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Elevated atmospheric CO2and ozone (O3) may affect productivity of legumes in part by altering symbiotic N2fixation. To investigate this possibility, measurements of plant biomass, N levels and natural 15N abundance (δ15N) were used to examine the effects of elevated CO2and O3on N acquisition in field‐grown peanut (Arachis hypogaeaL.) using open‐top chambers. Seasonal 12‐h daily average CO2treatment concentrations were 376, 550, and 730 μmol mol−1Carbon dioxide treatments were applied in reciprocal combinations with seasonal 12‐h daily average O3concentrations of 21, 49, and 79 nmol mol−1At mid‐vegetative growth, elevated CO2significantly reduced leaf N concentrations by up to 44%, but not δ15N values. Elevated O3did 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 CO2–O3treatment. Plant N accumulation varied in proportion with treatment effects on biomass production, which was increased with elevated CO2when averaged over the O3treatments and suppressed by high‐level O3at ambient CO2Elevated CO2reduced plant δ15N values in low‐ and mid‐level O3treatments while mid‐ and high‐level O3increased them at ambient CO2The changes in δ15N values suggested that N2fixation activity was stimulated with elevated CO2and inhibited by elevated O3Elevated CO2ameliorated detrimental O3effects to varying extents depending on the concentrations of the two gases. These results indicated that interactions between CO2and O3on plant physiology can alter N acquisition processes, with impacts on peanut productivity likely dependent in part on these changes.

Prior work showed that CO2enhancement ratios (ER) were similar for plants grown in open‐top chambers (OTCs) whether grown in the ground or in insulated containers aboveground. Per plant comparisons were suspect since the ground‐grown plants were cultivated in rows at normal densities making it difficult to separate the effects of plant competition from the variables of interest. Soybean [Glycine max(L.) Merr. cv. Essex] was grown in the ground and in aboveground containers in OTCs in ambient and elevated CO2at equal planting densities. The hypothesis was that at equal densities, container‐ and ground‐grown plants would exhibit both equivalent ERs and equivalent per plant yields. Although the only differences in net photosynthetic rate (An:μmol m−2s−1) and conductance to water vapor (gs:mol m−2s−1) were due to CO2and container‐ and ground‐grown plants had similar ERs (mean = 20%), per plant yields were still less in the container‐grown plants at both levels of CO2(mean = −17%). Reproductive measures, except mass per seed, as well as total stem biomass were significantly reduced in the containers. High CO2increased seed oil concentration and the level of fatty acid saturation. The only observed environmental difference was higher daytime root zone temperatures in containers (2–6°C). The robust ERs suggest that neither above‐ nor below‐ground resource limitations was the cause of the yield discrepancies.

Many adverse effects of tropospheric O3on C3crop plants are ameliorated by elevated concentrations of atmospheric CO2, 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 hypogaeaL.) by testing the effects of O3and CO2mixtures on yield and seed quality. Treatments were ambient CO2(375 μmol mol−1) and CO2additions of approximately 173 and 355 μmol mol−1in combination with charcoal‐filtered (CF) air (22 nmol O3mol−1), nonfiltered (NF) air (46 nmol O3mol−1), and NF air plus O3(75 nmol O3mol−1). At ambient CO2, pod number was suppressed 16% in NF air and 44% in elevated O3Pod and seed mass were not significantly affected in NF air but were lowered 33 to 37% in elevated O3Elevated CO2increased yield parameters 7 to 17% for plants grown in CF air and restored yield in NF air and elevated O3treatments 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 O3will suppress yield of O3–sensitive peanut cultivars, while elevated CO2will moderate this response. Elevated O3and CO2are not expected to have major effects on peanut seed composition and quality.

The effects of elevated CO2and ozone (O3) 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 CO2and O3mixtures on leaf gas‐exchange, harvest biomass, and leaf chemistry in peanut (Arachis hypogaeaL.), an O3–sensitive species, using open‐top field chambers. Treatments included ambient CO2(about 375 μmol mol−1) and CO2enrichment of approximately 173 and 355 μmol mol−1in combination with charcoal‐filtered air (22 nmol O3mol−1), nonfiltered air (46 nmol O3mol−1), and nonfiltered air plus O3(75 nmol O3mol−1). Twice‐ambient CO2in charcoal‐filtered air increased Aby 23% while decreasing seasonal stomatal conductance (gs) by 42%. Harvest biomass was increased 12 to 15% by elevated CO2In ambient CO2, nonfiltered air and added O3lowered Aby 21% and 48%, respectively, while added O3reduced gsby 18%. Biomass was not significantly affected by nonfiltered air, but was 40% lower in the added O3treatment. Elevated CO2generally suppressed inhibitory effects of O3on Aand harvest biomass. Leaf starch concentration was increased by elevated CO2and decreased by O3Treatment effects on foliar N and total phenolic concentrations were minor. Increasing atmospheric CO2concentrations should attenuate detrimental effects of ambient O3and promote growth in peanut but its effectiveness declines with increasing O3concentrations.

