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Crop Science 42:848-852 (2002)
© 2002 Crop Science Society of America

TURFGRASS SCIENCE

Kentucky Bluegrass Cultivar Root and Top Growth Responses When Grown in Hydroponics

K. S. Erushaa, R. C. Shearman*,b, T. P. Riordanb and L. A. Witb

a United States Golf Association Green Section, P.O. Box 708, Far Hills, NJ 07931-0708
b Dep. of Agronomy and Horticulture, 377 Plant Science, Univ. of Nebraska-Lincoln, Lincoln, NE 68583-0724

* Corresponding author (rshearman1{at}unl.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIAL AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
The fine texture and fibrous nature of turfgrass root systems make it difficult, tedious, and time consuming to measure root production, distribution, and plasticity. Timely, labor-efficient methods for turfgrass root assessment would be helpful. The objectives of this study were to: (i) evaluate Kentucky bluegrass (Poa pratensis L.) cultivars and experimental lines for root production, depth, and distribution in hydroponics; (ii) determine ability to redistribute roots as solution levels decline; and (iii) assess production of clipping yield, verdure, and top growth. Fifteen Kentucky bluegrass cultivars and one experimental line were studied in hydroponics experiments. ‘Georgetown’ produced the most root mass at 1460 mg, while ‘Kenblue’ and NE 80-88 had the least production at 1122 and 1099 mg, respectively. Total root production varied by 25% among entries. Root production was greatest for all cultivars in the range of 0 to 300 mm, declining with depth, but all entries produced roots at depths >600 mm. NE 80-88 had 90% of its root growth in this range, while ‘Touchdown’ had 71%. Root production declined considerably at depths >450 mm. ‘Birka’, ‘Dormie’, ‘Eclipse’, and Touchdown had the greatest roots, while ‘Aspen’, Georgetown, NE 80-88, and ‘Park’ had the least. Touchdown had the most roots (i.e., 27% of root mass) remaining in the hydroponic solution at the end of each experiment, while NE 80-88 had the least at 8.7%. Total top growth varied among the Kentucky bluegrass cultivars by 42%. ‘America’ produced the most total top growth at 9.2 g, while Kenblue had the least at 5.4 g. Aspen produced the most clippings with a yield of 3.7 g, while ‘Challenger’ and Eclipse had the lowest production at 2.1 g. Results from these experiments indicate hydroponic systems can be used to effectively separate Kentucky bluegrass genotypes for rooting and top growth responses. Furthermore, these results support the potential to select Kentucky bluegrass genotypes with improved rooting characteristics, such as root distribution and plasticity. Additional research is needed to relate results developed from hydroponics studies with field conditions.

Abbreviations: ET, evapotranspiration


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIAL AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
KENTUCKY BLUEGRASS is one of the most widely used cool-season turfgrass species in the USA (Beard, 1973). There is considerable interest in the turfgrass industry in developing Kentucky bluegrass cultivars with improved turfgrass performance, as is evidenced by the two most recent National Turfgrass Evaluation Program Kentucky bluegrass trials. The 1995 and 2000 trials had 103 (USDA, 1998) and 173 (K.N. Morris, 2001, unpublished data) entries, respectively. Kentucky bluegrass is widely used in cool, semi-arid regions of the USA, where water conservation is a concern (Beard, 1973). Identification of water-conserving and drought-avoiding Kentucky bluegrasses would be beneficial to turfgrass users in these areas.

Turfgrasses with deep extensive root systems, canopy resistance factors, and reduced evapotranspiration (ET) rates have been reported to be drought avoidant (Beard, 1989; Bowman and Macaulay, 1991; Carrow and Duncan, 1996; Ebdon and Petrovic, 1998; Kim and Beard, 1988; Salaiz et al., 1991; Shearman, 1986; Shearman, 1989; White et al., 1993). Deep rooting characteristics, as determined by root length density and root mass by depth, were reported as important traits associated with drought avoidance (Carrow, 1996a, 1996b; Hays et al., 1991; Salaiz et al., 1991; Marcum et al., 1995; White et al., 1993). The ability to distribute root mass and maintain a viable, multi-layered root system (i.e., root plasticity) under drought stress is also important (Carrow, 1996a; Huang et al., 1997a, 1997b; Kim et al., 1999; Sullivan and Ross, 1979). Identifying whether Kentucky bluegrass cultivars exhibit similar responses would be of interest to plant breeders and the turfgrass industry.

