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

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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