Structure and Function in Agroecosystem Design and Management - Chapter 18 doc

CHAPTER 18
Growth and Yield of Paddy Rice
Under Free-air CO
2
Enrichment
Kazuhiko Kobayashi, Mark Lieffering, and Han Yong Kim
CONTENTS
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
Atmospheric CO
2
and Rice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
Objectives of the Rice FACE Experiment . . . . . . . . . . . . . . . . . . . . . . 373
Growing Crops under Elevated [CO
2
]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
Chamber Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
FACE Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
Rice FACE System: Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
Ring Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376
CO
2
Control and Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376
Temporal and Spatial Control of [CO
2
] (1999) . . . . . . . . . . . . . . 377
The Effects of FACE on the Growth and Yield of Paddy Rice . . . . . . . . . . . 378
Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387
Conclusions and Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392

INTRODUCTION
Atmospheric CO
2
and Rice
It is estimated that up until the industrial revolution in the eighteenth
century,atmospheric CO
2
concentrations ([CO
2
]) were about 280 ppmV (parts
371
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© 2001 by CRC Press LLC
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372 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
per million by volume). Since then, the [CO
2
] has risen to 370 ppmV at pres-
ent and is expected to keep increasing at a rate of about 15 ppmV per decade.
The increase in [CO
2
] is attributed to human activities such as fossil fuel burn-
ing and deforestation (Houghton et al., 1996). It is predicted that the increase
will continue into the twenty-first century, resulting in a [CO
2
] concentration
somewhere between 450 and 550 ppmV around the year 2050 (Houghton
et al., 1996). Because CO
2
is a “greenhouse” gas, the increase in [CO

2
] is pre-
dicted to affect the global radiation energy balance and thereby climate. The
predicted changes in climate most notably include an increase in the Earth’s
mean surface temperature and alterations in rainfall patterns, both factors
which strongly affect biomass production in both agricultural and natural
ecosystems worldwide (Reilly, 1996).
Besides the indirect effects on plant growth induced by climate change,
elevated [CO
2
] can also directly alter plant processes, most importantly pho-
tosynthesis and stomatal conductance. Because photosynthesis in plants uti-
lizing the C
3
pathway is limited by current [CO
2
] levels, elevating [CO
2
]
increases rates of carbon (C) fixation, leading to greater plant biomass pro-
duction (Drake et al., 1997). Elevated [CO
2
] also tends to reduce stomatal con-
ductance which, coupled with the increase in photosynthesis, leads to an
increase in water use efficiency.
In terms of both area and tonnage harvested, rice, oryza sativa, h, is the
primary crop in Asia and is among the world’s three major crops (the other
two are wheat, Triticum aestivum L., and maize, Zea mays L.). Rice is unique in
that 95% of the world’s total production occurs in developing countries, and
the majority of that grown is consumed locally (Alexandratos, 1995). In most

of the countries where it is produced, rice provides a major part of the human
dietary needs, and its production is usually a large factor in the economy.
Rice production in Asia has increased almost linearly since 1960 and had
risen by 150% by 1995 (FAOSTAT; The harvested area
has increased by only 20%, hence the increased production has mostly come
from a 100% increase in yield per unit harvested area. This large yield
increase can be ascribed to technological advances such as the breeding of
new, high-yielding varieties, the development and expansion of irrigation
systems, increased fertilizer use and efficiency, and improved pest manage-
ment (Greenland, 1997).
It has been estimated that in the next 30 years the growing population in
Asia may need nearly 70% more rice (Hossain, 1997). Because the area avail-
able for cultivation is predicted to decrease, yield per unit harvested area
must increase more than the growth in population. However, there is evi-
dence that the impressive yield increases since 1960 may be plateauing
(Cassman et al., 1997), and there appears to have been little increase in poten-
tial crop yields in recent times (Khush and Peng, 1996). Therefore, it is spec-
ulated that further increases in yield may be achieved only by optimizing the
supply of resources limiting crop growth, such as water and nitrogen (N)
(Sinclair, 1998a).
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2
ENRICHMENT 373
The effects of elevated [CO
2
] on rice growth have been studied since the
1960s (e.g., Murata, 1962). In these early experiments, higher [CO
2
] was

shown to enhance both biomass growth (Imai and Murata, 1976) and yield
(Yoshida, 1973). It was also found that environmental variables such as N
(Imai and Murata, 1978) and temperature (Imai and Murata, 1979) can affect
growth enhancement due to higher [CO
2
]. In these studies, plants were
grown under higher [CO
2
] for only a portion of the growth duration. It was
later confirmed that rice yield also increases when plants are grown under
higher [CO
2
] throughout the growth duration (Imai et al., 1985; Baker et al.,
1990; Ziska and Teramura, 1992; Baker and Allen, 1993a, b; Seneweera et al.,
1994; Kim et al., 1996a,b; Ziska et al., 1997; Moya et al., 1998).
The studies cited above have identified some common factors which
result in the increase in yield with elevated [CO
2
]. Individual leaf area and the
number of leaves per stem are usually decreased but a greater tiller number
results in an increase in leaf area per plant (Imai, 1995). Photosynthesis per
unit leaf area is usually increased with elevated [CO
2
], though rates may
decrease as the leaf matures (photosynthetic acclimation) (Imai and Murata,
1978b). The net result is an increase in photosynthesis per plant, resulting in
greater carbohydrate accumulation and dry matter production (Rowland-
Bamford et al., 1990; Baker et al., 1993). Frequently, the increase in root dry
weight (d.wt) with elevated [CO
2