Turfgrass root investigations are difficult to conduct. The fine texture and fibrous nature of turfgrass root systems make it difficult, tedious, and time consuming to measure root depth, extent, and distribution. A number of methods have been used to assess root growth. Root samples are often obtained by a technique of washing roots from soil cores, drying the root material, and comparing weights (Bennett and Doss, 1960; Ensign and Weiser, 1975; Ervin and Koski, 1998; Madison and Hagan, 1962; Stuckey, 1941; Troughton, 1951). Willard and McClure (1932) used an additional step in this procedure by ashing roots to minimize variability among samples. This step has been eliminated in the more recent rooting assessments, most likely due to the additional time and difficulty the ashing procedure adds to the root sampling process. Root washing procedures are also tedious, time consuming, and labor intensive (Bohm, 1979; Smucker et al., 1982), adding the ashing procedure may reduce variability by accounting for mineral matter that is not removed from the root mass during washing; however, it is apparent that researchers have eliminated this step because it adds even more labor and time to the already tedious process.

Several researchers reported the use of rhizotron facilities to allow continuous, nondestructive observation of roots under field conditions (Barber, 1986; Huck and Taylor, 1982; Karnok and Kucharski, 1982). Rhizotron facilities are expensive to build and operate, and have limitations in numbers of treatment observations. Minirhizotrons are an option to rhizotrons for viewing root growth under field conditions (Branham and Smucker, 1986; Upchurch and Ritchie, 1983). They are relatively inexpensive to construct and operate, and allow for numerous treatment observations, but root production quantification remains a difficulty with this procedure (Merrill and Upchurch, 1994). Soilless and hydroponic root production assessment methods have also been used to determine plant root growth (Blum et al., 1977; Jordan et al., 1979; Kim et al., 1999; Lehman and Engelke, 1985; Sullivan, 1983).

With these thoughts in mind, a study to determine Kentucky bluegrass rooting responses was initiated, using a hydroponic technique. The objectives of this study were to: (i) evaluate Kentucky bluegrass cultivar and experimental lines for root production, depth, and distribution in hydroponics; (ii) determine ability to redistribute roots as solution levels decline; and (iii) assess production of clipping yield, verdure, and top growth.


   MATERIAL AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Fifteen cultivars and one experimental line of Kentucky bluegrass were selected for this study based on their potential genetic diversity. Cultivars were established from seed. Individual seedlings were transferred 2 wk after germination to 38-mm outside diameter x 210-mm depth plastic containers, filled with washed silica sand. Seedlings were fertilized every second day with the same nutrient solution later used in the hydroponics experiments (Table 1). Distilled water was applied weekly to leach the profile and minimize potential salt accumulation. Plants were clipped weekly at 50 mm during both experiments.


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  		Table 1. Stock solutions and concentrations for nutrient solution based on Hoagland and Arnon (1950).

 
Root growth reached the bottom of containers 8 wk after transplanting. Six plants that visually appeared to have similar shoot and root growth development were selected to represent each entry. These plants were washed to remove sand particles from the root system, and were transferred to the hydroponics system in the greenhouse.

The hydroponics system was previously described by Erusha (1986). A single plant was placed in the hole of the cap at the top of the hydroponics container. Hydroponics containers were constructed from 102-mm outside diameter x 750-mm length polyvinyl chloride pipe, and were the same as those described by Kim et al. (1999). Containers were capped at the bottom with 102-mm inside diameter sewer pipe caps. Caps were sealed with silicone sealant to prevent leakage. Tops were made with 19-mm thick styrofoam cut to tightly fit the inside diameter at the top of the container. A center hole, 25-mm diameter, was cut in the top to allow insertion of plants. The cap was painted with white latex paint. The containers were painted on the outside with black latex paint to reduce light penetration. A coat of white latex paint was added to cover the black surface to reduce the potential for heat build-up. The crown portion of the plant was wrapped with a narrow strip of saran wrap for support and to allow for expansion due to tiller formation and growth during the experiment.

