South Pacific Studies Vol. 25, No. 2, 2005

Effects of Enhanced Level of CO2 on Photosynthesis,
Nitrogen Content and Productivity of Mungbean(Vigna radiata L. WILCZEK)
Razia Sultana CHOWDHURY1, Md. Abdul KARIM1, 2, M. Moynul HAQUE1,
Abdul HAMID 1 and HIDAKA Tetsushi 3
1: Bangabandhu Agricultural University, Salna, Gazipur-1706, Bangladesh
3: Research Center for the Pacific Islands, Kagoshima University, 1-21-24 Korimoto,Kagoshima 890-8580, Japan

Abstract
Enhanced level of CO2 increases overall growth of crop plants, especially of C 3 species. This study was,however, initiated to quantify the effect of elevated CO2 on changes in photosynthesis and N uptake, and theirassociation in biomass accumulation pattern at different growth stages in mungbean (Vigna radiata L. WILCZEK).
Mungbean (var. BUmug 1) plants were grown in pots under i) open top chamber (OTC) with 570 ± 50 ppm CO2 ,ii) OTC + ambient CO2 which was about 370 ppm and iii) open field (about 370 ppm CO2 ) conditions. Elevated
CO 2 increased stomatal density in leaf. Photosynthetic rate (Pn) was faster due to elevated CO 2. The difference in
Pn among the treatments was very large at flowering than that during pod maturation. N content in leaf as well as
in whole plant, at different growth stages, was lower with elevated CO2 . Moreover, the content was higher at
vegetative stage than that at reproductive stage, irrespective of levels of CO2 . Leaf and stem dry weights wereconspicuously higher with elevated CO2 ; consequently, above ground biomass was also maximum in thattreatment. Yield components were favored dramatically by elevated CO2 . As a result the yield advantage withelevated CO 2 was 119 and 29% of that under field and OTC + ambient CO 2 conditions, respectively.
Key words: carbon dioxide, gas exchange, mungbean productivity

Introduction
The current trend of increasing atmospheric CO 2 indicates that the level might bedoubled from the present level, around 350 ppm, by the middle of this century (W ATSON etal., 1990; HOUGHTON et al., 1996). The global mean temperature will also rise to 3 - 4℃
with doubling of the CO 2 concentration (REDDY et al., 1995). Such a change of theatmosphere will obviously bring a shift in overall agriculture globally.
Enhanced level of CO 2 increases productivity, from 10 to 40%, of different food crops(KIMBALL 1983; M ITCHELL et al., 1993; WEIGEL et al., 1994; H AMID et al., 2003). The
variation in the range of productivity among the crop species attributed to differences in
Received Sept. 19, 2004
Accepted Jan. 10, 2005
2: Corresponding author, email: akarim1506@yahoo.com

98South Pacific Studies Vol. 25, No. 2, 2005

photosynthetic performance and sink strength. C 3 species responds more to high level of
CO 2 than C 4 species. This is largely because the oxygenase activity of CO 2 fixing enzyme,
ribulose biphosphate carboxylase/ oxygenase (Rubisco), of C 3 plants reduced at a high levelof CO 2 . Whereas, the CO 2 fixing process of C 4 plants does not competitively inhibited by
CO 2 (PEARCY and BJÖRKMAN 1983).
Mungbean (Vigna radiata L. W ILCZEK) is a tropical legume, extensively grown indiverse agro-climatic conditions of Asia and Australia. As a C 3 legume, mungbean also
responded positively to enhanced CO 2 (UPRETY et al., 1996). The positive response of
mungbean was related to high water use efficiency, greater photosynthesis and higher
nutrient use efficiency. However, insufficient information exists on a stage dependent
pattern of biomass accumulation and gas exchange activities. To develop a practical crop
performance model of mungbean, under changing atmospheric CO 2 conditions, more
precise information on each plant character is required. The present study was designed toquantify the pattern of biomass accumulation, nitrogen uptake and gas exchange activities
at different growth stages of mungbean, exposed to long term enhanced level of CO 2.