] is greater than the increase in shoot d.wt
(Imai et al., 1985). The greater tiller number leads to an increase in the pro-
duction of panicles, an important determinant of grain yield (e.g., Ziska et al.,
1997). Increased carbohydrate supply leads to an increase in both grain num-
ber per panicle and the percentage of mature grains that develop (Yoshida,
1981). Elevated [CO
2
] rarely increases individual grain weight because of the
physical limitations imposed by the grain and husk characteristics (Yoshida,
1981).
Objectives of the Rice FACE Experiment
In view of the importance of rice in the lives of a large proportion of the
world’s population and the anticipated decreases in per capita yield, there is
a need to determine the effects of the predicted elevated [CO
2
] on rice growth
and yield under field conditions. An important question is by how much will
elevated [CO
2
] increase rice yields under field conditions and to what extent
will these increases satisfy the predicted demand? Also, will there be interac-
tions between elevated [CO
2
] and the other factors that limit rice yields, and
if so how can these be utilized to maximize yields? In this chapter we briefly
review the techniques that have been used in past research efforts on the
effects of elevated [CO
2
] on rice growth and yield. All these studies have
grown rice in some kind of enclosure fumigated with air containing elevated

[CO
2
]. We highlight some of the drawbacks in using enclosures to grow
plants and then introduce the free-air CO
2
enhancement (FACE) technique as
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374 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
a method to grow large areas of crops under elevated [CO
2
]. We then present
some results from the first experiment to grow rice using the FACE technique
and discuss their implications.
GROWING CROPS UNDER ELEVATED [CO
2
]
Chamber Studies
Much information on the response of rice to elevated [CO
2
] has come from
experiments conducted using chambers or enclosures which were fumigated
with either ambient or CO
2
enriched air. The types of enclosures that have
been used include temperature gradient chambers (TGCs); (Kim, 1996a,b),
soil-plant-air research (SPAR) units (e.g., Baker et al., 1992; Gesch et al., 1998)
and open-topped chambers (OTCs) (e.g., Moya et al., 1998). However, to iso-
late the effects of elevated [CO
2
] on plant growth, it is important that the

experimental system imparts minimal effects on other abiotic environmental
parameters that may influence growth. In many of the early experiments con-
ducted in fumigated glasshouses (e.g., Imai et al., 1985), plants were grown
in pots. The soil environment in pots differs markedly from that under field
conditions, with differences in factors such as nutrient availability, water
drainage, and soil temperature. In fact, the response of plants to elevated
[CO
2
] has been shown to decrease with decreasing pot size (Arp, 1991).
Chambers and enclosures can affect abiotic environmental factors such
as temperature, solar radiation, humidity, and wind (McLeod and Long,
1999). Frequently, compared to outside conditions, within the chamber there
is less light, the air is drier, and temperatures are higher. These differences can
affect plant growth (commonly called a “chamber effect”) to as large an
extent as the effect of the elevated [CO
2
] (e.g., Knapp et al., 1994). The cham-
ber effect can influence many aspects of the response of plants and crops to
elevated [CO
2
], including photosynthesis, metabolism, biomass production,
and crop water and energy balances (McLeod and Long, 1999), making the
translation of results to outside conditions difficult. For example, Van Oijen
et al., (1999) found that the response of wheat grain yield to elevated [CO
2
]
was less in OTCs cooled to very close to the ambient temperature compared
to uncooled OTCs. However, the yield of plants in the cooled OTCs was still
different from those grown outside, suggesting that abiotic factors other than
temperature also contributed to the chamber effect.

FACE Systems
To overcome the limitations of chamber methods, the FACE method was
developed in the mid 1980s (Lewin et al., 1994). The first full scale field exper-
iment was established at Maricopa (Arizona, U.S.) using cotton as the crop
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GROWTH AND YIELD OF PADDY RICE UNDER FREE-AIR CO
2
ENRICHMENT 375
(Nagy et al., 1994). Generally, FACE systems involve fumigating a circular
area of vegetation with pure CO
2
or CO
2
/air mixtures, thereby generating a
zone having [CO
2
] higher than that of the surrounding ambient atmosphere.
The CO
2
is usually emitted from a structure (sometimes referred to as a ring)
constructed from pipes or tubes that surrounds the crop. The CO
2
is emitted
from the upwind direction of the ring, relying on the wind to mix and dis-
perse it over the whole ring. The target [CO
2
] in the fumigated zone may be
either static (e.g., a constant 500 ppmV) or dynamic, whereby the target is set
at a certain level (e.g., 200 ppmV) above the real-time ambient [CO
2

]. A con-
trol system regulates the amounts of CO
2
emitted by monitoring and inte-
grating wind speed and direction together with [CO
2
] levels at ring center. The
system must be able to deal with short-term changes in the weather, most
notably differences in wind speed and direction, both of which may change
over very short periods of time. The control system must also be able to cope
with longer term temporal variations in [CO
2
], which may be caused by fac-
tors such as diurnal and seasonal changes in the relative amounts of crop
photosynthesis and respiration.
The FACE method has been successfully used to study the effects of ele-
vated [CO
2
] on a variety of vegetation types. These include agriculturally
important crops such as cotton (Lewin et al., 1994), wheat (Kimball et al.,
1995), and pastures (Hebeisen et al., 1997), as well as harvestable tree species
(Hendrey et al., 1999). FACE has also been used in more natural vegetation
types such as desert vegetation (Jordan et al., 1999).
The most important advantage of FACE systems over other methods of
growing vegetation under elevated [CO
2
] is that the vegetation is not unduly
influenced by the effects of enclosures on environmental factors such as solar
radiation, temperature, and wind (McLeod and Long, 1999). Also, relatively
large areas of vegetation can be treated, meaning that a large number of sam-

ples can be collected for analyses and a range of experiments can be con-
ducted in one season. The major disadvantage of FACE systems is their
relatively high cost, both to build and run, the latter due primarily to the large
amounts of CO
2
required for fumigation. However, expressed on the basis of
cost per usable fumigated crop area, FACE systems can be more cost effective
than other methods of growing plants under elevated CO
2
(Kimball, 1992).
Rice FACE System: Description
The Rice FACE project was established in 1996 to study the effects of ele-
vated [CO
2
] concentrations on rice crop growth, yield, and ecosystem
processes. It is the first FACE experiment to be conducted on rice. After
design trials in 1997, a facility consisting of four FACE rings and their associ-
ated ambient (control) plots was constructed for use during the 1998–2000
rice growing seasons.
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376 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Ring Description
Each Rice FACE ring consists of a CO
2
emission structure, a CO
2
moni-
toring system, and a computerized control system. In order to minimize
atmospheric contamination of the control plots, there is at least 90 m between
the controls and the nearest ring. Each emission structure consists of a 12-m

