RESPONSE OF TWO OLD FIELD PERENNIALS TO INTERACTIONS OF CO2 ENRICHMENT AND DROUGHT STRESS

Aster pilosus Willd. (aster, C3) and Andropogon virginicus L. (broomsedge, C4) were grown in growth chambers at 26/20 C day/night temperatures with a PPFD of 1,000 ,umol s-' m-2. Water was withheld for a 2-wk drought period under three CO2 concentrations (approximately 380, 500, and 650 Atl 1-1). There were significant effects of CO2 enrichment on aster so that drought stress did not occur in plants grown with CO2 enrichment. Non-watered plants with CO2 enrichment had greater leaf water potentials, greater photosynthetic rates, and greater total dry wt than non-watered plants grown at 380 ,l 1-1 C02. The response of broomsedge to drought was the same in all CO2 treatments and there was no significant interaction of CO2 enrichment and drought. The decreased severity of drought stress and the increased growth of aster with CO2 enrichment may increase its competitive ability during droughts, allowing it to persist for longer periods during succession in abandoned fields. IN OLD-FIELD succession of the North Carolina piedmont, Aster pilosus Willd. (aster), a C3 perennial, is present in low numbers during the first year and becomes dominant during the second summer (Oosting, 1942). Within three or four yr, Andropogon virginicus L. (broomsedge), a C4 perennial grass (Downton, 1975), replaces aster until 10 to 15 yr after site abandonment when Pinus taeda (loblolly pine) becomes dominant. Keever (1950) concluded that broomsedge replaces aster as an old field dominant because it is more drought tolerant than aster. Aster appears to persist for longer periods in wetter than in drier fields. When aster and broomsedge roots share the soil volume, there is a lower survival of aster plants as compared to aster grown without broomsedge (Keever, 1950). Aster presumably dominates abandoned fields only until broomsedge, the better competitor during droughts, becomes established. Greenhouse studies have shown that growth and photosynthesis of plants with the C3 photosynthetic pathway tend to increase with an increase in atmospheric CO2 concentrations (Wong, 1979; Patterson and Flint, 1980; Rogers, Thomas and Bingham, 1983; but see Morison and Gifford, 1984a), whereas C4 species I Received for publication 9 September 1985; revision accepted 22 April 1986. This research was funded by NSF Grants BSR 82-1 5533 to B. R. Strain, and BSR 83-14925 to the Duke University Phytotron. We gratefully acknowledge Drs. W. Schlesinger, N. Sionit, and T. Sasek for their helpful reviews of the manuscript. vary in their responses to CO2 enrichment (Wong, 1979; Patterson and Flint, 1980; Sionit and Patterson, 1985; Potvin and Strain, 1985). Drought stress may be avoided or lessened by elevated CO2 in two ways. With decreased conductance, less water is used, delaying the onset of stress responses. Thus plant water potential remains high when plants are grown at elevated CO2 concentrations (Paez, Hellmers and Strain, 1984; Rogers, Thomas and Bingham, 1984; Sionit and Patterson, 1985). This allows continued growth during droughts, increasing overall biomass production (Sionit et al., 198 1; Morison and Gifford, 1984a). Also, under stess conditions, the higher concentrations of CO2 in the atmosphere can allow continuation of greater photosyntehtic rates than at ambient CO2 levels. The magnitude of the response to increased CO2 concentrations varies among species (Kimball, 1983). Thus plant interactions within communities may be altered as CO2 concentrations increase as a result of fossil fuel combustion and other sources (Clark, 1982). The CO2 enrichment can alter competitive interactions between C3 and C4 plants grown in controlled environments (Carter and Peterson, 1983; Bazzaz and Carlson, 1984; Patterson, Flint and Beyers, 1984; Zangerl and Bazzaz, 1984). Therefore, changes in the distribution of species may occur in areas where C3 and C4 plants grow in competition due to enhanced growth of C3 species under CO2 enrichment. Changes in competitive interactions could occur in abandoned fields and roadsides of the southeast where aster and broomsedge grow This content downloaded from 207.46.13.105 on Wed, 25 May 2016 06:25:28 UTC All use subject to http://about.jstor.org/terms October, 1986] WRAY AND STRAIN-EFFECTS OF C02 ENRICHMENT AND DROUGHT 1487 together. This study examines the responses of these two species to decreased water availability and CO2 enrichment. It was hypothesized that the C3 species, aster, would show stronger physiological responses to CO2 enrichment than broomsedge, a C4 species, and that this would result in a significant interaction of CO2 enrichment and drought in aster but not in broomsedge. As seen in previous competition studies of C3 and