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Rice yield enhancement by elevated CO2 negatively correlates with hastened occurrence of heading –Results form five years chamber studies-

Hidemitsu Sakai1, Han-Yong Kim2, Toshihiro Hasegawa1 and Kazuhiko Kobayashi3

1 National Institute for Agro-Environmental Sciences, http://www.niaes.affrc.go.jp, 3-1-3 kannnondai, Tsukuba, Ibaraki 305-8604, Japan, Email hsakai@niaes.affrc.go.jp, thase@niaes.affrc.go.jp
2
Division of Applied Plant Science, College of Agriculture & Life Science, Chonnam National University, http://www.chonnam.ac.kr, 300 Yongbong-dong, Bukgu, Gwangju 500-757, Korea, Email hyk1020@chonnam.ac.kr
3
Graduate School of Agriculture and Life Sciences, The University of Tokyo, http://www.u-tokyo.ac.jp, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan, Email aclasman@mail.ecc.u-tokyo.ac.jp

Abstract

Rice (Oryza sativa L.) yield enhancement by elevated CO2 concentration ([CO2]) has often been reported to a varying degree ranging. To identify possible reasons for the variation in yield enhancement by elevated CO2, we analysed the results from the experiments conducted in six naturally sunlit, controlled environment chambers for five years (from 1998 to 2002). Rice plants (cv. Nipponbare) were grown season-long under ambient (354-383/397-448 mol mol-1; day/night) and elevated (670-721/702-780 mol mol-1) [CO2] each in three chambers. Air temperature inside the chambers was controlled at the outside level. Relative humidity was kept 77-80 %. Total nitrogen application was 8 g m-2 in 1998 and 1999, and 12 g m-2 in 2000-2002. In 2001, two subplots in which the timing of top-dressing was different were made in all chambers.

Final total dry weight was significantly increased by 8.0-18.7% by elevated [CO2] in all five years. Enhancement of final total dry weight by elevated [CO2] at each N level was similar across years. Days to heading were significantly shortened by 2.6-8.0 days. Grain yield was significantly increased by elevated [CO2] by a varying degree ranging from 4.1 to 22.4 %, but the relationship between grain yield and final total dry weight enhancements was not clear. On the other hand, a strong negative relationship was found between grain yield enhancement and the days to heading hastened by elevated [CO2]. These suggest that degree of acceleration of plant development can have a significant impact on the rice yield responsiveness to elevated [CO2].

Media summary

Grain yield enhancement of rice by elevated CO2 was negatively correlated with hastened occurrence of heading.

Key Words

Elevated CO2, Rice, Yield, Growth

Introduction

Elevated CO2 concentration ([CO2]) increases growth and yield of many agricultural crops but with a varying degree (Kimball 1983). Rice (Oryza sativa L.) is generally less responsive to elevated [CO2] compared to other crop species (Kimball et al. 2002), but variation exists in the degree of enhancement by elevated [CO2] among previous studies of rice, ranging from 4 % up to 44 % (Baker and Allen 1993, Nakagawa and Horie 2000, Kim et al. 2003a). The difference in the magnitude is partially due to experimental methods, but the range in responses of growth and yield to elevated [CO2] also depends on factors such as nitrogen (N) level, air temperature and cultivar.

We have been examining the response of rice growth and gas exchange to elevated [CO2] under controlled environment (Climatron) chambers (Sakai et al. 2001). During the last five growing seasons, we have observed a varying degree of responses to elevated [CO2] even under the same experimental conditions. Identifying reasons for this variability in yield response will provide us with options for better adaptability of rice to possible future environments.