Materials and Methods
The experiment was conducted at the farm of Bangabandhu Sheikh Mujibur Rahman
Agricultural University (BSMRAU), Bangladesh during March- May, 2003. The soil usedin this experiment was loamy in texture, with pH 7.10 and 0.12% total nitrogen. The-1amount of available phosphorus and exchangeable potassium was 0.10 and 0.30mg 100 gdry soil, respectively.
Mungbean (var. BUmug 1) plants were grown in pots of 24cm in diameter and 27cm
in height filled up with 12kg air-dried soils. Three growing conditions were created, viz. i)
open top chamber (OTC) with elevated CO 2 (570 ± 50 ppm), ii) OTC with ambient CO 2
(about 370 ppm) and iii) open field (about 370 ppm) near the OTC.
Details of the construction and operation of open top chamber (OTC) can be foundelsewhere (LEADLEY and DRAKE 1993; U PRETY 1998). It is made of an iron frame of 3min diameter and 3m in height. It was installed on the ground and covered with transparent
polyvinyl chloride sheet. The top of the chamber was open to ensure near natural conditions. The CO 2 gas was supplied to the chamber from gas cylinder using a manifold, gas
regulator, pressure gauge and underground pipeline for using natural air with the help of ablower. The air blower, 30cm diameter, thoroughly mixed the supplied CO 2 gas withatmospheric air and blew to the chamber.
A fertilizer dose of 90kg urea, 130kg triple super phosphate (TSP) and 70kg muriate
of potash (MP) per hectare (recommended dose in Bangladesh) corresponding to 540mg
urea, 780mg TSP and 420mg MP per pot were used. Entire amount of TSP and MP, and one
half of urea were mixed into soil before sowing. The other half of urea was applied 3 weeks
after sowing. Seeds were soaked in water for six hours and then four bold seeds were sownstin each pot. Thinning was done at appearance of the 1 trifoliate leaf to keep two uniform

CHOWDHURY et al.: Effects of CO2 on Photosynthesis of Mungbean

99

and healthy plants in each pot. Weeding and other plant protection measures were done as
and when those were necessary.
Stomatal density was measured at flowering stage from fully expanded uppermost leaf.
In a preliminary study we noticed that number of stomata in abaxial side of leaf was higher
but well correlated with that in adaxial side. Therefore, we measured the stomatal frequency
only from the adaxial side. Cover slips of slide were attached to leaf with super glue forgetting stomatal impression. After 15 minutes, the cover slips were removed. The numberof stomatal impressions was counted from 20 microscopic fields per genotype per treatment
under 40X magnification. Since the photosynthesis at reproductive stage (current photosynthesis) influences greatly on grain yield, we measured leaf photosynthetic rate (Pn) during
flowering and pod maturation at 11:00 am in clear days with a portable photosynthesis
system (LICOR 6200, Lincoln, Nebraska). Pn was measured from the uppermost fully
developed leaves of four plants from each growing condition. N content at different growthstages were estimated by colorimetric method following LINDER (1944).
Plants were sampled at vegetative (four weeks after sowing), flowering and maturity
to determine matter partitioning into different components of mungbean plant. Reproductive
organs were considered during measurement of above ground biomass. Yield and yieldcomponents were recorded from six plants per treatment.
The data recorded on different parameters were statistically analyzed with help of“MSTAT” program. Differences between the treatment means were compared by leastsignificant difference (LSD) test at 5% level of significance after performing ANOVA.