C4 species (Bazzaz and Carlson, 1984; Patterson et al., 1984), an enhancement of aster's growth may increase its competitive ability during periods of drought, allowing it to persist for longer periods during old-field succession. MATERIALS AND METHODS-Seeds of Aster pilosus and Andropogon virginicus were collected from a field in the Blackwood Division of the Duke Forest, Orange County, NC. Broomsedge seeds were stratified at 5 C for 10 wk before germination. Seeds were germinated in a 1:1:1 by volume mixture of Vermiculite, gravel and Turface (Jaeger, Hellmers and Teare, 1981) in 500 cm3 pots. Turface is a tradename for arcillite prepared by International Minerals Corporation. Eleven days after emergence, plants were thinned to one per pot. As young seedlings can receive nutrient bums if subjected to a nutrient solution too soon, pots were watered 3 times a day with deionized water until establishment 11 days after emergence. Subsequently, plants were watered with a l/, 5th strength modified Hoagland's solution (Downs and Hellmers, 1975) in the morning and with deionized water in the afternoon, flushing the pots of accumulated salts. This nutrient level was used to control the growth of these species and simulate field conditions under which the plants grow. Plants were grown in controlled-environment chambers in the Duke University Phytotron (Kramer, Hellmers and Downs, 1970). Carts containing the pots were rotated daily within the chamber and every week between chambers to minimize chamber effects. The temperature regime was 26/20 C with a 14-hr photoand thermo-period. Vapor pressure deficit of the air was maintained at 1 kPa during the day. Light was provided by equal numbers of metal halide and high intensity discharge sodium lamps with photosynthetic photon flux density (PPFD) at the substrate surface of 1,000 ? 50 ,umol sm2. The CO2 concentrations in the three chambers were set at 350, 500, and 650 ,l 1-1 CO2. These levels correspond to present ambient levels, a midpoint, and an approximate doubling of present levels, which could be reached in the latter half of the next century (Edmonds et al., 1984). However, plant photosynthetic rates were not sufficient to reduce chamber levels below 380 Al 1-1 C02, and no scrubbing system was used, so actual chamber levels were maintained at 378 ? 25, 498 ? 8 or 648 ? 15 Al l-l by a computer-controlled injection system (Hellmers and Giles, 1979). The CO2 used was produced by a process that does not produce ethylene, and no abnormal ethylene concentrations were measured in the growth chambers (cf. Morison and Gifford, 1984b). Irrigation was withheld 35 days after emergence on a portion of the plants, while the other portion was watered with deionized water twice a day. Predawn leaf water potentials were measured on the youngest fully expanded leaf with a pressure chamber (Scholander et al., 1965) on five plants per treatment every other day during the 14-day drought period. These plants were then discarded and not used for harvest data. The original watering regime was resumed 49 days after emergence on all plants and continued for 7 days. Photosynthesis was measured on fully expanded leaves of three plants in each treatment in an open system by infrared gas analysis as described by Potvin and Strain (1985). Stomatal conductance was calculated based on simultaneous measurements of transpiration. The measurements of photosynthesis and stomatal conductance were made on plants grown at approximately 380, 500, and 650 l 11 C02 on days 10, 11, and 12 of the drought stress period, respectively. Well-watered plants were watered 30 min before placement in the cuvette whereas non-watered plants were taken directly from the chambers without watering. Light, vapor pressure deficit, temperature and CO2 concentration in the cuvette duplicated growth coiiditions of the plant being measured. Ten plants per treatment were harvested 1 1, 35, 49, and 56 days after emergence. These harvests corresponded to the start of the nutrient application, the start of the drought period, the end of the drought period, and the end of a 7-day recovery period, respectively. Roots were washed and the mica gently crushed and pulled from the roots. Dry wt of aboveand below-ground parts were determined separately after drying to constant wt at 65 C. All above-ground biomass was leaf tissue; no stems were formed in either species during this experiment. Leaf area was measured with a LiCor model 3,1 00 leaf area m. The responses of the species to CO2 to enrichment, drought stress, and their interaction were tested with one-way and two-way analysis This content downloaded from 207.46.13.105 on Wed, 25 May 2016 06:25:28 UTC All use subject to http://about.jstor.org/terms 1488 AMERICAN JOURNAL OF BOTANY [Vol. 73