2 0 1 0 ) 1558-1565 1561
400 i i m o * CO: 700 Mmol m o l ' CO2
OmM 10mM 15nnM 0 mM lOmM 15mM
Treatments
Fig. 1. Effect of CO2 (ambient CO2. approximately 400)xmolmol-' and elevated
CO2, 700nmolmol-') and increasing doses of NH4NO3 (0. 10 and 15mM) on net
photosynthesis (A„„, (imol CO2 m^^ s ') of nodulated alfalfa at the age of 44 (A and
C, corresponding to 2 weeks of treatment) and 60 days (B and D. corresponding to 4
weeks of treatment), at 400 and 700 (imol m o l " ' CO2. Bars represent the mean ± SE;
n - 4. Bars with the same letter are not significantly different (P > 0.05) according LSD
test.
significant differences were evaluated using Lees Significant Difference
post hoc tests (LSD) (P< 0.05).
Results
Elevated CO2 did not modify plant dry weight (DW) at the second
week (44 days old), but at the fourth week (60 days old)
plant production was enhanced by elevated CO2 (Table 1) (CO2
F= 7.945, P=0.011). NH4NO3 increased DW at the second and fourth
weeks (Table 1) (NH4NO3 F=8.85, P= 0.002 and NH4NO3 F= 24.64,
P<0.001, respectively). Increasing doses of NH4NO3 reduced nodule
DW in both weeks (second and fourth) of treatment (NH4NO3
F=21.29, P< 0.001 and NH4NO3 F= 27.66, P< 0.001, respectively).
At the second and fourth weeks, total leaf area was increased
by NH4NO3 (NH4NO3 F=29.9, P<0.001 and NH4NO3 F=123.90,
P< 0.001, respectively), but elevated CO2 did not affect it (Table 1).
At the second and fourth weeks of treatment, the root/shoot ratio
was not modified by elevated CO2 or NH4NO3, w i th the exception
of the OE versus OA treatment in the fourth week of treatment
(Table 1). At the second week, the specific leaf area (SLA) decreased
with elevated CO2 (Table 1) except for plants irrigated w i th 15 m M
NH4NO3 (CO2 XNH4NO3 F=7.13, P=0.005). Nevertheless, at the
fourth week, the SLA decreased w i th elevated CO2 in all treatments
(CO2 F= 137.08, P< 0.001), but was enhanced w i t h increasing doses
of NH4NO3 (NH4NO3 F= 7.57, P = 0.004).
Net photosynthesis {Anet) measured at 400|jimolmol"' CO2
after the second week of treatment (44 days old plants) (Fig. lA)
was decreased by NH4NO3 (NH4NO3 F=4.97, P=0.028) but not significantly
affected by CO2 (Fig. l A ) . When Anet was measured at
700 p,mol m o l ~ ' (Fig. 1C) no effect of CO2 or NH4NO3 was observed.
After 4 weeks of treatment (60 days old plants), Anet measured at
400 jj,mol m o l " ' (Fig. 1B) was unaffected by NH4 NO3, but decreased
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