tant to consider both elevated CO
2
concentrations and differences in soil water in order to assess the
possible effects of climate change on crops. Numerous experiments have demonstrated that
in many C
3
species high atmospheric [CO
2
] leads to increases in the photosynthetic rate, whole
plant growth and water use efficiency WUE and decreases in stomatal conductance and transpira-
tion and photosynthesis is the most sensitive pro- cess to CO
2
enrichment Kimball, 1983; Drake and Leadley, 1991; Bowes, 1993; Poorter, 1993;
Idso and Idso, 1994; Jiang, 1995; Wang et al., 1998. While results of studies on the plant
canopy water use requirements are conflicting Al- len, 1990 water deficit, on the other hand, is well
established to constrain leaf photosynthesis, plant growth and water use requirements with the most
sensitive process being cell growth Hsiao, 1973; Turner, 1987. However, on the interactive effects
of CO
2
and other environmental factors on plants, publications are relatively fewer, and
among these there are two contradictory views. Some authors proposed that high [CO
2
] effects on plants were not affected by environmental stress
factors Idso and Idso, 1994 whereas other au- thors have reported or theoretically concluded
that high [CO
2
] effects vary among plant species under different environmental conditions Kim-
ball, 1983; Poorter, 1993, 1998; Thompson and Woodward, 1994; Hunt et al., 1995; Ziska et al.,
1996; Bunce, 1998. Some authors have even sug- gested that the positive effects of CO
2
can not be maintained when other environmental factors are
limiting Kramer, 1981; Poorter, 1998. So plant growth and yield response to CO
2
can depend on the availability of soil water Stronach et al.,
1994. However, judging by the available data on the interactions between CO
2
and other environ- mental factors, water stress, which is probably the
most important of the environmental interactions with elevated CO
2
, is one of the least well studied Bowes, 1993; Picon et al., 1997.
In this study, broad beans were grown under different combinations of CO
2
concentration and soil water levels and focused on the effect of
long-term exposure of plants to elevated CO
2
and drought on photosynthesis, growth and water use.
It was hypothesized that: 1 there would be inter- action between CO
2
and drought on growth and yield, and the effects of CO
2
enrichment on plants depend on soil water status; 2 CO
2
enrichment would promote plant canopy water use require-
ments due to the decrease in transpiration being over-offset by an increase in leaf area; 3 WUE
i
and WUE would be increased by CO
2
enrichment.
2. Materials and methods
2
.
1
. Plant materials and growth conditions The experiment was conducted in field open-top
chambers OTC
f
, at Lanzhou 103.9° E, 36.0° N, Gansu, in semi-arid region of Loess Plateau of
China. Seeds of the local common-used broad bean cultivar Vicia faba L., Lincan II were sown
in March 1997 in 17.2 l black plastic pots 27 cm in diameter, and 30 cm in height 14 beans per
pot, filled with field loessial soil. Fertilizer com- pound fertilizer, carbamide and ammonium phos-
phate was applied prior to planting to reach local favourable nutrient level. Then, the soil was sam-
pled and analyzed at the Soil and Plant Chemical Testing Laboratory in the State Key Laboratory
of Arid Agroecology. Based upon the results of that analysis, the soil properties are: pH 7.5,
organic matter 3.2, available N 170.7 mg kg
− 1
i.e. hydrolytic N, 1 N NaOH hydrolysis, avail- able P 214.8 mg kg
− 1
0.5 M NaHCO
3
extrac- tion,
available K
228.5 mg
kg
− 1
1 N
CH
3
COONH
4
extraction, field water capacity 33. Before sowing the soil was irrigated to 80
field water capacity FWC favourable soil water level for broad bean. Six treatments consisting of
factorial combinations of two [CO
2
] levels and three soil water levels commenced 20 days after
sowing DAS. Treatments are designated HA, MA, LA, HD, MD, and LD, where H, M, and L
stand for high, medium, and low soil water levels, A and D stand for ambient and double ambient
[CO
2
], respectively. Due to the use of field-col- lected soil sufficient Rhizobium infection was
found. Plants were grown in six OTC
f
s F1.5 m × 2 m, three with ambient [CO
2
] a seasonal average of 350 ppmv, the other three with double ambi-
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