Methods

Controlled environment chambers

The experiments were conducted in six naturally sunlit, controlled environment chambers (Climatron) for five years (from 1998 to 2002). Details of the chamber system were described in Sakai et al. (2001). Briefly, chamber dimensions were 4x3x2 m (LxWxH). Each chamber housed two stainless-steel containers (1.5x1.5x0.3m) filled with paddy soil. Air temperature and relative humidity in each chamber were controlled by electrical resistive heaters (with a bubbling system for humidification) and cold-water heat exchangers. Daytime [CO2] was maintained by a computer-controlled pure CO2 injection system, which compensated for CO2 uptake by rice canopy. Night-time [CO2], which increased due to plant respiration, was kept below 100 mol mol-1 higher than the daytime [CO2] by a computer-controlled air ventilation system, which introduced ambient air to reduce [CO2].

Plant culture and growth condition

In all five years, rice plants (cv. Nipponbare) were grown season-long under ambient (354-383/397-448 mol mol-1; day/night) and elevated (670-721/702-780 mol mol-1) [CO2] each in three chambers. Germinated seeds of rice were sown to seedling trays (cell size, 1.5x1.5cm) on 20 April (1998-2001) and 23 April (2002), and seedlings were grown in Climatron chambers under 350 or 650 mol mol-1 [CO2], with air temperature and relative humidity controlled at 23 C and 80 %, respectively for about a month. They were then transplanted in the containers in chambers with 3 seedlings per hill at a 2020 cm spacing on 15 May (1998-2000 and 2002) and 25 May (2001). Total nitrogen (N) application was 8 g m-2 in 1998 and 1999, and 12 g m-2 in 2000-2002. All containers received the same amount of N at the same timing in each year (Table 1), except in 2001, where the timing of second topdressing was different between containers in all chambers: the second top-dressing was applied at the panicle initiation stage in one container and at the heading stage in the other container. In all years, 15 g of P2O5 m-2 and 15 g m-2 of K2O were applied just before transplanting. Air temperature inside the chambers was controlled at the outside level. Relative humidity was kept 77-80 %. When rice plants in ambient [CO2] chambers reached physiological maturity (always later than in elevated [CO2]), rice plants were harvested simultaneously in both [CO2] treatments.

Table 1. Summary of CO2 concentration, average air temperature and relative humidity, and nitrogen application for five years of the experiment (1998-2002).

Year

 

[CO2]

Mean

Mean

Basal N

Top-dressed N

Growth

Treatment

Day

Night

air T1)

RH2)

applied

duration4)

 

(μmol mol-1)

(C)

(%)

(g m-2)

(gN m-2)

(days)

1998

Ambient

354

397

23.4

79.6

5.0

3.0 (56) 3)

151

 

Elevated

670

703

           

1999

Ambient

366

409

25.1

78.6

5.0

1.5 (47)

1.5 (57)

142

 

Elevated

695

750

           

2000

Ambient

370

435

25.3

79.6

7.0

2.5 (31)

2.5 (60)

136

 

Elevated

721

779

           

2001

Ambient (NT)5)

377

446

24.4

77.3

6.0

3.0 (32)

3.0 (61)

131

 

Elevated (NT)

700

761

           
 

Ambient (LT)

377

446

24.4

77.3

6.0

3.0 (32)

3.0 (84)

131

 

Elevated (LT)

700

761

           

2002

Ambient

383

448

24.0

76.8

6.0

3.0 (33)

3.0 (57)

138

 

Elevated

706

780

           

1) Mean air temperature during the growing season, 2) Mean relative humidity during the growing season, 3) Values in parenthesis are days after transplanting when nitrogen was top-dressed. 4) Rice plants were harvested at the day of maturity under ambient CO2 concentration simultaneously in both CO2 treatments. 5) NT: normal top-dressing, LT: late top-dressing; nitrogen was applied at panicle initiation stage and at heading stage, respectively.

Total dry weight and yield determination

Three hills from each chamber were destructively sampled at harvest. At each hill, a block of soil with a size of 20x20x15 cm (LxWxD) was dug up, and the soil was gently washed away from the roots with running water. Plant samples were then oven-dried at 80 C for three days and the dry weights were measured. To determine grain yield, 12 hills (1998), 15 hills (1999, 2000) and 10 hills (2001, 2002) from each chamber (from each container only in 2001) were harvested. After air drying for one month, panicle number was counted and the spikelets were threshed carefully. Filled spikelets were selected using a salt solution with specific gravity of 1.06 (1998-2000) or an airflow machine (2001, 2002), and counted. Filled spikelets were them oven-dried at 80C in for one week. Grain yield was expressed as the dry weight of filled unhusked grain.

