ent [CO
2
]. CO
2
was supplied from three 25 t storage tanks with vaporization facilities. Elevated
atmospheric CO
2
was maintained for 10 h day
− 1
at photoperiod 08:00 – 18:00 h, local time. Indi- vidual blowers made the air inside each chamber
changing twice per minute. Nine pots were placed into each chamber. CO
2
concentrations were con- tinuously monitored by CO
2
infrared gas analyzer CID, USA and controlled by a computer. A
WHM3 thermo-hygrograph Tianjin, China was fixed in each chamber to record temperature and
relative humidity continuously. In the mean time the photosynthetic active radiation, leaf and air
temperature and relative humidity in chambers were
periodically examined
using a
CI-301 portable photosynthesis system CID, USA. Dur-
ing the broad bean growth season April – July in the chambers, average photosynthetic active radi-
ation PAR was 672 mmol m
− 2
s
− 1
, average daynight temperature was 24.512.9°C, average
relative humidity was 39.5 Table 1. Three soil water levels, 40, 60 and 80 FWC,
were applied to each chamber three pots per water treatment from seedling stage onwards.
The soil water contents were controlled by com- mon-used weight method. Before sowing soil wa-
ter content and soil field water capacity were measured. The total control weight for each pot
was derived from the pot weight, soil dry weight in it and the expected soil water content level. The
pots were weighted every other day and supple- mented a determinate quantity of water calculated
from the controlled weight minus the actual weight. In the last growth phase of broad bean
when the estimated total plant wet weight in each pot was more than 0.5 percent of control weight
the pot control weight was periodically corrected by adding total plant wet weight in each pot to its
initial control weight.
2
.
2
. Measurements Leaf net photosynthesis, transpiration and
stomatal resistance were periodically measured. At each time, upper most fully expanded leaves of
nine plants in each treatment were selected for the photosynthesis
measurement using
a CI-301
portable photosynthesis system CID, USA. Pho- tosynthesis mmol CO
2
m
− 2
s
− 1
was calculated on a leaf area basis determined by the window
size of certain leaf chamber and manually put into photosynthesis system while measuring.
This bean cultivar is self-pollinating. Plant growth was assessed by periodical destructive
growth analysis of three plants randomly selected from each pot. All component dry weights were
obtained following
oven-drying to
constant weight at 85°C. Leaf area was determined using
CI-203 area meter CID, USA. Plants were har- vested on 10 July. Total shoot dry weight, bean
dry weight per plant, bean number per plant and average bean dry weight in each pot were deter-
mined at harvest.
Instantaneous water use efficiency WUE
i
, that is transpiration efficiency, is defined as the ratio of
photosynthetic ratetranspiration
rate. Whole
growth season water use efficiency WUE was calculated from shoot dry weight per plant at
harvest divided by cumulative consumption of water per plant, thus leaf transpiration and soil
evaporation.
Standard deviation S.D. of each treatment was calculated. The significance analyses of indi-
vidual and interactive effect of CO
2
and drought were performed using two-way analysis of vari-
ance ANOVA with replicates and t-test at P B 0.05 using software developed by Statistical W5.0
Statistics Inc. USA.
3. Results
3
.
1
. Leaf photosynthesis Leaf net photosynthesis P
n
in different combi- nations of [CO
2
] and soil water level treated plants at pod-bearing stage is shown in Table 2.
High [CO
2
] increased the leaf net photosynthesis by over 90 for plants grown under either fa-
vourable soil water level HD or less than that MD, LD P B 0.01. However, soil drought had
a significant negative effect on photosynthetic rate only under double [CO
2
] P B 0.01. Based on the results of two-way ANOVA with replicates, there
was a significant interaction between high [CO
2
] and drought on leaf photosynthesis P B 0.01.
D .-
X .
Wu ,
G .-
X .
Wang En
6 ironmental
and Experimental
Botany
43 2000
131 –
139
134
Table 1 The dynamics of photosynthetic active radiation PAR, temperature, and relative humidity in the chambers during growth season of broad beans
Average maximum Average maximum
Daytime mean Days after sow-
Daytime mean Daytime mean rel-
Average minimum Night mean tem-
ative humidity temperature °C
perature 08:00– temperature
temperature °C PAR 08:00–
ing PAR mmol m
− 2
s
− 1
08:00–20:00 °C 20:00 °C
18:00 mmol m
− 2
08:00–20:00 s
− 1
661 24.1
5.4 15.5
1110 19.1
0–30 7.1
9.0 24.5
27.2 23.2
11.8 31–60
684 1250
31.7 689
14.5 33.0
1260 27.6
15.9 61–90
18.8 32.5
16.8 36.1
643 91–110
1070 29.7
3
.
