Published in Crop Sci. 44:1746-1753 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TURFGRASS SCIENCE
Evaluation of Drought Resistance for Texas Bluegrass, Kentucky Bluegrass, and Their Hybrids
Eleni M. Abrahama,
Bingru Huangb,*,
Stacy A. Bonosb and
William A. Meyerb
a Lab. of Range Sci., Aristotle Univ. of Thessaloniki, 54006 Thessaloniki, Greece
b Dep. of Plant Biology and Pathology, Rutgers Univ., New Brunswick, NJ 08901
* Corresponding author (huang{at}aesop.rutgers.edu).
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ABSTRACT
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Kentucky bluegrass (Poa pratensis L.; KBG) has good turf quality with adequate irrigation, but moderate to low drought resistance. Interspecific hybridization of Kentucky bluegrass with Texas bluegrass (Poa arachnifera Torr.; TBG), a drought resistant grass native to Texas, has been made to transfer genes of drought resistance from TBG to KBG. The objectives of this study were (i) to investigate whether the hybrids have improved drought resistance compared with either TBG or KBG by examining physiological responses to drought stress; and (ii) to determine major physiological factors associated with recuperative ability of those plants from drought stress. Plants were exposed to drought stress by withholding irrigation for 42 d in a greenhouse. The experiment consisted of 29 fourth-generation backcrossed hybrids, one third-generation hybrid (BDF), two KBG parents (‘C-74’, ‘Midnight’), and one TBG parent. The genotypes classified into three cluster groups of drought resistance (high, moderate, and low) based on the responses of relative water content (RWC), electrolyte leakage (EL), and photochemical efficiency (Fv/Fm) to drought stress. The RWC and Fv/Fm declined with drought stress, but were maintained at higher levels in the high resistance group than the other group, while EL increased during drought and was lower in the high resistance group. Texas bluegrass, Midnight KBG, and two hybrids were ranked highest in drought resistance. Other hybrids varied in drought resistance, with more hybrids of BDF x Midnight ranked in the high drought resistance group than the hybrids of BDF x C-74. Among all physiological parameters examined, EL was the most sensitive indicator to drought stress, as demonstrated by its most rapid increase in response to the stress. High Fv/Fm and retaining of green leaves during drought stress contributed to fast recovery from drought stress following rewatering.
Abbreviations: EL, electrolyte leakage • Fv/Fm, photochemical efficiency • RWC, relative water content
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INTRODUCTION
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KENTUCKY BLUEGRASS forms attractive turf when supplied with adequate water (Meyer and Funk, 1989), which is used extensively for lawns, athletic fields, and golf courses (Turgeon, 2002). It has moderate to low drought resistance (Beard, 1989), although there is significant variability among the cultivars within the species (Murphy et al., 1995). Therefore, the lack of drought resistance restricts its use under water deficit conditions. Water is becoming increasingly limited for irrigation. The development of improved water-saving varieties is a high priority in turfgrass breeding programs.
Interspecific hybridization is a strategy to induce desirable genes of drought resistance and to increase the existing variability from a genetically close species that is more drought resistant (Humphreys and Thorogood, 1993). Interspecific hybrids between Lolium perenne and L. multiflorum (Jones and Humphreys, 1993) and between L. multiflorum and Festuca arundinacea (Humphreys and Thomas, 1993) increased productivity of Lolium hybrids during summer months.
Texas bluegrass is a more drought- and heat-resistant species relative to Kentucky bluegrass. Texas bluegrass is a rhizomatous, dioecious grass native to the southern USA (Gould, 1975), and used mainly as a forage grass. It is characterized by low turf quality and poor seed production. Texas bluegrass x KBG crosses were first made by George H. Oliver in 1908 (Vinall and Hein, 1937), who noticed a large variation in first generation hybrids in terms of heat and drought tolerance between the hybrids and KBG. Some hybrids were more heat and drought tolerant and more productive than KBG (Vinall and Hein, 1937). Recently, a hybrid between TBG and KBG, ‘Reveille’, was released as a heat-resistant variety for the southwestern USA (Read et al., 1999). More recently, many hybrids have been developed by the New Jersey Agricultural Experiment Station to improve the performance of KBG under drought and heat stress (Bonos et al., 2000). However, the information on the relative drought resistance of these hybrids with their parents is lacking and the physiological parameters involved in the interspecific variation in drought resistance and the recuperative ability are not well understood.