diameter octagon made of eight 5-m long, 3.8-cm diameter polyethylene
tubes. Each tube is horizontally supported by a 5-m long, 2.2-cm diameter
galvanized steel pipe, which is supported at each end by similarly sized,
upright pipes dug 40 cm into the soil. The polyethylene tubes have 0.6–0.9-
mm diameter CO
2
-release holes located approximately every 4 cm on the side
facing into the crop. The height of the emission tubes above ground level is
set at approximately 50 cm above the canopy. Liquid CO
2
contained in a hold-
ing tank passes through a vaporizer, and the CO
2
gas is delivered to the emis-
sion tubes via valves to the emission tubes. Pure CO
2
at a maximum pressure
of 0.13 MPa is “sprayed” from the tubes; preliminary simulation studies have
shown that, depending on wind speed and emission pressure, concentrations
drop from 100% to 2000 ppmV within 20 cm of the emission tube (M.
Yoshimoto, pers. comm.). The use of pure CO
2
in the Rice FACE experiment
is different from that used in many other FACE designs, which emit a
CO
2
/air mixture into the ring using blowers. Under some circumstances this
can influence the microclimate within the FACE ring (“blower effects”)
(McLeod and Long, 1999), and the control plots must have blowers installed
to cancel out the blower effects. There is no such problem with the pure-CO

2
FACE.
The total area within each FACE ring is approximately 120 m
2
.
Walkways, situated approximately 15 cm above the paddy water level,
extend from one of the surrounding earth dikes to the ring center and pro-
vided access to the crop and monitoring equipment. Preliminary studies
indicate that canopy microclimate such as wind and canopy temperature do
not appear to be affected by the presence of the ring structures (M.
Yoshimoto, personal communication).
CO
2
Control and Monitoring
The main objective of the Rice FACE experiment is to determine the influ-
ence of elevated [CO
2
] on various crop and ecosystem processes. It is there-
fore crucial to have control over the amounts of CO
2
applied and also to know
with confidence what the level of [CO
2
] is at any time and location within the
ring over the duration of the experiment.
Because a dynamic target (200 ppmV above ambient) is being used, both
ambient and ring [CO
2
] levels must be monitored. Ambient [CO
2

] concentra-
tions are measured at the center of the two distal control plots using infrared
CO
2
gas analyzers. [CO
2
] in the FACE rings is monitored at ring center,
together with wind speed and direction, which are measured every second.
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GROWTH AND YIELD OF PADDY RICE UNDER FREE-AIR CO
2
ENRICHMENT 377
Table 18.1 Mean CO
2
above ambient (target ؍ 200 ppmV) at ring center
and 2.5 m (average of 4 locations) and 5 m (average of 8
locations) from the center for the 4 Rice FACE rings from
May 21 until August 20, 1999. Ambient [CO
2
] concentration
during the same time period was 391.1 ppmV.
CO
2
concentrations above ambient (ppmV)
Ring center 2.5m 5m
A 185.9 201.8 251.8
B 201.5 216.6 278.9
C 191.2 213.8 273.5
D 213.5 238.0 309.6
mean 198.0 217.6 278.5

The data generated is sent to data acquisition and control equipment which
determines the target and regulates how much and from which emission
tubes CO
2
is emitted, with the latter depending on wind direction. When
speeds are above 0.3 ms
Ϫ1
, the three tubes in the upwind direction emit the
required amount of CO
2
, while at wind speeds below 0.3 ms
Ϫ1
emission is
switched between every other tube every 10 sec.
Because a number of different experiments are conducted in various sub-
plots within each FACE ring, it is important to know what the [CO
2
] levels are
at these sites over the season. For each ring a separate infrared CO
2
analyzer
samples the atmosphere at canopy height at 13 locations. Sampling tubes are
located at the center and equidistantly spaced in two concentric circles 2.5 m
(4 locations) and 5 m (8 locations) from the center. [CO
2
] levels at any location
within the ring can be estimated by interpolating the actual [CO
2
] at each of
the sampling locations.

Temporal and Spatial Control of [CO
2
] (1999)
The ability of the FACE system to control [CO
2
] can be assessed by com-
paring the actual and target [CO
2
] at any location for a given time period.
Performance can be expressed as the average [CO
2
] concentration above
ambient for the time period, or the percentage of the time that all the actual
values were within 10 and 20% of the target can be calculated. During the first
half the 1999 season (up to the time of writing), [CO
2
] levels at ring center
were between 185 and 213 ppmV above ambient (Table 18.1) and about 55
and 85% of the samples were within 10 and 20% of the target, respectively
(data not shown). Within 2.5 m of the center, [CO
2
] averaged 220 ppmV above
ambient, and around 50% of the samples were within 10% of the target. At 5
m fromthe center, [CO
2
] averaged 280 ppmV above ambient, and only about
30% of the samples were within 10% of the target. (A different control algo-
rithm was used in 1998 which resulted in less satisfactory performance, with
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378 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT

CO
2
levels averaging 224 and 340 ppmV above ambient at the center and out-
lying yield plots respectively.)
CO
2
levels at 2.5 and 5 m from the center were around 3.5 and 13% greater
than at the center (Table 18.1), resulting in a “bowl shaped” [CO
2
] distribu-
tion pattern. CO
2
is released from the peripheries of the ring and dispersed
towards the center, and, as long as wind speeds and directions are evenly dis-
tributed over the season, such a distribution pattern is typical for FACE rings.
The size and shape of the [CO
2
] gradient from the ring edge to the center will
depend on factors such as ring architecture, the force of CO
2
emission, wind
speed, and the control algorithm used.
The CO
2
control performance of the Rice FACE in terms of the percent-
age of observations that were within 10% of the target at ring center was not
as good as those reported for other FACE systems of similar size. For exam-
ple, for the Maricopa FACE experiment, [CO
2
] levels at ring center were within