Table 2. The effects of elevated [CO2] on rice growth, yield and their related factors over five years.

Year

N applied
(g m-2)

CO2
Treat-
ment

Days to
heading

LAI at
heading

Final
total DW
(g m-2)

Panicle
density
(m-2)

Spikelet
density
(103 m-2)

Filled spikelet
(%)

Single
grain wt.
(mg)

Grain
yield1)
(g m-2)

1998

8

Ambient

100.3

5.77

1785

431

28.1

86.9

22.4

552

   

Elevated

97.7

5.22

1940

467

32.6

92.8

22.4

676

   

%change2)

-2.7

-9.4

8.7

8.4

15.8

6.8

-0.1

22.4

1999

8

Ambient

93.0

5.52

1797

502

30.2

92.7

23.5

658

   

Elevated

87.3

5.91

1940

499

33.9

89.7

23.5

713

   

% change

-6.1

7.1

8.0

-0.6

12.2

-3.2

-0.1

8.4

2000

12

Ambient

86.5

8.32

1810

543

35.2

93.7

23.0

771

   

Elevated

80.7

7.28

2149

529

41.1

88.9

23.2

846

   

% change

-6.7

-12.5

18.7

-2.6

16.7

-5.1

0.7

9.5

2001

12

Ambient

81.7

8.41

2067

519

37.7

93.9

21.5

762

 

(NT)3)

Elevated

77.3

7.80

2285

529

39.0

95.9

22.5

842

   

% change

-5.3

-7.3

10.6

1.9

3.5

2.2

4.7

10.5

 

12

Ambient

80.3

8.83

2375

531

35.8

96.0

23.0

793

 

(LT)

Elevated

75.0

7.41

2616

563

37.6

96.7

23.1

839

   

% change

-6.6

-16.1

10.1

6.1

4.9

0.8

0.2

5.8

2002

12

Ambient

89.0

9.09

2020

484

40.0

95.7

22.1

846

   

Elevated

81.0

9.11

2236

499

40.2

94.6

23.1

880

   

% change

-9.0

0.2

10.7

3.2

0.6

-1.2

4.6

4.1

ANOVA4)

                   

CO2

   

**

ns

**

**

*

ns

ns

*

Year

   

**

**

**

**

**

*

*

**

N

   

**

**

**

**

**

*

ns

**

CO2*Year

   

ns

ns

ns

ns

ns

ns

ns

ns

CO2*N

   

ns

ns

ns

ns

ns

ns

ns

ns

1) grain yield is expressed on a dry mass basis. 2) % change due to elevated [CO2]. 3) See table 1. 4) * and **, significant at P<0.05 and P<0.01; ns, not significant.

Results

The seasonal mean air temperature during five years ranged from 23.4 (1998) to 25.3C (2000). It was extremely cool summer in 1998. Contrary to 1998, it had been hot through the growing season in 1999 and 2000. In 2001, it was hotter than in 1999 and 2000 until panicle initiation stage, but was as cool as in 1998 after then. In 2002, air temperature was on a level with that in 1999 and 2000, but with some cool periods in tillering and grain filling stage.

Results of destructive sampling and yield determination of five years experiments are shown in Table 2. Days to heading were significantly shortened by elevated [CO2] by 2.6-8.0 days. Although we did not determine the definitive dates of physiological maturity, hastened occurrence of heading events by elevated [CO2] likely resulted in shorter growth duration in the elevated [CO2] than in the ambient [CO2] because grain filling in the elevated [CO2] progressed under hotter condition than in the ambient [CO2].

Leaf area index (LAI) at the heading stage was largely influenced by total N applied, but the effect of elevated [CO2] was not significant though LAI was reduced in three years in the elevated [CO2].

