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TURFGRASS SCIENCE

Selection for Deep Root Production in Tall Fescue and Perennial Ryegrass

^a Dep. of Plant Biology and Pathology, Cook College, Rutgers Univ., 59 Dudley Rd., Foran Hall, New Brunswick, NJ 08901-8520
^b Advanta Seeds Pacific, 33725 Columbus St. SE, Albany, OR 97321-0452

^* Corresponding author (bonos{at}aesop.rutgers.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The identification and selection of germplasm with improved drought tolerance will play an important role in developing turfs with better performance and persistence during drought stress periods. The objectives of the study were to: (i) determine the feasibility of simultaneously selecting plants with low-shoot-to-high-root ratios and increased root mass in lower sand profiles using flexible tubes under greenhouse conditions, and (ii) determine gain from two cycles of selection for increased root production within the flexible tubes. Seeds from two populations of tall fescue (Festuca arundinacea Schreb.) and perennial ryegrass (Lolium perenne L.) were germinated in 29.5% (–1.4 MPa osmotic potential) and 28.5% (–1.2 MPa osmotic potential) polyethylene glycol (PEG), respectively, for each cycle of selection. The most vigorous seedlings were transferred to flexible root tubes, 63.5 cm long filled with silica sand, to evaluate for deep root production in the greenhouse. Clippings were collected weekly and root weights were determined after approximately 8 to 12 wk of growth in the flexible root tubes. The top 2 to 4% of the populations were selected for the following characteristics: clipping weights at or below the mean of the population and root weights (in the bottom 30 cm) at least 1 SD above the mean of the population for the turf-type tall fescue and perennial ryegrass populations. After two cycles of selection for increased root mass in the lower 30 cm, gain from selection was approximately 41% in a narrow population and 81% in a broad population of tall fescue and 130% in a turf-type and 367% in a forage-type perennial ryegrass. This technique should be very successful in developing turfgrasses with improved rooting characteristics.

Abbreviations: PEG, polyethylene glycol • PCV, polyvinyl chloride

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
THE IDENTIFICATION and selection of germplasm with improved drought tolerance will play an important role in developing turfs with better performance and persistence during drought stress periods. Drought avoidance characteristics include root penetration into deeper portions of the soil profile and osmotic regulation in plant tissues in the presence of low water potentials. This study was conducted to determine the selection efficiency of screening for deep root production in tall fescue and perennial ryegrass populations and its practical implementation into a turfgrass breeding program.

Screening seedlings or cell cultures for drought tolerance in a high osmotic solution such as PEG has been evaluated by a number of researchers with various results (Blum and Ebercon, 1981; Blum and Sullivan, 1986; Bouslama and Schapaugh, 1984; Handa et al., 1983; and Heyser and Nabors, 1981). The PEG screening procedures could allow for the selection of plants with the ability to extract water from more negative water potential soils, but it does not allow for the selection of increased root growth. Successful selection for drought-avoidant plants may involve more than one parameter, such as selection of high root mass in the lower soil profile or altered shoot-to-root ratios.

Drought resistance has been associated with deeper root penetration for a number of turfgrass species (Burton et al., 1954; Carrow 1996; Huang et al., 1997; Sheffer et al., 1987; White et al., 1993). Lehman and Engelke (1991) developed a greenhouse screening technique using flexible root tubes to evaluate root extension and root length densities in creeping bentgrass (Agrostis stolonifera L.) genotypes. They found that narrow-sense heritabilities, at 41 to 50 cm depth, were 0.82 for root surface areas and were between 0.62 and 0.77 for root extension measurements. They suggest that progress could be made in breeding for these characters.

Ekanayake et al. (1985) studied the inheritance of root characteristics in rice (Oryza sativa L.) through parent progeny regression of F₂ and F₃ generations. Narrow sense heritability estimates of the F₃ generation for root length density were between 0.44 and 0.77. They also suggest that individual plant selection based on this character should be successful. The effect of selection for root weight (Pederson et al., 1984) and root system size (Chloupek et al., 1999) [defined as the electrical capacitance measured in relation to the surrounding soil (Chloupek, 1977)] was evaluated in alfalfa (Medicago sativa L.). These two studies found that progenies selected from plants with higher root weights and larger root system sizes had higher root weights and larger root system sizes than progenies selected from plants with lower root weights or smaller root system sizes. These studies also indicate that selection for deeper and stronger root systems should be possible. In addition, both studies also found that selection for increased root production resulted in increased shoot production. Barbour and Murphy (1984) also found similar results when they selected for long root lengths in oat (Avena sativa L.).