10% of the target 90% of the time (Nagy et al., 1994), compared to only 55%
for the Rice FACE experiment. This difference in performance can be partially
attributed to differences in the wind characteristics of the two sites. Greater
average wind speeds result in better CO
2
distribution and mixing. At the
Maricopa site, average daily wind speeds were about 1.7 ms
Ϫ1
(Nagy et al.,
1994) with calm periods (Ͻ 0.4 ms
Ϫ1
) occurring about 19% of the fumigation
time (Nagy et al., 1992). In contrast, at the Rice FACE site, average daily wind
speed ranged from 1.1 ms
Ϫ1
in June to 0.5 ms
Ϫ1
in September (season average
of 0.7 ms
Ϫ1
), while calm (Ͻ 0.3 ms
Ϫ1
) periods ranged from 30% of the time
early in the season to nearly 60% near the end (season average 45%). This
lower average wind speed and greater calm percentage makes effective tem-
poral control and uniform spatial distribution difficult and is probably a
major reason for the differences in CO
2
performance between the Rice FACE
and other FACE experiments.

THE EFFECTS OF FACE ON THE GROWTH AND YIELD
OF PADDY RICE
Materials and Methods
a. Site description. The Rice FACE experiment is located at Shizukuishi,
Iwate Prefecture, in the northern part of Honshu, Japan (39° 38’ N, 140° 57’ E).
It is situated in a valley at an altitude of about 200 m, surrounded by 600-m
high hills to the south, west, and north. The site was chosen because it is typ-
ical of the agroenvironment that grows a large proportion of the Japanese rice
crop. It is also close to existing research facilities at the Tohoku National
Agricultural Experiment Station near Morioka. The climate is best described
as humid continental with a summer precipitation maximum and a cold, dry
winter. Over the year, daily average air temperatures range from Ϫ2.5
(January) to 23.2°C (August); meteorological data from the 1998 growing
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2
ENRICHMENT 379
Table 18.2 Meteorological profiles of the Rice FACE site, 1998
Air temperature
a
Solar radiation Rainfall
c
mean min max mean daily
b
monthly
Month (°C) (MJ m
؊2
d
؊1
) (mm)

May
d
16.1 9.4 22.6 18.3 34.1
Jun 16.8 13.4 20.9 12.4 219.3
Jul 21.3 17.4 26.2 14.0 206.0
Aug 21.5 18.0 26.4 11.8 446.5
Sep 20.2 16.5 25.1 10.1 270.2
Season mean 19.7 15.9 24.5 12.5 1176.1
e
a
Monthly average of the daily mean, minimum, and maximum air temperatures.
b
Monthly average of the daily mean solar radiation.
c
Monthly accumulated rainfall.
d
For last 10 days of the month only.
e
Season accumulated rainfall.
season is shown in Table 18.2. The soils of the site are derived from volcanic
ash and have been tentatively classified as humic Andosols.
b. Experimental design. In both 1998 and 1999, the experiment was a
completely randomized block design with two levels of [CO
2
] (ambient [CO
2
]
(control) and elevated [CO
2
] within the FACE rings) replicated four times.

FACE and control plots were located in eight paddies blocked by location; the
four blocks consisted of paddies with similar agronomic histories and soil
characteristics.
c. Seedling establishment. In both years, presoaked seeds of rice cv.
Akitakomachi (a commonly grown variety in northern Japan) were sown into
seedling trays and grown under flooded conditions. Trays were placed in
plastic chambers fumigated either with air containing ambient or elevated
(ϩ200 ppmV) [CO
2
]. The duration of seedling growth was 14 and 23 days at
average air temperatures of 19.35°C and 18.25°C in 1998 and 1999, respec-
tively.
d. Crop establishment and management. Seedlings were hand-trans-
planted into either control or FACE plots on 21 May 1998 and 20 May 1999.
Although most Japanese farmers use mechanical transplanters in establish-
ing rice crops, hand transplanting was used in the experiment to ensure an
even number of seedlings per hill and regular hill spacing. In both years,
there were three seedlings per hill and 17.5 and 30 cm between hills and rows,
respectively (Ϸ 19 hills m
Ϫ2
). This spacing is commonly used by farmers in
this district. Three levels of N were supplied as ammonium sulfate: 4g (low),
8g (medium), and 12g N m
Ϫ2
(high) in 1998, and 4, 9, and 15 g N m
Ϫ2
in 1999.
The medium N level is typical of the standard rate used by local farmers. In
both years N was applied as a basal dressing (63% of the total), at mid-tiller-
ing (20%) and at panicle initiation (17%). Levels of phosphorus and potas-

sium fertilizer were similar for all N levels and adequate for the high N
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380 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
treatment. Flooded paddy fields were maintained throughout the season
except for a midsummer drainage conducted in mid-July in both years and
from 10 days prior to harvest in 1998. Herbicides, insecticides, and fungicides
were applied when necessary.
e. Sampling and harvesting. To determine the influence of elevated
[CO
2
] on vegetative growth, in both years seedlings were sampled on the day
of transplanting and established plants were sampled from the medium N
treatment of FACE and control plots at 25, 53, 81, 109, and 131 days after
transplanting (DAT) from three locations in 1998. In addition, plants in the
high N plots were harvested at 83 and 137 DAT, while low N plants were only
harvested at grain maturity. Plants were separated into living and dead leaf
blades, stems (including leaf sheath), panicles (when present), and roots;
d.wt of the plant parts as determined separately. The number of tillers and
panicles (when present) as determined and leaf area was measured. At final
harvest, the number of spikelets per panicle was also determined. To deter-
mine crop N uptake, the dried plant parts were milled and total N in each
part was determined (micro-Kjeldahl technique).
In order to determine flowering date, two or three locations within each
[CO
2
] plot were investigated daily for panicle appearance in 1999, but only
once in 1998. Flowering date was defined as when panicles had emerged
from 50% of the effective tillers (potential panicle bearing). The effect of
FACE on grain maturity was investigated by checking the color of the pani-
cles by eye during grain filling in 1998. The date of maturity was defined by

