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Drought Tolerance Associated with Proline and Hormone Metabolism TURF MANAGEMENT HORTSCIENCE 46(7):1027–1032. 2011. the most common naturally occurring cyto- kinins (Strivastava, 2002). Bano et al. (1993) noted that cytokinin concentration in xylem Drought Tolerance Associated with sap declined after water deprivation and in- creased again after re-watering. It has been Proline and Hormone Metabolism documented that the plants with higher cyto- kinins exhibited greater drought tolerance in Two Tall Fescue Cultivars and exogenous cytokinins (such as zeatin riboside) can improve turfgrass tolerance to Da Man, Yong-Xia Bao, and Lie-Bao Han1,2 drought stress (Zhang and Ervin, 2004; Turfgrass Research Institute, Beijing Forestry University, Beijing, China Zhang and Schmidt, 1999, 2000). IAA is 100083 associated with root initiation and growth (Nordstrom et al., 1991; O’Donnell, 1973) Xunzhong Zhang1,2 Recent study has shown that leaf tissue IAA Department of Crop and Soil Environmental Sciences, Virginia Polytechnic content was positively correlated with drought tolerance and exogenous indole-3-butyric Institute and State University, Blacksburg, VA 24041 acid increased endogenous IAA and im- Additional index words. drought tolerance, hormone, physiology, proline, tall fescue proved tall fescue drought tolerance (Zhang et al., 2005, 2009). It was also reported IAA Abstract. Drought stress is a major factor in turfgrass management; however, the content increased as plants adapted to drought underlying mechanisms of turfgrass drought tolerance are not well understood. This stress (Pustovoitova et al., 2004; Sakurai et al., greenhouse study was designed to investigate proline and hormone responses to drought 1985). stress in two tall fescue [Festuca arundinacea (Schreb.)] cultivars differing in drought Tall fescue [Festuca arundinacea tolerance. The two cultivars, Van Gogh (relatively drought-tolerant) and AST7002 (Schreb.)], a cool-season turfgrass, is widely (relatively drought-sensitive), were established and grown under either well-watered used for home lawns, recreational surfaces, (maintaining 90% container capacity) or drought stress (’26% container capacity) and and roadsides in the temperate to semitropi- then re-watered. Drought stress reduced turfgrass quality, relative leaf water content cal regions and experiences frequent summer (LWC), leaf indole-3-acetic acid (IAA) and cytokinin zeatin riboside (ZR) content, and drought stress in many areas (Zhang et al., increased proline and abscisic acid (ABA) content. ‘Van Gogh’ had greater turfgrass 2009). There are many different cultivars that quality rating, LWC, proline, ABA, and ZR content relative to ‘AST7002’ under drought vary widely in drought tolerance. ‘Van Gogh’ stress conditions. At the end of drought stress, leaf proline, ZR, and ABA content were is a cultivar with excellent drought tolerance 32%, 43%, and 50% higher in ‘Van Gogh’ relative to ‘AST7002’, respectively. No and ‘AST 7002’ is a cultivar with relatively cultivar difference was observed under well-watered conditions. The results of this study poor drought tolerance (National Turfgrass suggest that the proline, ABA, and ZR content are associated with drought tolerance. Evaluation Program, 2008). Little study has Selection and use of the cultivars with higher proline, ABA, and ZR content under been reported on mechanisms of drought drought stress may be a practical approach to improve tall fescue drought tolerance. tolerance-associated proline and hormone metabolism in different tall fescue cultivars. Understanding the physiological mechanism Drought is a major limiting factor of also functions as a free radical scavenger and of turfgrass drought tolerance is important for turfgrass culture in many parts of the world. suppresses free radical-mediated damage breeder to develop drought-tolerant cultivars Drought stress suppresses growth and causes during drought stress. Several studies have and turfgrass practitioners to improve turf- a loss of turf quality (Carrow and Duncan, indicated that proline content increases dur- grass quality under drought stress. Our objec- 2003; Zhang et al., 2009). Plants exhibit ing drought stress, and proline accumulation tives were to investigate proline and hormone various physiological and biochemical re- is associated with improvement in drought metabolism of the two tall fescue cultivars sponses to drought stress. Accumulation of tolerance in tall fescue and other plants (Seki (Van Gogh and AST7002) contrasting in osmotic compounds such as proline and et al., 2007; Zhang et al., 2009). drought tolerance and to examine if variation changes in phytohormones are some of the There is increasing evidence showing that between the cultivars differing in drought most common responses of plants to drought plant physiological fitness is largely gov- tolerance is associated with proline and stress (Hare et al., 1997). erned by hormonal balance (Itai, 1999; Zhang hormone metabolism. Proline, an amino acid, is a compatible et al., 2009). Abscisic acid plays an important solute involved in cell osmotic adjustment