Plants were allowed to equilibrate in the hydroponics system for 2 wk prior to starting experiments. Two experiments were conducted. The first experiment started on 15 Nov. 1985 and ended on 24 Jan. 1986. The second began on 15 Mar. 1986 and ended on 22 May 1986. Cultivar and experimental lines were arranged in a randomized complete block design with entries replicated six times. Mercury halide lights were used as lighting for a 14-hr photoperiod. Light intensity measurements ranged from a low of 270 W m-2 to a high of 730 W m-2, and greenhouse temperatures ranged between 22.0 and 28.0°C during these experiments.

Nutrient solution was the same as that reported by Shearman and Beard (1973). Nutrient solution was changed weekly to regulate pH, maintain nutrient concentrations, and minimize salt accumulation. The solution was replaced to the level of draw down resulting from ET for each treatment. This method exposed the cultivars and experimental line to declining water levels over the course of the experiments. Sullivan (1983) used this method when studying sorghum [Sorghum bicolor (L.) Moench] to identify genotypes that exhibited greater root growth distribution values associated with decreasing water levels. The hydroponics solution in each container was aerated using an aquarium bubble stone attached to a 2-mm inner diameter x 950-mm length tubing attached to a manifold that was connected to an air source. A pinch clamp located on each tube was tightened to achieve the same rate of bubbling for each container.

Growth parameters measured included clipping yield, verdure, top growth (i.e., total clipping yield plus verdure), root growth distribution, total root mass, and percentage of root growth in the hydroponics solution at termination of each experiment. Top growth was clipped to 50 mm every 5 d. Clippings were collected, dried at 70°C for 48 h, and weighed. Verdure was collected at the termination of the study, dried and weighted, using the same procedures reported for clippings. Top growth was determined as the sum of verdure and clipping yields. At the termination of each experiment, roots were separated from the crown portion of the plant when verdure measurements were made, and were divided into five 150-mm segments based on total rooting depth of the containers. Root materials were dried at 70°C for 72 h and weighed.

Data were subjected to analysis of variance. Data were combined from the two experiments after comparing mean square error terms and determining less than a two-fold difference (Steel and Torrie, 1980). Combined data were analyzed and means were separated using LSD at the 0.05 probability level.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
In this study, the 15 cultivars and one experimental line differed in their total root production and root distribution (Table 2). Kim et al. (1999) used a similar hydroponics system to effectively separate rooting responses of tall fescue cultivars and experimental lines. Total root production varied by 25% among entries, with Georgetown producing the most at 1460 mg, and Kenblue and NE 80-88 the least at 1122 mg and 1099 mg, respectively. Root production for all entries was greatest in the 0- to 300-mm range, declining with depth, but all production for all entries produced some rooting at >600 mm. Entries varied significantly in root distribution across the five rooting depths measured. For example, NE 80-88 produced 90% of its total root production in the 0- to 300-mm range, while Touchdown had only 70.6%. Root production declined considerably by the 450- to 600- and 600- to 750-mm ranges. Aspen, Georgetown, NE 80-88, and Park had the least root production, while Birka, Dormie, Eclipse, and Touchdown produced the most roots in this range. Deep rooting has been reported to be an important mechanism for drought avoidance (Sullivan and Ross, 1979; Kopec, 1985; Marcum et al., 1995; Ervin and Koski, 1998). These results support the potential for selection of genotypes with deeper rooting characteristics.


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  		Table 2. Total root production, root distribution, and percentage of roots in the hydroponics solution of 15 Kentucky bluegrass cultivars and one experimental line.