a “yellow index” in which maturity was defined as when 90% of the panicles
at a location had greater than 80% yellow grains.
For grain yield determination, subplots were set aside within both FACE
and control plots and not disturbed until final harvest. Plants were sampled
at grain maturity; total and fertile spikelet number per hill together with
mean grain weight were determined. Mean [CO
2
] (four replicates) in 1998 for
these grain yield plots over the season were 726 and 387 ppmV for FACE and
control, respectively.
In this chapter we present seedling and phenological data from both
years, but only 1998 data for crop dry matter and grain yield investigations.
Results
a. Seedling growth. When rice crops are established by transplanting,
early seedling growth and vigor under nursery conditions are important fac-
tors in the successful establishment and eventual yield of the crops. However,
there is little information on the effects of elevated [CO
2
] on the growth of rice
seedlings cultivated for transplanting using commercial agricultural condi-
tions and techniques. In both years, elevated [CO
2
] increased total and root
d.wt (Table 18.3). In 1998, leaf blade d.wt increased with elevated [CO
2
], while
leaf area decreased, leading to an increase in specific leaf weight (leaf d.wt
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GROWTH AND YIELD OF PADDY RICE UNDER FREE-AIR CO
2

ENRICHMENT 381
Table 18.3 The effect of ambient (AMB) and elevated (ELEV)CO
2
on the
characteristics of rice seedlings used in the Rice FACE
experiment in 1998 and 1999. Average air temperature and growth
duration are also shown. “
؉,”“*”, and “**” denote significance at
the p
Ͻ 0.1, 0.05, and 0.01, respectively. “ns” denotes not
significant.
Dry weight
LA LB CLS R T SLW Temp D
Year CO
2
cm
2
hill
؊1
mg hill
؊1
mg cm
Ϫ2
°Cdays
1998 ELEV 6.36 28.5 26.4 8.4 63.3 4.5 19.3 14
AMB 7.42 26.6 22.7 3.6 52.8 3.6 19.4 14
ns ns * ** * *
1999 ELEV 8.61 32.0 39.4 15.1 86.5 3.7 18.1 23
AMB 9.24 30.4 36.9 11.4 78.7 3.3 18.4 23
ns ns ns ϩ *ns

LA: Leaf area; LB: Leaf blade; CLS: Culm and leaf sheath; R: Root; T: Total biomass; SLW:
Specific leaf weight; Temp: Mean air temperature; D: Duration of seedling growth.
per unit leaf area; SLW). The increase in total d.wt was less in 1999 (10%)
compared to 1998 (20%); this may have been due to the higher air tempera-
ture and faster growth in 1998 (Table 18.3).
b. Crop vegetative growth. Rice crop growth consists of a vegetative
phase followed by a reproductive stage. The former entails the growth of
mainstem and tiller leaves; these combine to form the crop canopy where
photosynthesis occurs. The vegetative stage is important in determining
grain yield because the number of panicles at harvest is closely related to the
number of tillers that are produced. Also, photosynthate accumulated during
the vegetative stage can provide up to 40% of the material used for grain fill-
ing (Yoshida, 1981).
Tillers are important in determining final yield of rice in two ways: they
contribute to the extent of the canopy (and hence the level of canopy photo-
synthesis), and they bear panicles. In the 1998 Rice FACE experiment, at all
but the first sampling (25 DAT), FACE increased tiller number by 10 to over
20%, with the largest increases at around panicle initiation (Figure 18.1). The
lack of response for the first sampling may have been because CO
2
fumiga-
tion did not commence until 10 days after transplanting. Green leaf area of
the crop was calculated by measuring individual hill green leaf area at each
sampling and multiplying this by the plant population. At panicle initiation
(approximately 53 DAT), FACE increased green leaf area index (LAI) by 10%
(Figure 18.2). However, by flowering (81 DAT), when LAI had peaked at
around 4.0, there was no CO
2
effect.
For rice crops in general, green leaf area reaches a peak at around flow-

ering and then decreases gradually as materials are translocated to be used
for grain filling, and the leaves become senescent. At all harvests after pani-
cle initiation, FACE plants had far more dead leaf d.wt than control plants
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382 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Figure 18.1 The effect of ambient [CO
2
] (control) and free-air CO
2
enrichment
(FACE) on tiller number during the season. Plant population ≈ 19 m
Ϫ2
.
Error bars are Ϯ1 standard error of the mean.“ *”, “ **” denote significance
at the p Ͻ 0.05 and 0.01 levels, respectively. “ns” denotes not significant.
(data not shown; Kobayashi et al., 1999), suggesting a speeding up of leaf
development and senescence with FACE. Under FACE, green leaf area
declined more rapidly during grain filling compared to control, but the dif-
ference was not statistically different at P Ͻ 0.05 (Figure 18.2).
Total crop biomass was greater in FACE-grown plants compared to con-
trol plants at all harvests except the first one (again, possibly due to CO
2
fumi-
gation commencing only 10 days after transplanting) (Figure 18.2). The
greatest d.wt response to FACE was about 20% at panicle initiation. For all
harvests, the d.wt of most plant parts increased with FACE, including that of
the roots (data not shown). At all harvests the crop biomass response to FACE
was greater than that of leaf area. This suggests that the increase in biomass
was due to greater crop radiation use efficiency rather than an increase in
light interception.