Results and Discussion
Elevated CO 2 increased stomatal density to a great extent (Table 1). Leaves developed
under elevated CO 2 had 41 and 32% more stomata than those developed under OTC +ambient and field conditions. During microscopic study, it was noticed that epidermal cellsof the leaves under elevated CO 2 were smaller but grater in number than those under fieldand OTC + ambient CO 2 conditions. High stomatal frequency with elevated CO 2 could be
due to higher number of cells but smaller in size, although cell size was not recorded in thisstudy. In general photosynthetic rate (Pn) was faster during flowering than that during pod
maturation, under both OTC conditions (Table 1). The Pn was, however, not changed muchunder field conditions. During flowering stage, the Pn at elevated CO 2 was 46 and 104%higher, while that during pod maturation was 23 and 14 % higher than that measured at
OTC + ambient CO 2 and field conditions, respectively. Thus, the difference in the Pn,among the treatments, was much higher at flowering than that at later stage. Higher Pn
under elevated CO 2 in C3 plants was also reported by ZISKA and TERAMURA (1992), andU PRETY and M AHALAXMI (2000). According to P EARCY and BJÖRKMAN (1983), theincrease in Pn in C 3 plants under elevated CO 2 was related to CO 2 induced reduction inoxygenase activity of Rubisco, the primarily CO 2 fixing enzyme in C 3 species. Higher
photosynthesis during flowering and the subsequent decline during pod maturation could be

100South Pacific Studies Vol. 25, No. 2, 2005

related to sink demand for photosynthates. At flowering stage, the demand forphotosynthates increases due to onset of pod formation as well as continuation of vegetative
growth. During pod maturation, however, the demand decreases due to slow down ofgrowth processes, and the Pn may decline due to feed back effect (DELGADO et al., 1994).
The higher number of stomata under elevated CO 2 might have also contributed for higher
Pn under such conditions, as stomata regulates gas exchange activity in plants.
Irrespective of levels of CO 2, higher leaf N was observed at vegetative stage and theminimum at final harvest. N in leaf, under elevated CO 2 , was 14, 26 and 12% lower at
vegetative, flowering and maturity, respectively from that under field conditions. The
corresponding reduction under OTC + ambient CO 2 was 18, 26 and 17%, respectively.
Therefore, the difference in leaf N among the treatments was much higher at flowering. The
effect of elevated CO 2 on total N content in plant was more or less similar to that measured
in leaf. Perhaps, the low percentage of N under elevated CO 2 was due to dilution effect forhigh N use efficiency under such conditions (high biomass per unit N). Brassica plants also
showed reduction in N content by 30% due to enhanced CO 2 (U PRETY and M AHALAXMI
2000). According to H OCKING and M AYER (1990) elevated CO 2 could cause increased
biomass per unit uptake of N, even N is limiting in the substrate.
Table 1. Effects of elevated CO 2 on stomatal frequency and photosynthesis in mungbean.
Treatments

Photosynthesis ( μmol m -2s -1)
Flowering
Pod maturation
30.0
17.6
20.5
14.3
14.7
15.5
2.95
1.59
6.59
5.86

No. of stomata
-2
mm
20.8
14.8
15.8
1.96
5.93

OTC + elevated CO2
OTC + ambient CO2
Field conditions
LSD (0.05)
CV (%)

Notes: Elevated CO 2 level was 570±50 ppm; CO2 under OTC + ambient and field conditions was
about 370 ppm.
Table 2. Effects of elevated CO 2 on leaf N and total N at various growth stages in mungbean.
Treatment
OTC+elevated
CO 2
OTC+ambient
CO 2
Field conditions
LSD (0.05)
CV (%)

Vegetative
3.1

Leaf N (%)
Flowering
2.3

Maturity
1.5

Vegetative
2.3

Total N (%)
Flowering
1.4

Maturity
0.8

3.8

3.1

1.8

2.6

2.4

1.0

3.6
0.11
5.7

3.1
0.10
5.9

1.7
0.11
6.6

2.5
0.08
6.3

2.1
0.20
5.3

0.9
0.08
3.4

Notes: Elevated CO 2 level was 570 ± 50 ppm; CO 2 under OTC + ambient and field conditions was
about 370 ppm.