 
When the experiments were terminated, the amount of root growth in the hydroponics solution was measured and reported as a percentage of total root growth (Table 2). Sullivan (1983) used this procedure to separate sorghum lines for root redistribution and drought avoidance characteristics. In this study, the hydroponic solution diminished during the course of the experiments based on the ET rate of each entry. The mean overall ET rate for the 10-wk duration of the two studies was 430 mm, leaving {approx}320 mm of solution depth when the experiments were terminated. At the termination of the experiments, Touchdown had the highest percentage of total root mass remaining in the hydroponic solution with 27%, while NE 80-88 had the least with 8.7%. Root mass that was not in direct contact with the hydroponic solution appeared brown and suberized, while the portion in the solution appeared white and highly branched.

The ability of a turfgrass to redistribute its root system with declining moisture levels indicates a potential drought avoidance mechanism (Hays et al., 1991; Marcum et al., 1995; Beard and Sifers, 1997; Carrow and Duncan, 1996; Huang et al., 1997a; Ervin and Koski, 1998). Duncan and Carrow (1997) indicated that root plasticity involves the ability of plants to maintain a shallow root system to use rainfall and irrigation, as well as a deep root system to use available moisture deeper in the soil profile. Touchdown clearly demonstrated desirable root plasticity traits with relatively high root mass production at all levels measured in the rooting profile. On the other hand, Georgetown, Aspen, and NE 80-88 produced most of their root mass in the upper portion of the profile and demonstrated minimal root plasticity. Most research of this type has been conducted on interspecific rather than intraspecific comparisons (Fry and Butler, 1989; Carrow, 1995, 1996a, 1996b; Huang et al., 1997a, 1997b; Ervin and Koski, 1998). Results from this study indicate Kentucky bluegrass has considerable intraspecific differences in root depth and plasticity, and there is potential to select genotypes with rooting characteristics conducive to support drought avoidance mechanisms associated with differences in root distribution and plasticity.

Total top growth varied by 42% among the entries studied (Table 3). America, ‘Baron’, and Aspen produced the most top growth, while Challenger, Eclipse, ‘Nassau’, NE 80-88, Park, ‘Ram I’, and Kenblue produced the least. Cultivars differed in clipping yield and verdure. Aspen produced nearly twice the total clipping yield of either Challenger or Eclipse. Since clipping yields were a total accumulation of clippings taken from mowing done on a 5-d interval throughout the study, they reflect directly on the elongation rates of the grasses. Mean clipping yields per mowing for Aspen, Challenger, and Eclipse were 264, 150, and 151 mg, respectively. Horst et al. (1978) reported that yield potential of tall fescue forages could be predicted by leaf elongation rate.


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  		Table 3. Verdure, total clipping yield, and top growth of 15 Kentucky bluegrass cultivars and one experimental line grown in hydroponics solution.

 
America produced the greatest amount of verdure, and Kenblue produced the least. Shearman (1986) reported Kentucky bluegrass cultivars with high verdure and shoot density values, and low vertical elongation rates had lower ET rates than their counterparts. Ebdon and Petrovic (1998) reported leaf angle to be the most important discriminator influencing ET rates in the discriminant analysis evaluation they used. Evapotranspiration rates were not measured in this study. However, using the findings of Shearman (1986), Kim and Beard (1988), and Ebdon and Petrovic (1998), one could speculate that cultivars like Challenger and Eclipse might have low ET rates, while cultivars like Kenblue and Aspen would have high ET rates. However, more research is needed to support this speculation. The relative ratio of verdure to clipping yield might also be potential screening measurement to identify genotypes with greater canopy resistance and lower water use rates.

Additional research is needed to correlate results from hydroponics studies with field investigations. However, the results from these experiments indicate hydroponic systems can be used to effectively separate Kentucky bluegrass genotypes and their rooting responses. Furthermore, these results support the potential to select Kentucky bluegrass genotypes with improved rooting characteristics, such as root distribution and plasticity.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Agric. Res. Div., Univ. of Nebraska-Lincoln Journal Series no. 12904.

Received for publication March 16, 2001.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIAL AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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