e. Crop reproductive growth. The crop reproductive phase comprises
panicle initiation, development and heading, followed by flowering, grain
filling and finally grain maturity. Grains are composed mainly of carbohy-
drates which are derived from two sources: those stored in the vegetative
parts before flowering, and those produced after flowering. The contribution
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GROWTH AND YIELD OF PADDY RICE UNDER FREE-AIR CO
2
ENRICHMENT 383
Figure 18.2 The effect of ambient [CO
2
] (control) and free-air CO
2
enrichment
(FACE) on seasonal total crop dry weight and green leaf area index.
Error bars are Ϯ 1 standard error of the mean. “ϩ”, “ *”, and “**” denote
significance at the p Ͻ 0.1, 0.05, and 0.01 levels, respectively. “ns”
denotes not significant.
of post-flowering photosynthesis to grain filling ranges from 60 to 100% and
depends on the potential photosynthetic activity of the crop, longevity of
foliage, and the light environment after flowering (Yoshida, 1981).
In both years, FACE shortened the days to flowering (DTF) by 2–3 days
compared to plants in the control plots (Figure 18.3). However, for both CO
2
treatments flowering occurred 7 to 8 days earlier in 1999 compared to 1998. It
is likely that this difference was due to the 1.2°C higher air temperature in
1999. Final grain maturity date was about 2 days earlier with FACE in 1998
(Figure 18.4). This may simply be a result of the 2–3 days earlier flowering
with FACE, with the duration of the grain ripening unchanged.
Grain yield of rice crops is determined by panicle number, the number of

spikelets per panicle, percentage of filled grains, and mean grain weight.
FACE increased grain yield 16% compared to control (Table 18.4). This was
due almost entirely to an increase in panicle number per hill with a small con-
tribution by greater grain number per panicle. Spikelet fertility and grain
weight were similar for FACE and control (Table 18.4). There were no
changes in harvest index (HI) with FACE, indicating that elevated [CO
2
] does
not affect dry matter partitioning to the grain. Grain quality, as measured by
protein content, decreased slightly with FACE (S. Miura, pers. comm.),
though this did not affect the taste of the rice (data not shown).
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384 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Figure 18.3 The effect of ambient [CO
2
] (control) and free-air CO
2
enrichment (FACE)
on the time to flowering in 1998 and 1999 (see text for details). Average
temperature up to flowering is shown. Plant population ϭ 19 m
Ϫ2
. Error
bars are Ϯ 1 standard error of the mean. “ **” denotes significance at the
p Ͻ 0.01 level. (Note: no statistical analysis was possible in 1998.)
d. CO
2
and Nitrogen. In the previous section, we described the responses
of rice crops to FACE when plants were grown with moderate levels of
applied N. However, under otherwise optimal environmental conditions,
available soil N levels are usually limiting to crop growth. Hence, farmers

usually apply fertilizer N to maximize economic yields, with the rate
depending on factors such as cultivar, plant population, and environmental
conditions. Under controlled environment conditions, the response of plants
to elevated [CO
2
] has been shown to be influenced by applied N level (Imai
and Murata, 1978; Wong, 1979; Ziska et al., 1996). In this section, we discuss
the responses of rice to FACE when different levels of N are applied.
Regardless of N application level, FACE increased grain yield (Figure
18.5b). However, the yield response to FACE was greater with increasing
applied N level, with FACE increasing yields of low, medium, and high N
levels 12%, 16%, and 21%, respectively. These increases in yield were similar
to the increases in grain numbers (Figure 18.5a), indicating that higher yield
with elevated [CO
2
] depends mainly on increased grain number. Similar
responses have been found in a previous study conducted using temperature
gradient chambers (Kim et al., 1996b).
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GROWTH AND YIELD OF PADDY RICE UNDER FREE-AIR CO
2
ENRICHMENT 385
Figure 18.4 The effect of ambient [CO
2
] (control) and free-air CO
2
enrichment
(FACE) on the date of grain maturity in 1998 (as measured by yellow-
ness index; see text for details). Error bars are Ϯ 1 standard error of the
mean.

Table 18.4 The effect of ambient [CO
2
] (CONT) and free-air CO
2
enrichment
(FACE) on yield and its components. Total and fertile spikelet
number were determined from the yield plots. Harvest index (HI) is
also shown. “ns”, “
؉”, “*”, and “**” denote not significant and
significant at p Ͻ 0.1, 0.05, and 0.01, respectively.
Number of
Panicles Spikelets Spikelets Fertility Grain Yield
CO
2
m
Ϫ2
m
؊2
panicle
؊1
% wt. mg g m
؊2
HI %
FACE 402 33190 82.6 94.3 21.8 681.3 47.9
CONT 359 28813 80.2 93.9 21.7 588.3 46.1
** ** * ns ns ** ns
Change (%) 11.9 15.2 3.0 0.4 0 15.8 4.0
Total d.wt, leaf area, and/or tiller development of rice crops are closely
related to N uptake or content (Kim et al., unpubl.). Also, crop N content at
around panicle initiation largely determines total spikelet production and

survival per plant prior to anthesis (Kobayashi and Horie, 1994). In turn, final
spikelet number is a large determinant of grain yield. In this experiment,
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386 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Figure 18.5 The effect of CO
2
(ambient [CO
2
] (control) and free-air CO
2
enrichment
(FACE)) and nitrogen application on grain number and grain yield per
m
2
. Error bars are Ϯ1 standard error of the mean. “ns” and “**” denote
not significant and significant at p Ͻ 0.01, respectively.
despite the large effect of FACE on dry matter accumulation and grain yield,
for all harvests, FACE had little affect on N uptake where medium levels of
N were applied (Figure 18.6). Also, the increase in grain number with FACE
was not accompanied by an increase in N uptake. A possible reason for this
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GROWTH AND YIELD OF PADDY RICE UNDER FREE-AIR CO
2
ENRICHMENT 387
Figure 18.6 The effect of ambient [CO
2
] (control) and free-air CO
2
enrichment
(FACE) on nitrogen (N) uptake of crops supplied 8 g N m

Ϫ2
. Error bars
are Ϯ1 standard error of the mean. “ns” denotes not significant.
may be that FACE increased both crop biomass and grain production N use
efficiency (NUE) rather than N uptake or content.
Discussion
The results from the Rice FACE experiment presented here clearly
demonstrated that elevated [CO
2
] has a large positive effect on the growth of
rice crops grown under field conditions. Seedlings, which were grown using
commercial agricultural conditions and techniques, responded to elevated
[CO
2
] with an increase in d.wt. In both years, seedling root d.wt responded
more to elevated [CO
2
] than total d.wt. This may indicate that excess photo-
synthate due to elevated [CO
2
] was partitioned to the roots because of a greater
sink potential compared to the shoots. Previous studies have also shown that
for mature rice plants elevated [CO
2
] enhanced root biomass and/or
root:shoot d.wt ratio (Imai et al., 1985; Kim et al., 1996a; Ziska et al., 1997). It
is possible that this greater root biomass in seedlings grown under elevated
[CO
2
] may lead to greater rates of nutrient uptake after transplanting.