In studies of CO2–enrichment effects on plants, the applicability of results derived from experiments using container‐grown plants for predictions of future crop performance in a CO2–enriched atmosphere has been questioned. Concerns also have been expressed about plant growth studies with the air pollutant O3in pot‐grown plants. Further, since elevated CO2and O3co‐occur, studies are required with the combination of gases. In this 2‐yr experiment, soybean [Glycine max(L.) Merr.] plants grown in large pots (15 and 21 L) and in the ground were exposed to mixtures of CO2and O3in open‐top chambers. The CO2treatments were ambient and CO2enrichment of approximately 337 μmol mol−1added 24 h d−1Ozone treatments were charcoal‐filtered (CF) air (23 nmol mol−1) and approximately 1.5 times ambient O3levels (71 nmol mol−1) given 12 h d−1Relative effects of elevated CO2and O3on aboveground biomass and seed yield were quite similar for plants grown in pots compared with plants grown in the ground. Elevated CO2increased total seed mass and O3suppressed it to similar magnitudes in both rooting environments. Elevated CO2also reduced the toxic effects of O3Net photosynthesis (A) was similar while stomatal conductance (gs) was higher in pot‐grown compared with ground‐grown plants, possibly due to better soil moisture status. The results indicated that planting density and rooting environment affected plant morphology, but relative responses of seed yield to elevated CO2and O3were not fundamentally different between soybean plants grown in large pots and in the ground in open‐top chambers.

Erosion of the stratospheric ozone layer by anthropogenic emission of halogenated compounds may lead to increased UV-B radiation at ground level. In 1990 soybeans (Glycine max(L.) Merr. cv. Essex) were grown in open-top field chambers with 3 levels of UV-B replicated 3 times. UV-B treatments corresponded to changes in the total column ozone thickness of15% (low), -20% (medium) and -35% (high). Leaves were sampled during four intervals of the growing season and subjected to pressure-volume (P-V) analysis to determine symplastic volume (Vo); maximum turgor pressure(pMAX), symplastic solute content (Ns), tissue elasticity coefficient (z), and the potential at turgor loss (TLP). Leaf conductance, average specific leaf weight (SLW), and area per leaf were measured several times during the season. During the second sampling period at 58 days after planting (DAP), Vowas significantly decreased at the highest UV-B level but was not affected by UV-B treatments again. Also during the second sampling period the elasticity coefficient, z, in the medium and high UV-B treatments was significantly less than in the low UV-B treatment. In the fourth sampling period (100 DAP), z again was significantly affected by UV-B treatment; in this case, however, while z was lower in the high than the medium UV-B treatment, it did not diffet from the low UV-B treatment. The relative symplastic volume at the turgor loss potential (RSVTLP) was the only other parameter to show a significant UV-B effect, but only at the highest treatment level and only near the end of the season. Generally, the significant changes in all P-V parameters with plant age were much larger than any treatment effects. There were no significant differences in leaf conductance, leaf area or SLW, indicating that, although the leaf P-V relationships were noticeably altered, these sporadic treatment effects had little real influence on leaf water balance.

It has been suggested that increases in ground-level UV-B, as a result of stratospheric ozone depletion, may have major deleterious effects on crop photosynthesis and productivity. The direct consequences of such effects have been projected by some as a world-wide decrease in crop yields of 20–25%. Further losses, or unrealized gains, have also been suggested as a result of increased UV-B counteracting the beneficial effects of elevated atmospheric CO2. Deleterious UV-B effects may be largely partitioned between damage to the plant genome and damage to the photosynthetic machinery. Direct damage to DNA is a common result of absorption of high energy UV-B photons. However, most plants possess repair mechanisms adequate to deal with the levels of damage expected from projected increases in ground-level UV-B. In addition, most plants have the ability to increase production of UV-absorbing compounds in their leaves as a result of exposure to UV-B, UV-A and visible radiation. These compounds contribute substantially to reducing UV-B damage in situ. It has also been shown that in some plants, under the proper conditions, almost every facet of the photosynthetic machinery can be damaged directly by very high UV-B exposures. However, electron transport, mediated by Photosystem II (PS II) appears to be the most sensitive part of the system. Various laboratories have reported damage to virtually all parts of the PS II complex from the Mn binding site to the plastoquinone acceptor sites on the opposite surface of the thylakoid membrane. However, a critical review of the literature with emphasis on exposure protocols and characterization of the radiation environment, revealed that most growth chamber and greenhouse experiments and very many field experiments have been conducted at unrealistic or indeterminate UV-B exposure levels, especially with regard to the spectral balance of their normal radiation environment. Thus, these experiments have led directly to large overestimates of the potential for damage to crop photosynthesis and yield within the context of 100 year projections for stratospheric ozone depletion. Indeed, given the massive UV-B exposures necessary to produce many of these effects, we suggest it is unlikely that they would occur in a natural setting and urge reconsideration of the purported impacts of projected increases of UV-B on crop productivity.

Ferulic acid (FA) is commonly found in soils and is considered an allelochemical. Studies have suggested that FA and other phenolic acids decrease plant growth in part by decreasing the absorption of mineral nutrients and water. However, no studies have examined these parameters in a single experimental system to investigate how FA affected both ion uptake and plant-water relations in whole plants. Using intact cucumber (Cucumis sativus L. cv. Early Green Cluster) seedlings, we examined short-term effects of FA on ion uptake kinetics, transport promoters and inhibitors, and water relations as indicated by a pressure-volume analysis. We found that after 3 h of treatment, 200 μM FA inhibited net ion uptake, particularly NO⊟3, and promoted net K+ efflux from seedling roots. The addition of fusicoccin, a K+ transport promoter, counteracted the inhibitory effect of FA on net K+ uptake. Concurrent treatment of seedlings with FA and tetraethylammonium, a channel-blocking salt, reduced average K+ efflux by 66%. Treatment of seedlings with FA also decreased leaf water potential (ψ1 and turgor pressure (PT). However, decreased ψ1 and PT were not caused by changes in the osmotic properties of the symplast or stomatal conductance. A decrease in water absorption is a likely explanation for the loss of PT observed. The results of our experiments indicate that both ion uptake and plant-water relations can be adversely affected by FA.