The objectives of this study were: (i) to compare drought resistance among KBG, TBG, and their hybrids; and (ii) to examine physiological factors contributing to the recuperative ability of KBG, TBG, and their hybrids from drought stress following rewatering.
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MATERIALS AND METHODS
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Plant Materials
Interspecific hybridization first between TBG and KBG hybrids was used to transfer genes for drought resistance from TBG to KBG. The first generation hybrids were backcrossed several times to KBG to improve turf quality and seed production. A total of 33 genotypes were evaluated under drought stress conditions: (i) 29 fourth-generation (BC4) hybrids; (ii) one third-generation (BC3) hybrid, BDF, which was the female of the majority of the BC4 selected hybrids; (iii) two KBG genotypes, C-74 and Midnight, which were the pollinators of the majority of the BC4 selected hybrids; and (iv) one TBG accession (186), which was used in the original cross.
Growth Conditions and Treatments
Approximately 15 single tillers of each genotype were initially planted into plastic flats filled with a soil and organic matter mix. After 20 d, six uniform plants of each genotype were transplanted into each plastic pot (20 cm in diameter and 38 cm deep) filled with the soil mix. Plants were grown in a greenhouse for about 2 mo and fertilized twice with controlled-release fertilizer (N–P–K, 16–4–8) before the drying treatment was imposed. Pots were watered twice weekly until drainage occurred from the bottom of the container. Turf was hand-clipped weekly at a 5-cm height.
Plants in six pots (replicates) for each genotype were exposed to drought stress by withholding irrigation for 42 d until complete leaf wilting of most plants and then rewatered to allow for recovery for 21 d. This treatment was performed in April and May 2001.
The experiment was repeated for a second time for 28 d in June and July 2001 to examine the consistency of genotypic variation in drought resistance. All cultural factors remained the same as in the first test, except the greenhouse environmental conditions. The mean day/night temperature was 21/15°C during the first test and 27/21°C during the second test. Relative humidity was averaged (24 h) 75 and 65%, respectively, in the first and second test. Photoperiod was averaged 13 h in the greenhouse during both tests.
Measurements
All measurements were made at weekly intervals in the first test. During the last week of drought stress, only turf quality, leaf wilting, and percentage of green leaves were determined because of insufficient leaf samples for the physiological measurements. After rewatering, turf quality was rated weekly for 21 d. In the second test, turf quality and leaf wilting were visually rated, which was done mainly to evaluate the consistency in ranking different genotypes for drought tolerance between two tests using different parameters.
Turf quality was rated on a 0-to-9 scale, where 0 = brown, dead turf; 6 = acceptable quality for a home lawn; and 9 = optimum color, density, and uniformity (Turgeon, 2002). Leaf wilting was evaluated on 0-to-9 scale, where 0 = no observable leaf wilting and 9 = completely wilted. The percentage of green leaves was assessed visually on a 0 to 100% scale. The clippings were collected weekly and dried at 70°C for 48 h to determine the clipping yield, which was expressed in grams of shoot dry weight per pot.
Volumetric moisture content in the top 20 cm of soil was measured using the time domain reflectometry (Soil Moisture Equipment, Inc., Santa Barbara, CA). Soil content was 25% before drought stress, declined to below 5% at 28 d and 3% by 35 d of drought stress in the first test, and at 21 and 28 d of drought stress, respectively, in the second test.
Leaf RWC was determined according to the method of Barrs and Weatherley (1962) on the base of the following equation: RWC = (FW – DW)/(SW – DW) x 100, where FW is leaf fresh weight, DW is dry weight of leaves after being dried at 85°C for 48 h, and SW is turgid weight of leaves after soaking in water for 24 h at room temperature (20°C).
Electrolyte leakage of leaves was measured according to the method of Blum and Ebercon (1981) and Marcum (1998), with modifications. Leaves were excised and cut into 1-cm segments. After being rinsed three times with distilled deionized H2O, leaves were placed in each test tube containing 20 mL of distilled deionized H2O. Test tubes were shaken on a shaker for 17 to 18 h, and the initial conductivity (Ci) was measured (Model 32; Yellow Spring Instrument Co., Yellow Springs, OH). Leaves then were killed at 120°C for 30 min, and the conductivity of killed tissue (C2) was measured after tubes cooled down to room temperature. The relative EL was calculated as (Ci/C2). Leaf Fv/Fm, expressed as chlorophyll fluorescence, was determined on two randomly selected second fully expanded leaves in each container with a fluorescence induction monitor (Dynamax, Houston, TX).
