Influence of Ozone Stress on Soybean Response to Carbon Dioxide Enrichment: I. Foliar Properties

opposite responses to elevated O3, and include increased photosynthesis and decreased stomatal conductance Tropospheric O3 can cause foliar injury, decreased growth, and (Jones et al., 1984; Jones et al., 1985; Rogers et al., 1983a; decreased yield, whereas CO2 enrichment generally causes opposite effects. Little is known about plant response to mixtures of O3 and CO2. Sionit et al., 1984), development of larger, thicker, and Open-top field chambers were used to determine if foliar responses of heavier leaves (Thomas and Harvey, 1983), increased soybean [Glycine max (L.) Merr.] to CO2 enrichment are affected branching, increased numbers of nodes (Allen et al., by O3 stress and vice versa. Plants were grown in 14-L pots and 1988; Rogers et al., 1984), changed root/shoot ratios exposed to four CO2 and three O3 concentrations in 12 combinations. (Idso et al., 1988), and increased growth and yield (Allen The CO2 treatments were ambient (366 mL L2) and three treatments et al., 1988; Rogers et al., 1983a, 1984, 1986). with CO2 added for 24 h d2 at approximately 1.3, 1.6, and 2.0 times Carbon dioxide enrichment has been shown to inambient. The O3 treatments were charcoal-filtered air (CF), nonfilcrease or decrease foliar chlorophyll of various species tered air (NF), and NF with O3 added for 12 h d2 (NF1), resulting whether chorophyll content is measured on a weight in seasonal concentrations of approximately 20, 46, and 75 nL L21. per weight (w/w) or weight per leaf area (w/a) basis. Foliar effects of CO2 enrichment were dependent on the amount of stress caused by O3. In the CF treatment, plants were not stressed by On a w/w basis, decreases were reported by Allen et O3, and CO2 enrichment caused chlorosis and decreased chlorophyll. al. (1988), Cave et al. (1980), Delucia et al. (1985), In the NF and NF1 treatments, plants were stressed by O3, and CO2 Heagle et al. (1993), and Rao et al. (1995), whereas enrichment suppressed chlorosis and increased chlorophyll. Ozone increases were reported by Allen et al. (1988) and Vu decreased specific leaf weight, increased foliar N and C, and decreased et al. (1989). On a w/a basis, decreases were reported C/N ratios, whereas CO2 caused opposite responses for these meaby Delucia et al. (1985), Houpis et al. (1988), Wulff and sures. Ozone increased foliar S and B but did not affect P or K Strain (1981), and Wullschleger et al. (1992), whereas concentrations. Conversely, CO2 enrichment suppressed foliar S, B, increases were reported by Chen and Sung (1990) and P, and K concentrations. These interactions between O3 and CO2 Pinter et al. (1994). Several studies showed no effects emphasize a need to consider the amount of plant stress caused by on a w/a basis (Cave et al., 1980; Havelka et al., 1984a,b; O3 in studies to measure effects of CO2 enrichment. Reeves et al., 1994). Decreased foliar chlorophyll accompanied by visible chlorosis is a common response of soybean to O3 stress T O3 concentrations have increased rap(Brennan et al., 1987; Miller et al., 1991; Reich et al., idly over the past 50 yr (Altshuller, 1987). Concen1986). Effects of CO2 enrichment on soybean chlorotrations in many areas of the USA are now approxiphyll have been variable, however. For the cultivar mately twice as high as would exist without anthroBragg, the response (w/w) at 58 d after planting (DAP) pogenic influence (Heck et al., 1994b). Ambient concenwas curvilinear, with highest chlorophyll content at intrations of O3 in many areas can alter permeability of termediate CO2 levels and less chlorophyll at 800 mL plant cell membranes, disrupt metabolism (Heath, 1988, L21 than at 330 mL L21 (Allen et al., 1988). For Stonewall 1996), decrease foliar chlorophyll and photosynthesis, soybean, foliar chlorophyll (w/a) was not affected by change photosynthate allocation, and suppress growth 705 mL L21 CO2 as measured at the full bloom-early and yield (Heagle, 1989; Miller, 1988). Dose-response pod set stage (Reeves et al., 1994). Carbon dioxide enmodels, coupled with economic analyses, indicate that richment caused foliar chlorosis of Bragg soybean, but O3 effects on major agronomic crops cost the U.S. econchlorophyll per se was not measured (Rogers et al., omy approximately $3 billion annually (Adams et al., 1986). 1987; Heck et al., 1984a,b; Lesser et al., 1990). A recent review indicated that relative enhancing efTropospheric CO2 concentrations have increased fects of CO2 are greatest when resources limit growth, from an annual mean of 315 mL L21 in 1958 to 350 mL or when plants are grown in suboptimum environments, L21 in 1988, and further increases are expected (Allen, including those contaminated by air pollutants such as 1990). Plant responses to CO2 enrichment are generally O3 (Idso and Idso, 1994). An early report showed that CO2 at 500 mL L21 above ambient (approximately 850 A.S. Heagle, USDA-ARS, Air Quality Plant Growth and DevelopmL L21) partially protected tobacco (Nicotiana tabacum ment Research Unit, 1509 Varsity Drive, Raleigh, NC 27606, and Dep. of Plant Pathology, North Carolina State Univ.; J.E. Miller, L.), but not pinto bean (Phaseolus vulgaris L.), from USDA-ARS, Air Quality Plant Growth and Development Research foliar injury caused by exposure to O3 (Heck and DunUnit, 1509 Varsity Drive, Raleigh, NC 27606, and Dep. of Crop Scining, 1967). Recent reports of experiments with concurence, North Carolina State Univ.; and F.L. Booker, Air Quality Rerent exposures to CO2 and O3 showed that growth stimusearch Unit, 1509 Varsity Drive, Raleigh, NC 27606, and Dep. of Crop Science, North Carolina State Univ. Cooperative investigations of the USDA-ARS Air Quality Research Unit and the North Carolina Abbreviations: DAP, days after planting; CF, open-top field chamber State University. Funded in part by the North Carolina Agricultural receiving charcoal-filtered air; NF, open-top field chamber receiving Research Service. Received 16 Dec. 1996. *Corresponding author nonfiltered air; NF1, open-top field chamber receiving nonfiltered (E-mail: [email protected]). air with O3 added for 12 h d21; w/a, measured on a weight per area basis; w/w, measured on a weight per weight basis. Published in Crop Sci. 38:113–121 (1998).