Final total dry weight was significantly increased by elevated [CO2]. The enhancement of total dry weight was generally similar across years, but was slightly but consistently higher in years with larger N application, averaging 8 and 10 % for 8 and 12 g N m-2 application, respectively. Interestingly, the enhancement ratio was not altered when the timing of top-dressing was changed with the same total N application in 2001 though top-dressing at the heading stage increased total dry weight more than that at the panicle initiation stage in both [CO2] treatments.

Both panicle and spikelet densities increased as a result of elevated [CO2], but the effect was generally larger on spikelet density, indicating that spikelet per panicle was also increased by elevated [CO2]. Filled spikelet percentage was generally high but significantly different among years with no consistent effect of elevated [CO2]. The effect of elevated [CO2] on single grain mass was noted, but about 5% enhancement was observed in two occasions with relatively small enhancement of spikelet density.

Elevated [CO2] increased grain yields significantly, with a range of 4.1 to 22.4 % depending on the trials. The grain yield enhancement by elevated [CO2] in some cases was larger than in biomass enhancement, but lower in other cases, and there was no clear association between them.

Figure 1. The relationship between the increase rate (%) of grain yield and the shortening rate (%) of days to heading by elevated CO2 concentration in five years (1998-2002).

The variation in grain yield enhancement was closely correlated with the shortened days to heading by elevated [CO2] (Figure 1). The earlier occurrence of heading in the elevated [CO2] treatment seemed to have decreased the enhancement of spikelet density and filled spikelet percentage.

Implication and Conclusion

Final total dry weight was significantly increased by elevated [CO2] in all five years, as has been observed in other studies with rice (Baker and Allen 1993, Nakagawa and Horie 2000, Kim et al. 2003a). Final total dry weight enhancement by elevated [CO2] was similar among trials with the same N level despite a range of weather conditions experienced for the five years. This result suggests that conserved nature of the final total dry weight response to elevated [CO2] is due to the limitation set by the amount of N available for rice plants in the range of N applied in this experiment. Grain yield enhancement, on the other hand, was more variable than that in biomass.

The strong negative relationship between grain yield enhancement and hastened occurrence of heading likely involves a number of different responses to CO2. Shorter days to heading might not have ensured enough time for reproductive organ development, so that the spikelet density response can be one source of variation in grain yield enhancement. Filled spikelet percentage can also be a source of variation because different timing of heading can result in different environmental conditions during the grain filling.

The mechanism by which elevated [CO2] hastens occurrence of heading is not clear, but the present results suggest that degree of acceleration of plant development can have a significant impact on the rice yield responsiveness to elevated [CO2].

References

Baker JT and Allen LH, Jr. (1993) Effects of CO2 and temperature on rice: a summary of five growing seasons. Journal of Agricultural Meteorology 48, 575-582.

Kim HY, Lieffering M, Kobayashi K, Okada M and Miura S (2003a) Seasonal changes in the effects of elevated CO2 on rice at three levels of nitrogen supply: a free air CO2 enrichment (FACE) experiment. Global Change Biology 9, 826-837.

Kim HY, Lieffering M, Kobayashi K, Okada M, Mitchell MW and Gumpertz M (2003b) Effects of free-air CO2 enrichment and nitrogen supply on the yield of temperate paddy rice crops. Field Crop Research 83, 261-270.

Kimball BA (1983) Carbon dioxide and agricultural yield: an assemblage of 430 prior observations. Agronomy Journal 75, 779-788.

Kimball BA, Kobayashi K and Bindi M (2002) Responses of agricultural crops to free-air CO2 enrichment. Advances in Agronomy 77,293-368.

Nakagawa H and Horie T (2000) Rice responses to elevated CO2 and temperature. Global Environmental Research 3, 101-113.

Sakai H, Yagi K, Kobayashi K and Kawashima S (2001) Rice carbon balance under elevated CO2. New Phytologist 150, 241-249.

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