role in stress signal network and it is postu- Materials and Methods (OA) and protection of cell components lated to mediate stress-induced stomatal clo- during dehydration (Zhang et al., 2009). sure (Xiong et al., 2002; Zhu, 2002). It has Tall fescue culture and drought stress Osmotic adjustment helps to maintain cell been reported that ABA increased in re- treatment. This study was conducted at the turgor, which can allow cell enlargement and sponse to drought stress in tall fescue (Zhang Turfgrass Research Institute at Beijing For- plant growth during water stress; and it can et al., 2009) and other crop plants (Domash estry University, Beijing, China, from May allow stomata to remain at least partially et al., 2006; Lopez-Garbonell et al., 1994; 2009 to Mar. 2010. Two tall fescue cultivars, open and CO2 assimilation to continue at Pustovoitova et al., 2004; Yang et al., 2001). Van Gogh (relatively drought-tolerant) and water potentials that would be otherwise Desikan et al. (2004) pointed out that drought AST7002 (relatively drought-sensitive), inhibitory (Alves and Setter, 2004). Proline stress increases ABA synthesis and redistri- were planted in pots (15 cm diameter, 15 cm bution. The change in ABA initiates an deep) filled with silt loam soil (fine-loamy, intricate network of signaling pathways in mixed mesic Typic Hapludult) mixed with Received for publication 25 Feb. 2011. Accepted guard cells leading to stomatal closure and a small amount of sand (4:1) at a seeding rate for publication 29 Apr. 2011. conservation of water. of 30 g pure live seeds/m2. The grass was This research was supported by the Specific Programs Cytokinins exhibit antisenescence proper- grown in a greenhouse with photosyntheti- in Graduate Science and Technology Innovation of ties (Musgrave, 1994). Cytokinins delay the cally active radiation at 400 mmolÁm–2Ás–1 (at Beijing Forestry University (NO.BLYJ201101), Beijing, China. senescence process probably by maintaining 1400 Hr) and 24/21°C (day/night) and 1Da Man and Yong-Xia Bao contributed equally the integrity of the tonoplast membrane watered by hand three times a week. After to this work. (Thimann, 1987). Zeatin and its sugar de- the grass was grown for 8 weeks, half of the 2To whom reprint requests should be addressed; rivatives (zeatin riboside and zeatin ribotide– pots were subjected to dry down and the e-mail [email protected]; [email protected]. dihydrozeatin and isopentenyl adenine) are remaining pots were irrigated to approximately HORTSCIENCE VOL. 46(7) JULY 2011 1027 maintain container capacity. Nitrogen (20N– nitrogen and homogenized in 5 mL Na-phos- methane fractions were reduced to dryness 8.8P–16.6K) was applied at 5 gÁm–2 before phate buffer (0.05 M, pH 7.0) containing with nitrogen gas. Then samples were dis- planting and every month thereafter. The grass 0.02% sodium diethyldithiocarbamate as an solved in methanol (50 mL) and diluted with was mowed weekly to 9 cm with scissors. antioxidant and the hormones were extracted 0.95 mL 50 mM tris-buffered saline (pH 7.0), For drought stress treatment, the tall by continuous shaking for 2 h at 4 °C. The and filtered using an acrodisc 13-mm syringe fescue was subjected to dry down by allow- samples were transferred into test tubes after filter with a 0.2-mm nylon membrane (Fisher ing soil moisture to drop gradually to 26% extraction and pH was adjusted to 2.6 with Scientific Company, Pittsburgh, PA) be- of container capacity (6% soil water content) 1.0 M HCl. The sample was slurried with fore being analyzed. and holding soil moisture in this range for 2 d. Amberlite XAD-7 (Sigma, St. Louis, MO) for Zeatin riboside was extracted according to Then, all drought treatment pots were re- 30 min. After removal of the buffer, the XAD- the procedure by Zhang and Ervin (2004) with watered to 90% container capacity (25.8% 7 was washed two times with 4 mL of 1% some modifications. Leaf samples (300 mg) soil water content) on Day 10. It took 8 d for acetic acid before being slurried two times were used for extraction. The ZR samples soil moisture to decrease to 26% container with 5 mL dichloromethane for 30 min were dissolved in methanol (50 mL) and di- capacity in this study. Soil moisture content (Edlund et al., 1995). The combined dichloro- luted with 0.95 mL 50 mM phosphate-buffered (top 5 cm deep soil) was determined using aThetaProbesoilmoisturesensor(TypeML1; Delta-T Devices Ltd., Cambridge, U.K.). Measurements. Physiological parameters were determined during dry down and re- covery periods. Samples of leaf tissues for biochemical constituents were taken from the top 1.5 cm of the canopy at 0, 4, 8, 12, 16, and 20 d of treatment, placed in plastic sample bags, frozen with liquid nitrogen, and stored at 80 °C for analysis of proline, IAA, cytokinin, and ABA contents. Turfgrass quality, relative leaf water content, leaf wilting, and root growth. Turf- grass quality was rated on a visual scale of 1 to 9 with 9 indicating the best quality (very dark green, dense, uniform, turgid shoots) and 1 indicating completely dead or brown shoots (Waddington et al., 1992).
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