Photosynthesis and Water Relations of Big Bluestem (C 4 ) and Kentucky Bluegrass (C 3 ) under High Concentration Carbon Dioxide

As the carbon dioxide (CO2) concentration in the atmosphere increases, comparing how C3 and C4 plants will respond is important. The objective of this study was to determine the photosynthetic rate, intercellular CO2 concentration, transpiration rate, stomatal resistance, leaf temperature, water potential, and water requirement of a C3 grass (Kentucky bluegrass, Poa pratensis L.) and a C4 grass (big bluestem, Andropogon gerardii Vitman) growing in the fall in a tallgrass prairie in Kansas under two levels of CO2 (ambient and two-times ambient). Elevated CO2 increased the photosynthetic rate of Kentucky bluegrass by 141% but did not affect the photosynthetic rate of big bluestem. Intercellular CO2 concentrations of both grasses were increased by about the same amount, which was about half the increase in the atmospheric CO2 concentration. Doubled CO2 reduced the transpiration rates and increased stomatal resistances of both grasses, but big bluestem was affected more than Kentucky bluegrass. The twice-ambient level of CO2 increased (between 0.2 and 0.3 MPa) the water potential of both grasses. Doubled CO2 decreased the water requirements of big bluestem and Kentucky bluegrass by 41.6% and 158%, respectively. The increasing concentration of CO2 in the atmosphere is well known. It has risen from 315 pl/liter in 1958 to the present 350 pl/liter (Chaudhuri et al., 1990; Taylor and MacCracken, 1990). Several studies have documented the effect of elevated levels of CO2 on crop growth (Kimball, 1983). But little work has utilized native plants, and most of it has been done in con' Contribution No. 92-244-J from the Kansas Agric. Exp. Stn. , Manhattan, Kansas. This research was supported by U.S. Department of Energy Grant No. DE-FG02-84ER60253. 2 Corresponding author. 3 Current address: Texas AM Nie et al., 1991). During most of the growing season (May through September), big bluestem dominates, because temperatures are too hot for the growth of Kentucky bluegrass. However, in spring and fall, temperatures are usually cool enough for both grasses to grow. Apparently, the only work that has been done to determine the effect of elevated levels of CO2 on C3 and C4 plants under natural conditions involved marsh plants in the Chesapeake Bay in Maryland (Curtis et al., 1989a, 1989b, 1990; Ziska et al., 1990). Except for the study by Kirkham et al. (1991) in which the photosynthetic rates of big bluestem under wet and dry conditions are given, no one has reported the response of plants in the tallgrass prairie. Therefore, the objective of this study was to determine the effect of an elevated concentration of CO2 on the photosynthetic rate, intercellular CO2 concentration, transpiration rate, stomatal resistance, and leaf temperature of big bluestem and Kentucky bluegrass in the tallgrass prairie of Kansas. MATERIALS AND METHODS The experiment was conducted in 1989 on a tallgrass prairie in Manhattan, Kansas. The soil was a silty clay loam (fine, mixed, mesic Pachic Argiustoll). Sixteen circular, closed-top chambers (1.5 m in diameter; 1.8 m tall) in four rows oriented in the north-south direction were placed on the experimental area, which sloped upward (2%) to the east. The experiment was a randomized, complete block with two concentrations of CO2 (high-CO2 level or twice ambient and low or ambient), two watering regimes (high-water level or field capacity and low-water level or half-field capacity), and four replications (16 chambers). Carbon-dioxide injection into the chambers began on May 1989 or Day of Year (DOY) 139. Air was exchanged in the chambers every 20 sec. The centers of the inlet and exhaust holes in each chamber were 0.35 and 1.20 m above ground, respectively. Carbon dioxide was delivered from a tank to the chambers through tubing laid on the ground. Nie et al. (1991) explain in detail how the CO2 levels were regulated. The CO2 concentration in the air of each chamber was monitored by using a computerized system every 30 to 40 This content downloaded from 157.55. 39.54 on Fri, 01 Jul 2016 04:09:37 UTC All use subject to http://about.jstor. org/terms VOLUME 95, NUMBERS 1-2 141 minutes continuously throughout the study. Gas analyzers were calibrated every five to six hours. For enriched and ambient chambers, the chamberto-chamber variation at any one time was about 9 and 5 Al/liter, respectively. During the study, the average CO2 concentrations and standard deviations in the highand low-CO2 chambers were 658 ? 81 and 337 ? 32 Al/liter, respectively. We could not take simultaneous measurements of big bluestem and Kentucky bluegrass in the spring, because, by the time the experimental equipment was ready, the Kentucky bluegrass leaves were too small to be put in the chamber of the portable photosynthetic system. We could not study another C3 grass, as Kentucky bluegrass was the only C3 grass that occurred on the prairie and in each chamber. Kentucky bluegrass leaves began to grow in the fall and were big enough then for the chamber of the instrument that measured photosynthesis. This paper reports the measurements of both grasses made in the fall between 20 September (DOY 263) and 23 October (DOY 296). The measurements of big bluestem made during the summer months have been reported (Kirkham et al., 1991). The methods used for big bluestem in the summer were similar to those used for both grasses in the fall. Until heavy rains fell in late August and early September, the 1989 season was one of the driest on record. The departures from normal rainfall in May, June, July, and August were -58 mm, -49 mm, -63 mm, and +55 mm, respectively (NOAA, 1989). No rain fell into the covered chambers. Because the site was on a slope, there was lateral movement of water into the soil under the chambers when the rains came, and soil water content in the highand low-water chambers was similar after DOY 243 (Kirkham et al., 1991). Measurements of photosynthesis, transpiration, stomatal resistance, temperature, and intercellular CO2 concentration (also called internal CO2 concentration) of leaves of big bluestem and Kentucky bluegrass were made usually two times per week between DOY 263 and 296 with a portable leaf photosynthesis system (Model LI-6200, Li-Cor, Inc., Lincoln, Nebraska). (Measurements of transpiration on DOY 270 are not reported because the software of the LI-6200, used to calculate the transpiration rate, was not working.) Temperature of the air was also measured with the LI-6200 and subtracted from the temperature of the leaf to determine how much cooler or warmer than air the leaves were. At least two leaves of each species in eight chambers (two replications) were measured on each date at midday (between about 1100 and 1500 h). Two readings were taken on each leaf. The measured leaf was usually the second leaf from the top (recently matured leaf). If weather permitted, two leaves of each species in all 16 chambers (four replications) were measured. Measurements were not made during cloudy conditions. Because the soil-water level in the highand low-water plots was the same during the measurement period, it had no effect on the This content downloaded from 157.55. 39.54 on Fri, 01 Jul 2016 04:09:37 UTC All use subject to http://about.jstor. org/terms 142 TRANSACTIONS OF THE KANSAS ACADEMY OF SCIENCE 24 BIG BLUESTEM, LOW C02 cJ20 BIG BLUESTEM, I 20 HIGH CO2