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Drought and heat stress can limit the growth of cool-season grass species, whereas doubling ambient CO2 has been shown to promote plant growth. The objectives of this study were to examine differential responses of shoot and root growth as well as photosynthesis and respiration to doubling ambient CO2 during drought or heat stress alone or the two stresses combined and to determine the relative effectiveness of doubling ambient CO2 in mitigating negative effects of drought or heat stress alone and in combination in a cool-season perennial grass species. Kentucky bluegrass (Poa pratensis cv. Baron) plants were exposed to ambient CO2 (400 mL L) or doubling ambient CO2 (800 mL L) concentrations while subjected to the following stress treatments in growth chambers: drought stress by withholding irrigation, heat stress (35 8C), or the combined two stresses for 28 days. Doubling ambient CO2 increased root and shoot growth as well as root/shoot ratio under all treatments. Doubling ambient CO2 enhanced leaf net photosynthetic rate (Pn) to a greater extent under drought or heat alone, whereas it reduced respiration rate (R), to a larger degree under heat and the combined stress, leading to a greater ratio of Pn/R. Doubling ambient CO2 mitigated adverse physiological effects of drought or heat stress alone, whereas fewer effects were observed under the combined drought and heat stress. The positive effects of doubling ambient CO2 were associated with the development of roots biomass and the maintenance of a positive carbon balance under either stress alone or the combined drought and heat stress. Drought and heat stress are two major environmental constraints limiting the growth of cool-season plant species and simultaneous drought and heat can be more detrimental than either stress alone (Albert et al., 2011; Jiang and Huang, 2001a; Mittler, 2006). Major physiological damages under both drought and heat stress include the induction of stomatal closure; the decline in leaf photochemical efficiency, net photosynthetic rate, and leaf relative water content; the increase in electrolyte leakage as well as the induction of oxidative damages (Ebdon and Kopp, 2004; Jiang and Huang, 2001b; Mittler, 2006; Wang and Huang, 2004; Yu et al., 2012; Zhang et al., 2003). In addition to drought and heat stress, plants are exposed to increasing concentrations of atmospheric CO2, which is predicted to double the current concentration by 2050 (Solomon et al., 2007). Previous studies examined the effects of elevated CO2 on plant responses to drought or heat stress and concluded that elevated CO2 may mitigate either drought or heat stress alone as manifested by increased shoot and root growth, improved water use efficiency as well as promoting photosynthesis and decreased respiration and various other physiological activities (Alonso et al., 2009; Drake et al., 1997; Fleisher et al., 2008; Hamilton et al., 2008; Kirkham, 2011; Long and Drake, 1992; Prasad et al., 2009; Robredo et al., 2007; Wand et al., 1999). However, limited studies have examined the relative effectiveness of the mitigation of stress damages for drought or heat stress, and particularly little is known of the effects of doubling ambient CO2 under combined drought and heat stress conditions relative to either stress alone. Shoots and roots may respond differently to elevated CO2 under drought or heat stress alone or the combined stress. Under ambient CO2 conditions, roots exhibited better dehydration tolerance than shoots under drought stress (Erice et al., 2006), whereas roots were more sensitive to heat than shoots (Huang and Liu 2003; Lyons et al., 2007; Pote et al., 2006; Xu and Huang, 2000). Elevated CO2 has been found to increase root-toshoot ratio in various plant species, mainly under non-stress conditions, but few studies examined the relative sensitivity of shoots and roots to elevated CO2 under different abiotic stresses (Hunt et al., 1996; Luo et al., 2006; Nie et al., 2013; Wall et al., 2011). Carbon balance involving both photosynthesis and respiration is a determinant factor controlling shoot and root growth (Campbell et al., 2007). Drought and heat stress, applied singly or combined, inhibit photosynthesis, whereas heat stress alone increases respiration, both of which can lead to negative carbon gain or carbon depletion restricting plant growth (Larcher, 2003; Ohashi et al., 2006; Qaderi et al., 2006). The promotive effects of elevated CO2 on plant growth under Received for publication 25 Feb. 2014. Accepted for publication 4 Apr. 2014. We thank the Chinese Scholarship Council for providing stipend to Y. Song and thanks go to Center for Turfgrass Science at Rutgers University for funding support of the research project. Thanks also go to Patrick Burgess, David Jespersen, and Jillian Keough for critical review of the manuscript. Corresponding author. E-mail: huang@aesop.rutgers.edu. 364 J. AMER. SOC. HORT. SCI. 139(4):364–373. 