2
. Plant growth High [CO
2
] significantly increased the shoot dry weight at harvest at high and medium soil water
levels HD vs. HA, MD vs. MA Table 3. However, under low soil water level, the effect of
high [CO
2
] on growth was not significant LD vs. LA. The shoot dry weight at harvest was signifi-
cantly reduced by 40 by water deficit at ambient [CO
2
] LA vs. HA and by 69 at high [CO
2
] LD vs. HD. Based on the results of two-way
ANOVA with replicates, high [CO
2
] and drought showed significant interaction effect on growth
P B 0.01.
3
.
3
. Water use Since high [CO
2
] significantly increased photo- synthesis and decreased transpiration, high [CO
2
] significantly
increased transpiration
efficiency WUE
i
P B 0.001. On the other hand, drought had no significant effects on WUE
i
due to its simultaneous decreasing effect on photosynthesis
and transpiration. The negative effect of high [CO
2
] and drought on transpiration was closely associated with increase in stomatal resistance
Table 2. As for whole season water consumption and WUE, high [CO
2
] and drought had a positive and a negative effect, respectively Table 3.
3
.
4
. Yield and yield components CO
2
enrichment markedly increased the num- ber of beans per plant at high and medium water
levels HD vs. HA, MD vs. MA Fig. 1A, while under low soil water levels, the effect of high
[CO
2
] was not significant. Water deficit severely decreased the number of seeds per plant by 50
at ambient [CO
2
] LA vs. HA, by 78 at doubled CO2 LD vs. LD. There were significant interac-
tions between high [CO
2
] and drought on bean number. In the current experiment, the average
bean number per pod was 1.5, and relatively stable.
Both CO
2
enrichment and drought had no sig- nificant effect on individual bean weight Fig. 1B.
CO
2
enrichment significantly increased the total- bean weight per plant by 113 at high water level
HD vs. HA, 80 at medium water level MD vs. MA Fig. 1C, while under low soil water level,
the effect of high [CO
2
] was not significant. Water deficit significantly reduced bean production by
34 at ambient [CO
2
] LA vs. HA and 77 at high CO
2
LD vs. HD. Drought and high [CO
2
] showed significant interactive effects on bean
weight per plant. The proportion of plant dry biomass allocated
to beans harvest index, HI increased by over 10 by high [CO
2
] HD and water deficits LA not significant Fig. 1D.
Table 2 The individual and interactive effects of elevated CO
2
and soil water deficit on leaf photosynthetic rate, transpiration rate and WUE
i
of broad beans at 85 days after sowing DAS late pod-bearing stage
a
Photosynthetic rate Stomatal resistance
Treatments Transpiration rate
WUE
i
m
2
s mol
− 1
mmol m
− 2
s
− 1
mmol mmol
− 1
mmol m
− 2
s
− 1
Favourable soil water+ambient [CO
2
] 7.27a
4.24a 5.53a
1.51a HA
Medium soil water+ambient [CO
2
] 3.29b
1.94a 7.43a
9.54b MA
6.50a Soil water deficit+ambient [CO
2
] LA 3.20b
10.1b 2.35a
21.0b 25.3b
Favourable soil water+double ambient 1.43c
22.1c [CO
2
] HD 35.7d
19.0b Medium soil water+double ambient
21.8b 0.99d
[CO
2
] MD 36.1d
20.0b Soil water deficit+double ambient
12.6c 0.75d
[CO
2
] LD
a
During the measurements, average temperature was 26.2°C, average PAR 579 mmol m
− 2
s
− 1
, average relative humidity 35.2. Within columns, values followed by different letters are significantly PB0.05 different n = 9.
Table 3 The individual and interactive effects of elevated CO
2
and soil water deficit on growth and water use of broad beans
a
Cumulative consumption of Treatments
Final shoot dry Leaf area at early flowering
WUE g kg
− 1
weight g plant
− 1
stage cm
2
plant
− 1
water kg plant
− 1
116.0 18.2a
Favourable soil water+am- 9.24a
1.96a bient [CO
2
] HA Medium soil water+ambi-
99.2b 12.0b
7.10b 1.69b
ent [CO
2
] MA 83.0b
10.9b 6.68b
Soil water deficit+ambient 1.63b
[CO
2
] LA 2.50c
207.0c 33.9c
Favourable soil water+ 13.6c
double ambient [CO
2
] HD
117.0a Medium soil water+double
19.9a 10.3a
1.93a ambient [CO
2
] MD 89.1b
10.6b 5.88b
1.79ab Soil water deficit+double
ambient [CO
2
] LD
a
Within columns, values followed by different letters are significantly PB0.05 different n = 9.
4. Discussion