Experimental Design and Statistical Analysis
The experiment consisted of 33 genotypes with six replicates arranged in a completely randomized design with repeated measurements across time. Data were first subjected to cluster analysis. The genotypes were placed into three cluster groups of drought resistance (Table 1). The RWC, EL, and Fv/Fm at 0, 14, 28, and 35 d of drought stress were used as classification factors in a hierarchical cluster analysis (SPSS 10 for Windows). During the second test of drought stress, only turf quality and leaf wilting rate were evaluated, which were used as classification factors. General linear models procedure (SPSS 10 for Windows) was used for ANOVA. Variation was partitioned into genotypes, groups, and duration of treatment (time) as main effects and corresponding interactions. The LSD at the 0.05 probability level was used to detect the differences among means.
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Table 1. Ward's cluster analysis classification of 30 Texas bluegrass (TBG) and Kentucky bluegrass (KBG) hybrids, one TBG (186), and one KBG (‘C-74’) genotype based on relative water content, electrolyte leakage, and photochemical efficiency during the first test of drought stress.
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RESULTS AND DISCUSSION
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Genotype Classification in Drought Resistance
Generally, drought tolerant turfgrasses are able to maintain high turf quality, leaf water status (or less leaf wilting), and photosynthesis, but also low levels of EL (an indicator of cell membrane stability) during drought stress (Huang and Gao, 1999; Qian and Fry, 1997). These physiological parameters have been widely used as physiological indicators for the selection of drought tolerant plant materials in turfgrasses and other species (Blum and Pnuel, 1990; Bonos and Murphy, 1999; Jiang and Huang, 2001). Genotypes of TBG, KBG, and their hybrids varied dramatically in response to drought stress in terms of RWC, Fv/Fm, and EL (Fig. 1)
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Fig. 1. Variation in (A) relative water content, (B) photochemical efficiency, and (C) electrolyte leakage among 33 genotypes of Texas bluegrass, Kentucky bluegrass, and hybrids during the first cycle of drought stress. Each data point at a given day of treatment represents one genotype.
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All genotypes were classified into three cluster groups of drought resistance based on the level of RWC, EL, and Fv/Fm under drought stress in the first test (Fig. 2)
. One KBG genotype (C-74) was placed in the low drought resistance group (Table 1), whereas TBG (186) was placed in the high resistance group. The other KBG genotype (Midnight) was infected by powdery mildew [Blumeria graminis (DC.) E.O. Speer] during the first test of drought stress and excluded from the data analysis. The BC3 hybrid (BDF) was placed in the moderate group, but was close to the tolerant one. Seventy-three percent of the hybrids (Table 1) that placed in the high resistance group were derived from the cross BDF x Midnight.
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Fig. 2. Classification of Texas bluegrass, Kentucky bluegrass, and their hybrids in three cluster groups based on relative water content (RWC), photochemical efficiency (Fv/Fm), and electrolyte leakage (EL) under drought stress.
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Genotypes also varied in drought resistance based on the decline in visual rating of turf quality and the increase in the severity of leaf wilting in the second test (Fig. 3)
. During the second test, air temperature in the greenhouse was higher than that during the first one, which increased evapotranspiration demand and resulted in more severe drought stress. Only five genotypes were placed in the drought resistance group in the second test (Table 2), including TBG, Midnight, BDF, and two hybrids (855 and 861) from the cross of BDF x Midnight. In consistence with the grouping in the first test, TBG was placed in the high drought resistance group and KBG (C-74) was in the low drought resistance group in the second test. Also, KBG Midnight was found to belong to the high drought resistant group.
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Fig. 3. Turf quality and leaf wilting rates of Texas bluegrass, Kentucky bluegrass, and their hybrids during the second test of drought stress. Open squares represent the high resistance group, and diamonds represent the low and moderate resistance group.
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Table 2. Ward's cluster analysis classification of 30 Texas bluegrass (TBG) and Kentucky bluegrass (KBG) hybrids, one TBG (186), and two KBG (‘C-74’ and ‘Midnight’) genotypes, based on turf quality and leaf wilting rates during the second test of drought stress.
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On the basis of the cluster analysis, most hybrids from the cross of BDF x Midnight were more drought resistant than from the cross of BDF x C-74, although there were exceptions in both cases. For instance, the hybrids 835 and 837 from the cross BDF x C-74 were placed in the high and moderate groups of drought resistance during the first and second tests, respectively. On the other hand, 848 from the cross BDF x Midnight was consistently placed in the low resistance group. Humphreys and Thomas (1993) compared drought resistance of introgression lines from Lolium multiflorum x Festuca arundinacea hybrids and also found segregation among the progenies.
