interception, it
can escape to the atmosphere through one or the other
non-submerged part of the rice plants.
Impact of different rice plant traits
Since up to 90% of CH 4 released from a rice field during
a growing season could be emitted by rice plantmediated
transport, cultivar-specific properties may
have a strong impact on CH 4 emission. In a study with
six rice varieties, semi-dwarf varieties evolved 36% less
CH 4 than tall rice varieties (Lindau et al. 1995). Other
investigations in rice fields of India (Parashar et al.
1991, 1994; Adhya et al. 1994), China (Lin 1993), Japan
(Watanabe et al. 1995b), Italy (Butterbach-Bahl et al.
1997), and Texas, USA (Sigren et al. 1997) have also
indicated differences in the rate of CH 4 emission between
different varieties. These differences in CH 4 flux
rates could be attributed to differences in CH 4 production,
oxidation and gas transport capacities of different
cultivars. Recently Butterbach-Bahl et al. (1997) observed
that two Italian rice varieties differed by
24–31.5% in their CH 4 emission during two seasons.
This relative difference which was observed irrespective
of fertilizer treatment was not related to any difference
in CH 4 production or oxidation, but was attributed
to the different transfer capacities of the two cultivars.
High transfer capacity coincided with an increase in the
relative pore diameter of the root-shoot transition zone
of the aerenchyma system (Butterbach-Bahl et al.
1997).
Many measurements of CH 4 emission in rice fields
revealed seasonal patterns and variations in time and
space. Seasonal CH 4 emission patterns from rice fields
are the net result of the combination of many factors
such as reducing capacity of the soil, C source, nutrient
level, rice plant, temperature, and agricultural practices.
In a recent study, Wang et al. (1997c) observed

26
similar emission patterns for the cultivars Dular and IR
72 with one peak in the early and one in the late stage,
whereas the cultivar IR 65598 did not develop a second
emission peak. The distinct features of the new plant
type IR 65598 were low organic C in root exudates,
high oxidation power, few tillers and low dry matter per
plant. However, these findings (obtained with individual
plants) have to be confirmed for the situation encountered
in the field.
Relative importance of CH 4 transport pathways
The plant-mediated flux increases rapidly within the
first month after planting, which is also reflected in the
relative contribution of this pathway to total emission
(Table 1). In Italian rice fields, the aerenchyma transport
contributed 88–90% of the overall emission
throughout the reproductive and ripening stage (Butterbach-Bahl
et al. 1997). Over the entire season, plantmediated
transport can account for up to 90% of the
total CH 4 emission (Cicerone and Shetter 1981; Holzapfel-Pschorn
et al. 1985, 1986; Schütz et al. 1989; Butterbach-Bahl
et al. 1997). However, the relative contribution
of plant-mediated transfer is much lower under
high organic inputs (Wassmann et al. 1996). Organic
amendments result in high ebullitive fluxes during the
first few weeks when plants are very young and have
not developed their CH 4 transport mechanism. As a
consequence, the relative significance of plant-mediated
transport is decreased to values ranging from
48% to 85% (Table 1).
Conclusions and future research needs
The role of rice plants in the regulation of CH 4 emission
comprises many facets involving virtually every
level of the CH 4 budget in rice fields. However, it
should be noted that most of the findings on the effects
of plants on CH 4 production and oxidation are derived
indirectly from emission rates. With the advent of techniques
for inhibiting CH 4 oxidation, future studies
should address the influence of different plant parameters
on CH 4 production and oxidation. Such information
could be useful for formulating mitigation options
and for future rice breeding programs.
One individual trait of the rice plant can represent
an enhancing as well as a decreasing factor for CH 4 emission.
A high diffusion capacity of the aerenchyma entails
higher efflux of CH 4 plus higher oxidation power
of the root. Therefore, it is imperative to balance the
different impact mechanisms of rice plants for obtaining
a holistic view of the CH 4 budget in rice fields. Recent
achievements have considerably improved our understanding
the three decisive functions of plants, i.e.
(1) root exudation, (2) CH 4 transport to the atmosphere
and (3) CH 4 oxidation in the rhizosphere. But we
are still far away from a comprehensive understanding
of these phenomena, especially with respect to the interactive
relationships between the mechanisms involved.
Considering the enormous variety in physiology and
morphology of the genus Oryza, the impact of different
traits is most important for devising mitigation strategies.
Recent findings provide evidence that the desired
trait of a low emission potential can be reconciled with
a high yield potential (Wang et al. 1997a). High root
exudation rates represent a loss of assimilates for the
rice plants and can therefore be detrimental to yields.
However, information is lacking on the chemical composition
(e.g. amino acids, sugar C, organic acids) of
root exudates of different rice cultivars and its variations
in response to different management practices.
Comparative studies on cultivars may yield decisive
clues as to the identification of favourable traits for mitigation
and – at the same time – to improve our understanding
of the various functions of plants in the CH 4
budget of rice fields. Whereas other mitigation strategies
such as intermittent drainage require substantial
changes in farmers’ practices, new varieties can be introduced
within a given setting. The identification of
cultivars that combine low emissions with high yields is
the most challenging, but probably also the most promising,
strategy for a sustained reduction of CH 4 emissions
from rice fields.
Table 1 Contribution of plant-mediated CH 4 emission at different sites and under different treatments
Site/country Fertilizer/Cultivar Plant
age or
interval
Overall CH 4
emission rate
(mg CH 4 m P2 h P1 )
Plant-mediated
CH 4 emission
(% of overall
emission)
Source
Vercelli/Italy Urea/Roma 25 days 7.8 0 Schütz et al. (1989)
Urea/Roma 54 days 17.0 48
Urea/Roma 76–103 days 23–28 90–97
Vercelli/Italy Unfertilized/Roma Single season 11 88 Butterbach-Bahl (1992)
Unfertilized/Lido Single season 8.1 90
Los Baños/Philippines Urea/IR72 Dry season 1.1 85 Wassmann et al. (1996)
Straw/IR72 Dry season 9.4 65
Urea/IR72 Wet season 1.3 82
Straw/IR72 Wet season 6.3 48

27
Acknowledgement The review was conducted as part of the project
“Reduction of CH 4 emission from rice fields by screening for
low CH 4 transport capacity” funded by the German BMZ/GTZ
(Project No. 95.7860.0–001.05)
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