However, this idea has not been tested.
During vegetative growth after transplanting, crop biomass responded
positively to FACE. In previous studies on the effect of elevated [CO
2
] on rice
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388 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
growth, tiller number frequently shows a larger response to elevated [CO
2
]
than other measured growth parameters (e.g., Imai et al., 1985; Baker et al.,
1990; Ingram et al., 1995; Kim et al., 1996b; Ziska et al., 1997). In the Rice FACE
experiment, the tiller number response was greater than the green leaf area
response, suggesting that FACE may decrease individual leaf size and/or
leaf number per tiller. In many other species where individual leaf size has
been reported, elevated [CO
2
] generally increases individual lamina area
(Pritchard et al., 1999). More detailed studies on the effects of elevated [CO
2
]
on leaf morphology are warranted.
The magnitude of the increases in rice crop biomass with FACE ranged
from 19% early in the season to 12% at final harvest. One possible reason for
this decrease in response over time may be a decline in crop photosynthetic
capacity with season-long exposure to elevated [CO
2
]. This so-called accli-
mation or down-regulation of photosynthesis is due to a change in the pho-
tosynthetic characteristics of the leaves and an inhibition of photosynthetic

capacity (Drake et al., 1997). For rice, acclimation is accompanied by both a
drop in Rubisco activity and Rubisco amount relative to other leaf proteins
and an increase in nonstructural carbohydrates in the leaf blades and sheaths
(Rowland-Bamford et al., 1990, 1991). The increase in nonstructural carbohy-
drates leads to increases in SLW (Pritchard et al., 1999); for rice SLW increased
by approximately 2% with increasing [CO
2
] from 330 ppmV to 500 ppmV
(Rowland-Bamford et al., 1990). This is similar to the 3% increase in whole
canopy SLW in the Rice FACE experiment (Kim et al., 1999).
In the Rice FACE experiment, crop biomass and grain yield increased 12
and 16%, respectively, with FACE. However, these responses were smaller
than those reported for other investigations on the influence of elevated
[CO
2
] on rice growth. For example, in experiments conducted using cham-
bers, Baker and Allen (1993a), Kim et al. (1996b), and Ziska et al. (1997)
reported grain yield increases ranging from 20 to 30% under a comparable
[CO
2
] and optimal temperatures. There are a number of possible reasons for
the difference between the Rice FACE results and these other studies. Firstly,
plant photosynthesis and biomass responses to elevated [CO
2
] are greater
under high temperatures (Drake et al., 1997; Nakagawa et al., 1997). Thus, it
is also likely that the crop grain yield response is greater under high temper-
atures as long as HI, spikelet sterility, and grain ripening are not affected. In
the Rice FACE experiment, the seasonal mean temperature was 19.7°C,
whereas the other studies had much higher mean temperatures of 25 to 26°C.

A second possible reason for the smaller response to elevated [CO
2
] in the
Rice FACE experiment compared to previous studies is the importance of N
availability in determining the response to elevated [CO
2
]. For a number of
species (Wong, 1979), including rice (Ziska et al., 1996), the response to ele-
vated [CO
2
] is greater with increased N availability. For the growth and yield
data reported here, fertilizer N was added at a rate of 8 g m
Ϫ2
(medium N
level). In the other studies cited previously, N was supplied at rates of 11–30
g m
Ϫ2
, hence a greater response to elevated [CO
2
] could be expected. Indeed,
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GROWTH AND YIELD OF PADDY RICE UNDER FREE-AIR CO
2
ENRICHMENT 389
Figure 18.7 The relationship between nitrogen (N) uptake and crop biomass or
grain yield for plants supplied 8 or 12 g N m
Ϫ2
and grown under ambi-
ent CO
2

(open symbols) or FACE (closed symbols). “+”, “ *”, and “ **”
denote significance at the p Ͻ 0.1, 0.05, and 0.01 levels, respectively.
in both FACE and control, crop biomass and grain yield increased linearly
with increasing N uptake, but the slopes of the linear regression were greater
with FACE (Figure 18.7). This indicates that the positive effects of FACE on
rice crop production and yield could be greater with higher levels of avail-
able N. Other possible reasons for variations in the response to [CO
2
] include
the use of different cultivars and plant population.
Rice grain yield is a function of total crop d.wt and the percentage of this
that is partitioned to the grains as expressed by HI (Horie et al., 1992). FACE
had no affect on HI (Table 18.4). Thus, the increase in grain yield due to FACE
with increasing N uptake was due to the increase in crop biomass rather than
HI. An increase in grain number was the major contributor to the yield
increase with FACE. Grain number was also increased with increasing
applied N. It is important to provide high levels of plant N prior to anthesis
in order to produce and maintain spikelet number in rice crops (Kobayashi
and Horie, 1994). In both FACE and control plots, grain number increased lin-
early with increasing N uptake during vegetative development (from trans-
planting to flowering; Figure 18.8a). However, the grain number response to
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390 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Figure 18.8 The relationship between nitrogen (N) uptake and grain number (a) or
grain N use efficiency (NUE)(b) for plants supplied 8 or 12 g N m
Ϫ2
and
grown under ambient [CO
2
] (open symbols) or FACE (closed symbols).