The highly drought resistant BC3 hybrid BDF, as well the existence of resistant hybrids from the cross BDF x Midnight and BDF x C-74 are evidence that the hybrids contain genes of drought resistance derived from TBG. Additionally, these results suggested that the drought resistance of the elite hybrids was probably also derived from KBG gene pool, since drought resistance of most BDF x Midnight hybrids and the Midnight parent were better than those of BDF x C-74 hybrid and C-74. Susceptible progenies of BDF x Midnight would be expected since the parental plants would have been largely heterozygous.
There is, however, lack of consistency in ranking of individual genotypes between the first and second tests. Although the classification was based on different factors during the first and second test, this is indicative of the large genotype-by-environment interaction. The selection pressure for drought resistance may be higher in the second test, which may narrow down the genotypes in the drought resistant group. This result suggests that severe drought stress should be imposed to select for superior drought resistant plants. Inconsistent response of genotypes under different levels of drought stress, duration, and intensity were referred in many cases of breeding for drought resistance, such as in sorghum [Sorghum bicolor (L.) Moench; Jagtap et al., 1998], rice (Oryza sativa L.; Pantuwan et al., 2002), and maize (Zea mays L.; Bruce et al., 2002). Suplick-Ploense et al. (2002) compared salinity tolerance between TBG, KBG, and their hybrids in two experiments and placed seven KBG cultivars in the most tolerant group in the spring experiment and four KBG and three TBG cultivars in the most tolerant group in the summer experiments. They concluded that environmental conditions could affect bluegrass salt tolerance expression.
Physiological Responses to Drought Stress
The ANOVA showed that the main effects of drought duration and genotypic groups were significant at P < 0.05 for all physiological parameters except the main effect of genotypic group for turf quality and clipping yield during recovery, indicating drought responses varied with genotypes and stress duration (Table 3). The interactions between time and genotypic groups were significant for all parameters, indicating that genotypic variation in drought responses was affected by duration of drought.
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Table 3. Analysis of variance for relative water content (RWC), electrolyte leakage (EL), photochemical efficiency (Fv/Fm), percentage of green leaves, leaf wilting, turf quality, and recovery rating for Texas bluegrass, Kentucky bluegrass, and their hybrids growing under drought stress condition for 6 wk.
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Leaf RWC and Fv/Fm declined while EL increased compared with the initial nonstress level for all three groups of genotypes (Fig. 4)
. The changes in EL during drought stress were more dramatic than RWC and Fv/Fm. Significant differences among the groups for leaf RWC (Fig. 4A) and Fv/Fm (Fig. 4B) were detected at 28 and 35 d of drought stress, respectively. The differences in EL among the groups started earlier than RWC and Fv/Fm, at 14 d of treatment, which showed more pronounced differences among the three groups (Fig. 4C) than RWC and Fv/Fm. At this time, the low resistance group had significantly higher EL than the high and moderate group. Thus, the high resistance group maintained significantly higher RWC, Fv/Fm, and lower EL than low and moderate groups during a drought stress period. The high resistance genotypes had a significantly lower wilting rate and higher percentage of green leaves than the low and moderate at 28, 35, and 42 d of drought stress (Fig. 5A,B)
. The low wilting rate and the high percentage of green leaves could contribute to the higher turf quality during 28 to 42 d of drought stress (Fig. 5C).
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Fig. 4. (A) Relative water content, (B) leaf photochemical efficiency (Fv/Fm), and (C) electrolyte leakage as affected by drought stress in low, moderate, and high drought resistance group of Texas bluegrass, Kentucky bluegrass, and their hybrids during the first test. Bars on the lines represent standard error of mean and vertical bars at the top or bottom of the figure indicate LSD values (P  0.05) for group comparison at a given day of treatment.
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Fig. 5. Percentage of (A) green leaves, (B) leaves wilting, and (C) turf quality as affected by drought stress in the low, moderate, and high drought resistance groups of Texas bluegrass, Kentucky bluegrass, and their hybrids in the first test. Bars on the lines represent standard error of mean and vertical bars at the top or bottom of the figure indicate LSD values (P 0.05) for group comparison at a given day of treatment.