Although selection for deeper root production is important for drought tolerance, the associated increased shoot production is not advantageous for turfgrass culture. Turfgrasses have been selected for more than 40 yr for decreased shoot production and slower vertical growth rate (Funk and Meyer, 2001) to reduce mowing frequency and increase turf quality. If turfgrasses could be selected with low-shoot-to-high-root ratios with deeper root penetration, significant gain in drought avoidance should be possible.

The objectives of the study were to: (i) determine the feasibility of simultaneously selecting plants with low-shoot-to-high-root ratios and increased root mass in lower sand profiles using flexible tubes under greenhouse conditions, and (ii) determine gain from two cycles of selection for increased root production within the flexible tubes.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Plant Material
Two tall fescue (ATF188 and ATF192) and two perennial ryegrass (APR120 and ‘Bastion’) populations were subjected to two cycles of selection for germination in PEG and for deep root production in flexible root tubes under greenhouse conditions. The two tall fescue and perennial ryegrass base populations and the standard cultivars evaluated for each species are described in Table 1. The ATF188 tall fescue population represented a narrow germplasm base selected from one cultivar. The ATF192 tall fescue population represented a broad germplasm base selected from seven cultivars. APR120 and Bastion represented a diploid turf-type and a tetraploid forage-type perennial ryegrass population, respectively. During each selection cycle, ‘KY-31’ or Bastion was included as standards for each replication. Following two selection cycles, plants representing the base population and both cycles of selection were compared (in two separate runs) for deep root production in flexible root tubes to determine gain from selection. Additionally, four standard cultivars for each species were also included for comparison. The breeding scheme for both tall fescue and perennial ryegrass is illustrated in Fig. 1 and described below.

View larger version (26K): [in this window] [in a new window]   Fig. 1. Breeding scheme for selection of drought avoidance characteristics in tall fescue and perennial ryegrass populations.

Greenhouse Flexible Root Tube Evaluation
This selection procedure was developed following the procedure described by Lehman and Engelke (1991), with modifications. Clear polyethylene tubing (4-mm wall thickness, 7.62-cm diam. [U-Line, Waukegan, IL]) was cut to a 63.5-cm length, and heat sealed at one end. The sealed end was perforated for drainage. Each polyethylene tube was filled with silica sand grade No. 8 (Williamette Graystone, Corvallis, OR) (pH 6.9) mixed evenly with 1.0 g of Scotts micronutrients, 2.0 g of (19.2–1.8–9.9) (N–P–K) Scotts starter fertilizer, and 4.0 g (12–11.3–9.9) Scotts Poly S (Marysville, OH). The flexible polyethylene tube was inserted inside a polyvinyl chloride (PVC) (black coex acrylonitrile butadiene styrene cellular core drainage, waste, and vent Schedule 40) pipe, 5.1 cm in diameter, cut to 63.5 cm and maintained at an approximate angle of 30° from vertical. Six tables, each holding 169 PVC tubes and representing one replicate, were constructed with a 5.1-cm chain-link fence and 3.5-cm-diam. posts, 90 cm tall. Chain-link fence was placed across the top of the table and the PVC tubes were inserted in the holes for support and stability (Fig. 2) . A wire grid was also positioned on the lower end of the table to support the bottom of the tubes.

View larger version (168K): [in this window] [in a new window]   Fig. 2. Photograph illustrating irrigation system and design of the flexible root tube table used for deep root selection in tall fescue and perennial ryegrass populations.

Selection Criteria
Each table (each replicate) contained 39 plants (tubes) of the comparison cultivar, KY-31 or Bastion for tall fescue and perennial ryegrass, respectively, and 130 plants (tubes) of the population to be selected. Plants selected for each cycle were chosen from 390 plants total or three tables (replicates). Plants were clipped at 5.1 cm and fresh weights of clippings were measured weekly. After approximately 8 and 12 wk of growth for tall fescue and perennial ryegrass, respectively, or when 25% of the tubes had roots reaching the bottom of the tube, the flexible root tubes were cut at 30 cm and the roots in the lower 30 cm were harvested. Since tubes were kept at a 30° angle from vertical, roots pulled freely away from most of the silica sand medium. Roots were washed free of excess silica sand using a gentle stream of water over a sieve. Excess moisture was removed by patting roots with paper towels. Fresh weight of roots was measured immediately.

