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Elevated atmospheric C[O.sub.2] and ozone ([O.sub.3]) may affect productivity of legumes in part by altering symbiotic [N.sub.2] fixation. To investigate this possibility, measurements of plant biomass, N levels and natural [sup.15]N abundance ([delta]sup.15]N) were used to examine the effects of elevated C[O.sub.2] and [O.sub.3] on N acquisition in field-grown peanut (Arachis hypogaea L.) using open-top chambers. Seasonal 12-h daily average C[O.sub.2] treatment concentrations were 376, 550, and 730 [micro]mol [mol.sup.-1] Carbon dioxide treatments were applied in reciprocal combinations with seasonal 12-h daily average [O.sub.3] concentrations of 21, 49, and 79 nmol [mol.sup.-1]. At mid-vegetative growth, elevated C[O.sub.2] significantly reduced leaf N concentrations by up to 44%, but not [delta][sup.15]N values. Elevated [O.sub.3] did not significantly affect N concentrations or [delta][sup.15]N values. At harvest, plant N concentrations were similar among treatments except for a 14% reduction in the highest-level C[O.sub.2]-[O.sub.3] treatment. Plant N accumulation varied in proportion with treatment effects on biomass production, which was increased with elevated C[O.sub.2] when averaged over the [O.sub.3] treatments and suppressed by high-level [O.sub.3] at ambient C[O.sub.2]. Elevated C[O.sub.2] reduced plant [delta][sup.15]N values in low- and mid-level [O.sub.3] treatments while mid- and high-level [O.sub.3] increased them at ambient C[O.sub.2]. The changes in [delta][sup.15]N values suggested that [N.sub.2] fixation activity was stimulated with elevated C[O.sub.2] and inhibited by elevated [O.sub.3]. Elevated C[O.sub.2] ameliorated detrimental [O.sub.3] effects to varying extents depending on the concentrations of the two gases. These results indicated that interactions between C[O.sub.2] and [O.sub.3] on plant physiology can alter N acquisition processes, with impacts on peanut productivity likely dependent in part on these changes.

Many adverse effects of tropospheric [O.sub.3] on [C.sub.3] crop plants are ameliorated by elevated concentrations of atmospheric C[O.sub.2], but the extent of the interaction can vary, depending on the species, gas concentrations, and other experimental conditions. A 2-yr open-top field chamber experiment was conducted to examine this interaction in peanut (Arachis hypogaea L.) by testing the effects of [O.sub.3] and C[O.sub.2] mixtures on yield and seed quality. Treatments were ambient C[O.sub.2] (375 [micro]mol [mol.sup.-1]) and C[O.sub.2] additions of approximately 173 and 355 [micro]mol [mol.sup.-1] in combination with charcoal-filtered (CF) air (22 nmol [O.sub.3] [mol.sup.-1]), nonfiltered (NF) air (46 nmol [O.sub.3] [mol.sup.-1]), and NF air plus [O.sub.3] (75 nmol [O.sub.3] [mol.sup.-1]). At ambient C[O.sub.2], pod number was suppressed 16% in NF air and 44% in elevated [O.sub.3]. Pod and seed mass were not significantly affected in NF air but were lowered 33 to 37% in elevated [O.sub.3]. Elevated C[O.sub.2] increased yield parameters 7 to 17% for plants grown in CF air and restored yield in NF air and elevated [O.sub.3] treatments to control or higher levels. Gas treatment effects on peanut market grade characteristics were small. No treatment effects were observed on the protein and oil contents of seeds, but there were changes in fatty acid composition. Overall results indicate that increasing concentrations of tropospheric [O.sub.3] will suppress yield of [O.sub.3]-sensitive peanut cultivars, while elevated C[O.sub.2] will moderate this response. Elevated [O.sub.3] and C[O.sub.2] are not expected to have major effects on peanut seed composition and quality.

The effects of elevated C[O.sub.2] and ozone ([O.sub.3]) on net photosynthetic rate (A) and growth are generally antagonistic although plant responses are highly dependent on crop sensitivity to the individual gases and their concentrations. In this experiment, we evaluated the effects of various C[O.sub.2] and [O.sub.3] mixtures on leaf gas-exchange, harvest biomass, and leaf chemistry in peanut (Arachis hypogaea L.), an [O.sub.3]-sensitive species, using open-top field chambers. Treatments included ambient C[O.sub.2] (about 375 [micro]mol [mol.sup.-1]) and C[O.sub.2] enrichment of approximately 173 and 355 [micro]mol [mol.sup.-1] in combination with charcoal-filtered air (22 nmol [O.sub.3] [mol.sup.-1]), nonfiltered air (46 nmol [O.sub.3] [mol.sup.-1]), and nonfiltered air plus [O.sub.3] (75 nmol [O.sub.3] [mol.sup.-1]). Twice-ambient C[O.sub.2] in charcoal-filtered air increased A by 23% while decreasing seasonal stomatal conductance ([g.sub.s]) by 42%. Harvest biomass was increased 12 to 15% by elevated C[O.sub.2]. In ambient C[O.sub.2], nonfiltered air and added [O.sub.3] lowered A by 21% and 48%, respectively, while added [O.sub.3] reduced [g.sub.s] by 18%. Biomass was not significantly affected by nonfiltered air, but was 40% lower in the added [O.sub.3] treatment. Elevated C[O.sub.2] generally suppressed inhibitory effects of [O.sub.3] on A and harvest biomass. Leaf starch concentration was increased by elevated C[O.sub.2] and decreased by [O.sub.3]. Treatment effects on foliar N and total phenolic concentrations were minor. Increasing atmospheric C[O.sub.2] concentrations should attenuate detrimental effects of ambient [O.sub.3] and promote growth in peanut but its effectiveness declines with increasing [O.sub.3] concentrations.

Elevated atmospheric CO2 and ozone (O3) may affect productivity of legumes in part by altering symbiotic N2 fixation. To investigate this possibility, measurements of plant biomass, N levels and natural 15N abundance (δ15N) were used to examine the effects of elevated 002 and O3 on N acquisition in field-grown peanut (Arachis hypogaea L.) using open-top chambers. Seasonal 12-h daily average CO2 treatment concentrations were 376, 550, and 730 μmol mol1. Carbon dioxide treatments were applied in reciprocal combinations with seasonal 12-h daily average O3 concentrations of 21, 49, and 79 nmol mol-1. At mid-vegetative growth, elevated 002 significantly reduced leaf N concentrations by up to 44%, but not δ15N values. Elevated O3 did not significantly affect N concentrations or δ15N values. At harvest, plant N concentrations were similar among treatments except for a 14% reduction in the highest-level 002-O3 treatment. Plant N accumulation varied in proportion with treatment effects on biomass production, which was increased with elevated 002 when averaged over the O3 treatments and suppressed by high-level O3 at ambient CO2. Elevated 002 reduced plant δ15N values in low- and mid-level O3 treatments while mid- and high-level O3 increased them at ambient CO2. The changes in ö15N values suggested that N2 fixation activity was stimulated with elevated CO2 and inhibited by elevated O3. Elevated CO2 ameliorated detrimental O3 effects to varying extents depending on the concentrations of the two gases. These results indicated that interactions between CO2 and O3 on plant physiology can alter N acquisition processes, with impacts on peanut productivity likely dependent in part on these changes. [ABSTRACT FROM AUTHOR]