2014. non-stress conditions have been related to the increase in carbon assimilation through promoting photosynthesis and the decline in carbon loss through inhibiting respiration (Ainsworth et al., 2002; Drake et al., 1997; Long et al., 2004; Reddy et al., 2010). Limited information is available on the differential responses of photosynthesis and respiration to doubling ambient CO2 under drought or heat stress alone and particularly under combined drought and heat stress. Understanding differential responses of shoot and root growth and carbon balance involving photosynthesis and respiration to doubling ambient CO2 under drought or heat alone and simultaneous stress conditions is essential for improving plant productivity in adverse environmental conditions under the scenarios of global climate change and increasing atmospheric CO2 concentration. Kentucky bluegrass is a widely used turfgrass species common to sports fields and home lawns, which exhibits differential responses to drought or heat stress alone or the combined stress of drought and heat (Ebdon and Kopp, 2004; Jiang and Huang, 2000; Su et al., 2007; Wang and Huang, 2004; Zhang et al., 2003). The interactive effects of doubling ambient CO2 with drought or heat stress alone or with combined drought and heat stress have not been well documented in cool-season perennial grass species. The objectives of this study were to 1) examine differential responses of shoot and root growth and photosynthesis and respiration to doubling ambient CO2 under drought or heat stress alone and combined drought and heat stress; and 2) determine the relative effectiveness of doubling ambient CO2 on the mitigation of drought or heat stress alone and the combined stress in a cool-season perennial grass species. Materials and Methods PLANT MATERIAL AND GROWTH. Kentucky bluegrass (cv. Baron) plants were collected from established field plots located at the turfgrass research farm at Rutgers University in New Brunswick, NJ. Plants were propagated in pots (10 cm diameter, 40 cm height) filled with fritted clay and allowed to establish in a greenhouse with average day/night temperatures of 21/18 C and 12 h natural light at 750 mmol m s photosynthetically active radiation (PAR). During 38-d plant establishment, plants were trimmed once per week to maintain a canopy height at 10 cm, irrigated every other day, and fertilized every 3 d with half-strength Hoagland’s solution (Hoagland and Arnon, 1950). Plants were then transferred to growth chambers (Environmental Growth Chamber, Chagrin Falls, OH) for CO2, drought, and temperature treatments. The environmental conditions in the growth chamber were 70% relative humidity, 660 mmol m s PAR, and a 12-h photoperiod. TREATMENTS AND EXPERIMENTAL DESIGN. The experiment consisted of three factors, including drought, heat, and CO2 treatments. The treatments were arranged in a split plot design with CO2 treatment as the main plot and drought and temperature stress as subplots for each treatment. The CO2 treatments included ambient CO2 (400 ± 20 mmol mol) and doubling ambient CO2 (800 ± 20 mmol mol). The concentration of CO2 inside each growth chamber was maintained through an automatically controlled, open-chamber CO2 system connected to a 100% CO2 gas tank (Airgas, Piscataway, NJ) using the design described in Yu et al. (2012). The different CO2 levels were continuously monitored through an infrared gas analyzer (LI-820; LI-COR, Lincoln, NE) and controlled using an automatic system consisting of a programmable logic controller unit, solenoid valves, and a laptop computer with monitoring software accurate to within 20 mmol mol of the target levels (400 and 800 mmol mol). Plants exposed to ambient or doubling ambient CO2 treatments were maintained as a well-watered control or with drought Fig. 1. Soil water content (SWC) in pots with kentucky bluegrass plants under non-stress control (A), drought stress (B), heat stress (C), and the combined drought and heat stress (D + H) (D) during 28 d of treatment. Vertical bars on the top represent the values of least significant difference at P # 0.05 for comparison of CO2 treatment effects at a given day of control, drought, heat, or D + H. Vertical bars on the right side represent the values of least significant difference at P # 0.05 for comparison between days of treatment. J. AMER. SOC. HORT. SCI. 139(4):364–373. 2014. 365 stress (D), heat stress (H), or the combination of drought and heat stress (D + H). Plants in the control treatment were irrigated every 2 d to replace 100% evapotransporative (ET) water loss at 20 C day/17 C night. Plants under D were irrigated every 2 d to replace 50% ET water loss. Plants under H were exposed to 35 C day/32 C night temperature and irrigated same as the control treatment. The D + H treatment was set at 35/32 C and irrigated every 2 d to replace 50% ET. Control plants and D plants were placed in four growth chambers controlled at ambient or doubling ambient CO2 with temperature set at 21/1

journal of the american society for horticultural science

Differential Effectiveness of Doubling Ambient Atmospheric CO2 Concentration Mitigating Adverse Effects of Drought, Heat, and Combined Stress in Kentucky Bluegrass