The objective was to select plants with deeper root production without significantly increasing shoot production. Therefore, to improve selection efficiency, summary statistics (mean, SD, variance, and minimum and maximum values) were calculated separately for each table (replication) to determine average clipping yield and average root weight in the lower 30 cm for the population. Two to four percent of the population was selected with root weights in the lower 30 cm at least 1 SD above the mean of the population and average clipping yields at or below the mean of the population. The selected plants were transferred to a field nursery, and interpollinated to produce seed for the next cycle of selection.

Bastion was originally selected as the standard cultivar for each replication of the perennial ryegrass selection cycles. However, during the first cycle of selection it was evident that the root systems of the plants representing Bastion were very short compared with the turf-type diploid population. At that time, it was decided to select for deep root production in Bastion as well. Since it was the standard cultivar, only 117 (39 x 3) plants were available to select from for the first cycle of selection. The population was increased to the 390 plants used for the other populations for the second cycle of selection. Additionally, since Bastion is used for forage, and high shoot production is an important attribute, the selection criteria were modified for this population to select plants with increased shoots and increased roots. Plants were selected with both average clipping weights and average root weights in the lower 30 cm greater than 1 SD above the mean of the population.

After the two cycles of selection in all populations, the greenhouse flexible root tube evaluation was repeated (in two separate runs) to compare the original base populations, the two cycles of selection for each population and the standard cultivars (Table 1). The tubes were arranged in a randomized complete block design with six replications. This evaluation was conducted to determine the gain from selection for deep root production in the flexible root tubes.

Statistical Analysis
All replicated data were subjected to ANOVA. The gain from selection was calculated from the equation µ or R = µ_o – µ = ß(µs – µ) = ßS, as described in Lynch and Walsh (1998), where R = response to selection (equivalent to the gain from selection), µ_o = mean of the selected population, µ = mean of the base population, S = selection differential, and ß = realized heritability. Gain from selection was expressed as a percentage of the mean [(µ/µ) x 100]. Root-to-shoot ratios were determined by dividing root weight by shoot weight.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Turf-Type Tall Fescue and Perennial Ryegrass Populations
Selection for increased root production in the lower 30 cm of the flexible root tubes resulted in deeper root production in subsequent generations of both ATF188 and ATF192 tall fescue populations (Fig. 3) and the APR120 perennial ryegrass population (Fig. 4) . The ATF188 tall fescue population, one with a narrow germplasm base, had a gain of 41% in root production in the lower 30 cm after two cycles of selection (Fig. 3). The ATF192 tall fescue population, one with a broad germplasm base, had a gain of 81% in root production after two cycles of selection (Fig. 3). Interestingly, root-to-shoot ratios did not increase in the narrow tall fescue population (5.58, ATF188 C0 to 5.36, ATF188 C2), but they did increase in the broad tall fescue population (4.21, ATF192 C0 to 5.98, ATF192 C2). The ATF188 population was selected from only one cultivar, ‘Tulsa’, while the ATF192 population was selected from seven cultivars. These results indicate that selection was more effective in the tall fescue population with a broader germplasm base (ATF192) most likely because of the higher degree of recombination and genetic variation that is associated with highly heterozygous heterogeneous populations.

View larger version (23K): [in this window] [in a new window]   Fig. 3. Fresh root and shoot weights of tall fescue cultivars grown in flexible root tubes under greenhouse conditions. (A) Standard cultivars. (B) ATF188 base population vs. cycles of selection for deep root production. (C) ATF192 base population vs. cycles of selection for deep root production. Different letters indicate a significant difference at P 0.05 using Fisher's protected least significant difference mean separation. Connecting lines indicate the selection cycles over which the total percentage gain in each panel was calculated.

View larger version (28K): [in this window] [in a new window]   Fig. 4. Fresh root and shoot weights of perennial ryegrass cultivars grown in flexible root tubes under greenhouse conditions. (A) Standard cultivars. (B) APR120 base population vs. cycles of selection for deep root production. (C) Bastion base population vs. cycles of selection for deep root production. Different letters indicate a significant difference at P 0.05 using Fisher's protected least significant difference mean separation. Connecting lines indicate the selection cycles over which the total percentage gain in each panel was calculated.