N uptake was greater under FACE compared to control. For control, grain
number never exceeded 40,000 m
Ϫ2
, regardless of N uptake. In contrast, with
FACE, approximately 50,000 grains m
Ϫ2
were produced when N uptake
reached 18 g m
Ϫ2
. This was due to an increase with FACE in N uptake with
high levels of applied N level during vegetative development (data not
shown). FACE resulted in a higher grain number per unit absorbed N (grain
number production NUE) compared to control, though, for both FACE and
control, NUE declined exponentially with increasing N uptake (Figure
18.8b).
Overall, FACE resulted in a significant increase in tiller number, crop bio-
mass, and grain yield. The number of days to flowering was significantly
decreased by FACE, but there was no difference in the grain filling period.
The increase in grain yield with FACE was greater with higher levels of
applied N. This yield increase was due primarily to greater panicle and grain
number. Because FACE increased crop biomass but had little effect on N
uptake in plants under medium levels of N, grain number NUE increased
significantly.
CONCLUSIONS AND IMPLICATIONS
The Rice FACE facility provides a unique opportunity to study the
growth of rice crops when exposed to [CO
2
] at levels similar to those pre-
dicted for the middle of the twenty-first century. FACE technology permits
crops to be grown under field conditions with minimal perturbations to other

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GROWTH AND YIELD OF PADDY RICE UNDER FREE-AIR CO
2
ENRICHMENT 391
abiotic environmental factors. All other factors being equal, it is likely that the
data presented in this chapter for the Rice FACE experiment will be repre-
sentative of the growth and yield of rice cultivated in northern Japan under
the levels of [CO
2
] predicted for the mid-twenty-first century.
The results of the FACE experiment clearly indicated that rice crop
growth is stimulated and grain yield is increased by elevated [CO
2
]. The
responses of rice plants to higher [CO
2
] with FACE, e.g., an increase in the
number of tillers, were similar to those commonly found in experiments con-
ducted using chambers to contain elevated [CO
2
]. However, the magnitude
of some of the Rice FACE responses to elevated [CO
2
], including that of yield,
were smaller than those found in chambers experiments. The likely reasons
for this difference in response are the lower temperature and smaller
amounts of fertilizer N in the Rice FACE experiment.
As noted earlier, rice yields will have to increase substantially to meet the
requirements of the increasing population in the future. The FACE experi-
ment showed that there was a positive interaction between N fertilization

and [CO
2
]: the response to elevated [CO
2
] was greater with higher levels of
applied N. The greater yield increase with increasing N supply also suggests
the possibility of taking advantage of elevated [CO
2
] by changing agronomic
practices to obtain even higher yields. For example, cultivars with higher
rates of N uptake could benefit more from elevated [CO
2
]. Moya et al. (1998)
suggested the possibility of selecting varieties that are more responsive to
CO
2
in order to increase yields. Their results showed that a rice variety with
greater tillering response to the elevated [CO
2
] exhibited a greater yield
increase. In the Rice FACE project, too, varietal differences are being studied,
although the results are yet to be analyzed.
Apart from the importance of rice production for food supply, paddy
fields (which grow a large proportion of the rice crops) are among the major
anthropogenic sources of atmospheric methane (CH
4
), which is a more pow-
erful greenhouse gas than CO
2
(Prather et al., 1995). In the 1998 FACE exper-

iment, root biomass was significantly increased by CO
2
enrichment, which
implies an increased C input to the soil and therefore increased supply of
substrate to the methanogenic microbes. Methane emissions from the paddy
fields could therefore be higher under higher [CO
2
]. Indeed, Ziska et al. (1998)
reported increases of 49–60% in methane emission under elevated [CO
2
] in
open-top chambers. In contrast, Schrope et al. (1999) reported a marked
decrease in methane emission under elevated [CO
2
] in temperature-gradient
chambers. Measurements in the Rice FACE experiment in 1998 showed
reduced methane emission early in the season, but increased emission late in
the season (K. Inubushi, pers. comm.). Further studies on the effects of ele-
vated [CO
2
] on methane emission and soil microbial activity are in progress.
It is also important to know if the amounts of C stored in paddy soils are
significantly altered by the enhanced C supply to below ground under ele-
vated [CO
2
]. Soil C storage is one of the key processes in the prediction of C
budgets and atmospheric [CO
2
] in the future. Changes in the soil C pool under
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392 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
elevated [CO
2
] are being studied in the Rice FACE experiment by utilizing
13
C
abundance to detect net C input to the soil. In addition, the ecosystem C
budget is being studied by measuring CO
2
exchange rates across the soil-
water and water-air interfaces in the FACE rings (H. Koizumi, pers. comm.).
An advantage of the Rice FACE project is that large areas of crop can be
grown under elevated [CO
2
], and hence a number of different experiments
can be carried out at one time. In addition to the investigations on growth and
yield, other studies that are also being undertaken include the effects of FACE
on photosynthesis, photosynthate partitioning, plant morphology, nutrient
content, and root development. Other investigations include the effects of
FACE on the canopy energy budget, the sensitivity of rice plants to fungal
disease, and the behavior of insect populations within and around the crop.
FACE is essentially a field experiment with CO
2
as the control variable.
Results of the investigations are therefore subjected to the influences of the
agronomic practices used and the environment at the field site. The interac-
tions of elevated [CO
2
] with temperature and N supply imply that the
responses of rice crops to FACE may depend on climate and agronomic prac-

tices. Because more than a half of the world’s rice is grown in China and India,
where the climate, soils, and agricultural practices are quite different from
those in northern Japan, we believe that FACE experiments must also be done
in these major rice-producing countries. Indeed, there is a rice-wheat cropping
system FACE project at the designing and testing stage in India (A.P. Mitra
pers. comm.). A FACE experiment in China is also being set up.
Knowledge obtained from Rice FACE experiment will improve our
understanding of the rice crop and ecosystem processes in a world with ele-
vated [CO
2
]. A better understanding of these processes will hopefully help us
predict the changes in rice production and rice ecosystems, thereby enabling
planning for, adapting to, and possibly utilizing the future elevated CO
2
.
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