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These results indicated that the high resistance group controlled the leaf water status better than the low and moderate groups. Optimum leaf RWC is about 85 to 95% for most species when water uptake by roots equals the leaf transpirational water loss; the critical RWC is approximately 50% (varies among species and tissue types), below which tissue physiological injuries and death occurs (Taiz and Zeiger, 1998). It is remarkable that RWC of the high resistance group was approximately 60% (Fig. 4A), even when the soil water content was <3% at 35 d of treatment. At the same soil moisture level, RWC of the low resistance group was <50%. The high RWC of resistant genotypes was probably the result of their better ability for water uptake at low soil water potential (Volaire et al., 1998), along with better dehydration tolerance of their tissue (Volaire and Lelierre, 2001). It is well documented that a critical component of the dehydration tolerance for grasses is cell membrane stability (Crowe et al., 1987; Volaire and Lelievre, 2001). In fact, the resistant genotypes exhibited better membrane stability than susceptible ones under severe drought stress, as demonstrated by the lower EL. Additionally, this ability of resistant genotypes to maintain cell membrane stability probably contributed to their higher Fv/Fm under drought stress (Jiang and Huang, 2001).
The variation among the genotypes within each group (Table 4), as was measured by the CV, increased with stress duration or severity for all the parameters. The only exception was the CV for EL, which was high even at the 14 d of drought stress and remained at the same level until 35 d. These results suggested that differences in drought resistance among genotypes were better expressed by exposing plants to prolonged, severe drought stress conditions.
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Table 4. Coefficients of variance for relative water content (RWC), electrolyte leakage (EL), photochemical efficiency (Fv/Fm), percentage of green leaves, and turf quality for low, moderate, and high resistance groups under drought stress conditions.
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Recovery from Drought Stress Following Rewatering
Turf quality and clipping yield increased when plants were rewatered following drought stress for all three groups of genotypes (Fig. 6) . The high resistance group maintained higher turf quality than the other two groups during rewatering (Fig. 6A), which could be due to its maintenance of higher turf quality under drought stress. The recovery rate for the low and moderate groups, however, was greater than the high group. The high resistance group also had higher clipping yield than the other groups at 7 d of rewatering (Fig. 6B).
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Fig. 6. (A) Turf quality and (B) dry weight of clippings yield of drought-stressed plants following rewatering in low, moderate, and high drought resistance groups of Texas bluegrass, Kentucky bluegrass, and their hybrids. Bars on the lines represent standard error of mean and vertical bars at the top or bottom of the figure indicate LSD values (P 0.05) for group comparison at a given day of treatment.
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The Fv/Fm ratio (Fig. 7A)
, percentage of green leaves (Fig. 7B), RWC (Fig. 7C), and leaf wilting status (Fig. 7D) under drought stress were significantly correlated with recovery in turf quality following rewatering (Fig. 7). The correlation was closer for Fv/Fm (r2 = 0.75) and percentage of green leaves (r2 = 0.79) than RWC (r2 = 0.42) and leaf wilting (r2 = 0.48). These results indicated that the ability of retaining photosystem II function and green leaves during drought stress had major contributions to their faster recovery from drought stress after rewatering. Maintaining green leaves with high Fv/Fm would allow rapid recovery in photosynthesis upon rewatering. This has been observed in Miscanthus "stay-green" genotypes (Clifton-Brown et al., 2002) under field conditions. Development of genotypes which have delayed leaf yellowing, or retain green leaves under water deficit conditions, and have rapid recuperative ability following rewatering is of great importance for turfgrass breeding programs. The exploitation of these genotypes will help select for stay-green turfgrass varieties suitable for nonirrigated turf when water deficits occur.
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Fig. 7. Correlation of physiological parameters under drought stress [(A) leaf photochemical efficiency (Fv/Fm) at 35 d of drought stress, (B) percentage of green leaves at 42 d of drought stress, (C) relative water content at 35 d of drought stress, (D) leaf wilting score at 42 d of drought stress] and turf quality recovery following rewatering for Texas bluegrass, Kentucky bluegrass, and their hybrids. * Significant at P = 0.05. ** Significant at P = 0.01.
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In conclusion, hybridization between KBG and TBG could improve drought resistance of KBG. Selecting highly drought resistant, first generation hybrids and backcrossing them with elite drought resistant genotypes of KBG would achieve further improvements in drought resistance of the hybrids. Cell membrane stability was a sensitive physiological indicator, which could be used to screen drought tolerant plants in the early stages of drought stress. Maintaining high photochemical efficiency and green leaves during drought stress contributed to rapid recovery of growth from drought stress for TBG, KBG, and their hybrids.
Received for publication September 4, 2003.
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TOP
ABSTRACT
INTRODUCTION