Chloupek et al. (1999) and Pederson et al. (1984) also found positive results when they selected alfalfa for increased root weight and size, respectively. Lamb et al. (1999) found realized heritabilities from 21 to 48% for fibrous root mass selection and from 11 to 43% for lateral root number in alfalfa. Additionally, Pantalone et al. (1996) found a 26% gain from selection for phenotypic root score (visual index of fibrous root area) in soybean [Glycine max (L.) Merr.]. Ekanayake et al. (1985) indicated that both additive and dominance affects contributed to the inheritance of maximum root length, thick root number, and root volume in rice. The large gains observed in this study suggest that recurrent selection for increased root production in lower soil depths should be effective in improving deeper root growth in subsequent generations of tall fescue and perennial ryegrass.

The more important question to answer is whether deeper root production results in better drought tolerance under field conditions. Several field studies identified a correlation between deeper root growth and better drought tolerance (Burton et al., 1954; Hurd, 1968; Price et al., 1997). Additionally, Ekanayake and coworkers (1985) found five root characters were significantly correlated with visual field drought resistance scores. On the contrary, Barbour and Murphy (1984) were unable to predict superior drought tolerance by selecting longer root length in oat seedlings. Huang et al. (1997) emphasized that root vitality (or physiological function) may also play an important role in drought resistance. It will be important to evaluate these populations of tall fescue and perennial ryegrass cycled for increased root production under drought stress conditions to determine the effectiveness of this selection technique.

Many researchers found a high positive correlation between root and shoot characteristics and noticed that selection for increased root characteristics resulted in increased shoot characteristics in a number of plant species (Barbour and Murphy, 1984; Chloupek et al., 1999; Ekanayake et al., 1985; Palazzo and Brar, 1997; Pederson et al., 1984). Interestingly, Ekanayake et al. (1985) realized that tall rice plants tended to have deeper root systems and noticed that recombinants exhibiting short stature and deep root systems were observed with moderate frequency. They indicated that it should be possible to obtain short- or intermediate-statured segregants with deep root systems. Our research supports these conclusions. Selection for segregants with deep root production without the associated increase in shoot production proved effective in both tall fescue (ATF192) and turf-type perennial ryegrass (APR120) populations (Fig. 3 and 4). However, the lack of improvement in root-to-shoot ratios and modest gains associated with the ATF188 narrow tall fescue population indicate the importance of using broad heterozygous populations when breeding cross-pollinated crops such as tall fescue. Improvements in deep root production without the associated increase in shoot production should have positive implications for improving drought tolerance in turfgrasses.

Forage-Tetraploid Perennial Ryegrass
The forage-type tetraploid perennial ryegrass cultivar, Bastion, had a 367% gain in root production in the lower 30 cm after two cycles of selection and a 79% gain in shoot weight after two cycles of selection (Fig. 4). Relatively dramatic gain from selection for deep root production in Bastion was observed even though the first cycle of selection was based on a small number of plants (117). It is likely that the dramatic gains observed are a function of the poor initial root system observed for Bastion (0.05 g). From Fig. 4 it is evident that Bastion (base population) had a very small root system compared with the standard turf-type cultivars. It is much easier to make dramatic gains if the initial starting point is low, as was the case for Bastion. It would be much more difficult to make such a dramatic gain had the initial root weight been much larger. The final root weight (after two cycles of selection) was still only comparable to the standard turf-type cultivars.

As stated previously, the turf-type cultivar APR120 was selected for increased root production and mean shoot production while Bastion, the forage cultivar, was selected for both increased root and increased shoot production. These results indicate that, within the species and cultivars tested, selection for increased root production can be separated from shoot production when selection for both characteristics is applied. It also indicates that selection for deep root production in both turf and forage types are possible.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The results of this study indicate that selection for low-shoot-to-high-root ratios in turf-type tall fescue and perennial ryegrass populations using sand culture in flexible root tubes was very successful. Furthermore, the selection for increased shoot and root production simultaneously in forage perennial ryegrass was also successful in improving deep root production in subsequent generations. The technique of using flexible tubes to select for deeper root production may be a feasible selection technique that could be used in turfgrass breeding programs. Studies are underway to see if the improved root growth observed in these selected populations will result in improved drought tolerance under field conditions.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Research was supported by Advanta Seeds Pacific, the Rutgers Center for Turfgrass Science, and the New Jersey Agricultural Experiment Station (Journal No. D-12180-1-03).

Received for publication February 11, 2003.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by Bonos, S. A. Articles by Meyer, W. A. Search for Related Content PubMed Articles by Bonos, S. A. Articles by Meyer, W. A. Agricola Articles by Bonos, S. A. Articles by Meyer, W. A. Related Collections Turfgrass Management Turfgrass Crop Genetics