Leaf dry matter increased with ontogeny (Table 3). Elevated CO 2 increased leaf dry
mass by 74, 40 and 50% compared to that under field conditions, at vegetative, reproductive
and maturity stages, respectively. The corresponding increase was 51, 16 and 14%,respectively under OTC + ambient CO 2 conditions. Similarly, stem dry mass under elevated
CO 2 was 91, 11 and 14% higher at vegetative, flowering and maturity, respectively than that

CHOWDHURY et al.: Effects of CO2 on Photosynthesis of Mungbean

101

under field conditions. The corresponding stem biomass was 62, 7 and 7% higher than that
under OTC + ambient CO 2 conditions, respectively. Above ground biomass also showed asimilar response to leaf and stem mass. The large difference in biomass accumulation
among different growth stages was largely due to a very fast growth of plants at early stage,brought about by elevated CO 2 . The higher Pn due elevated CO 2 presumably contributed forhigher biomass production under such conditions. According to PAL et al., (1997) higher
dry matter accumulation in stem and leaves under elevated CO 2 was caused by higher
photosynthesis under such conditions. Elevated CO 2 induced increase in biomass was also
observed in cotton and soybean (KIMBALL 1983).
Table 3. Effects of elevated CO 2 on weights of leaf, stem and above ground biomass at different
growth stages in mungbean.
Lea f dry weight (g plant -1 )Treatments

Vegetative Flowering
OTC +eleva ted
1.22
3.53
CO 2
OTC +ambient
CO 2
Field
conditions
LSD (0.05)
CV (%)

M aturity
4.24

Stem dry we ight (g plant -1 )Vegetative Flowering
0.42
1.18

M aturity
5.40

Above ground biomass (g plant -1 )Vegetative Flowering
1.64
5.75

M aturity
23.80

0.81

3.04

3.71

0.26

1.10

5.03

1.07

5.14

20.08

0.07

2.52

2.82

0.22

1.06

4.75

0.92

4.49

15.10

0.15
3.5

0.22
4.32

0.33
3.52

0.08
5.69

2.07
5.36

NS
8.69

0.174
10.81

0.272
4.45

1.637
6.71

Notes: Elevated CO 2 level was 570 ± 50 ppm; CO 2 under OTC + ambient and field conditions was
about 370 ppm.
Table 4. Effects of elevated CO2 on plant height, yield components and grain yield of mungbean.
Treatments
OTC + elevated CO2
OTC + ambient CO2
Field conditions
LSD (0.05)
CV (%)

Plant height
(cm)
65.6
58.3
52.5
5.68
6.8

Pods
plant-1
26.9
24.3
20.5
2.1
6.13

Seeds
pod -1
9.00
8.10
5.90
0.85
6.3

100-seedweight (g)
5.32
4.85
3.66
0.45
4.64

Yield plant
(g)
11.96
9.26
5.45
1.27
7.93

-1

Notes: Elevated CO 2 level was 570 ± 50 ppm; CO 2 under OTC + ambient and field conditions was
about 370 ppm.

Elevated CO 2 increased plant height by 13 and 25% compared to that under OTC +ambient CO 2 and field conditions, respectively (Table 4). Number of pods per plant wasmuch higher under elevated CO 2 than that observed under OTC + ambient CO 2 (11%) andfield conditions (31%). Elevated CO 2 also increased number of seeds per pod and seed size
(100- grain weight) by 53 and 45%, respectively compared to those measured under field
conditions. The corresponding increase was 11 and 10%, respectively compared to that
under OTC + ambient CO 2 conditions. Naturally the better off yield components in plants
with elevated CO 2 increased the grain yield conspicuously by 119 and 29% compared tothat under field, and OTC + ambient CO 2 conditions, respectively. The faster rate of current
photosynthesis (photosynthesis at reproductive stage) under elevated CO 2 might havecontributed appreciably for higher grain yield. However, the large difference in Pn among

102South Pacific Studies Vol. 25, No. 2, 2005

treatments during flowering might have contributed mostly to the difference in grain yieldof mungbean under different growing conditions.
In conclusion, elevated CO 2 enhanced greatly the mungbean productivity. The better
performance of mungbean plant due to high level of CO 2 was supported by the faster rateof photosynthesis, especially at flowering stage. Elevated CO 2 also increased the productivity of mungbean per unit amount of N. Perhaps the vigorous growth of mungbean plantcaused by elevated CO 2 demands for more nitrogen application than that recommended forfield conditions in Bangladesh.

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