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Crop Science 40:196-203 (2000)
© 2000 Crop Science Society of America

TURFGRASS SCIENCE

Root Physiological Characteristics Associated with Drought Resistance in Tall Fescue Cultivars

Bingru Huanga and Hongwen Gaob

a Department of Horticulture, Forestry & Recreation Resources, Kansas State University, Manhattan, KS 66506 USA
b Shanxi Academy of Agricultural Science, Taiyuan, Shanxi 030031, People's Republic of China

bhuang{at}oz.oznet.ksu.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Knowledge of root traits associated with drought tolerance is important for further understanding drought tolerance mechanisms of the whole plant. The experiment was designed to investigate effects of drought stress on root physiological activities of six cultivars (Kentucky-31, Falcon II, Houndog V, Phoenix, Rebel Jr., and Bonsai) of tall fescue (Festuca arundinacea Schreb.) varying in drought resistance. Grasses were grown in well-watered or drying (nonirrigated) soil for 35 d in a greenhouse. Drying reduced root length and dry mass in the 0- to 20-cm layer for all six cultivars. Root length and dry mass in the 40- to 60-cm layer was enhanced for Houndog V, Falcon II, and Kentucky-31; was not affected for Phoenix and Bonsai; and was reduced for Rebel Jr. by soil drying. Water uptake rates for Falcon II and Kentucky-31 decreased with soil drying in the 0- to 20-cm layer but increased in the 40- to 60-cm layer. Soil drying limited water uptake by Rebel Jr. in both layers. Drought stress increased root mortality in the 0- to 20-, 20- to 40-, and 40- to 60-cm layers, but the increase was most dramatic in the surface soil layer. The increase in root mortality in each soil layer was most severe for Rebel Jr. and least severe for Kentucky-31. Root death of tall fescue cultivars during drought was positively correlated with root desiccation, as evidenced by severe leakage of organic solutes from roots in drying soil. Carbohydrate supply to roots was not a contributor to root death during drought stress. This was supported by the increased or unaffected total nonstructural carbohydrates in both shoots and roots, and the increased C allocation to roots under soil drying conditions.

Abbreviations: PAR, photosynthetically active radiation • PVC, polyvinyl chloride • SWC, soil water content • TNC, total nonstructural carbohydrates • TTC, triphenyltetrazolium chloride


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
EFFICIENT WATER UPTAKE is an important determinant of drought resistance. Water uptake depends on root size (length or mass), activity, and spatial distribution. Most existing research on roots has focused on morphology and growth. Extensive, deep rooting often has been emphasized in relation to drought resistance (Taylor, 1983; Hays et al., 1991; Marcum et al., 1995). However, little is known about the genetic variations in physiological responses of roots to limited soil moisture. Moreover, extremely limited information is available about physiological factors controlling root mortality under drought stress.

Tall fescue is better able to avoid drought than other cool-season turfgrasses such as perennial ryegrass (Lolium perenne L.) or Kentucky bluegrass (Poa pratensis L.) (Sheffer et al., 1987). Within tall fescue species, cultivars also vary in drought resistance (White et al., 1993; Carrow, 1996a). Both intraspecific and interspecific variations in turfgrass drought resistance have been attributed mainly to differences in total root length density and rooting depth (Youngner et al., 1981; Beard, 1989; White et al., 1993; Marcum et al., 1995; Carrow, 1996b; Huang and Fry, 1998). Although total root length influences water and nutrient uptake, maintenance of roots that are viable and active in water and nutrient uptake during drought may be more important for plant tolerance to drought (Huang et al., 1997). Drought has been among the main causes of root death in the field (Smucker et al., 1991). Persistent root growth of perennial grasses is a characteristic that greatly enhances the adaptation of a grass to semiarid and arid climates (Weaver and Zink, 1955).

Understanding the mechanisms of root tolerance to drought stress may further our understanding of root physiological traits associated with drought tolerance. Such traits could be incorporated into breeding programs to improve drought tolerance. Therefore, our study was designed (i) to evaluate genetic variations in root activities and spatial distribution among tall fescue cultivars in response to soil drying and (ii) to determine physiological factors influencing root survivability in drying soils.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
Six tall fescue cultivars were examined in this study: Kentucky-31, Houndog V, Phoenix, Falcon II, Bonsai, and Rebel Jr. Sod pieces, 15 cm in diameter and 3 cm deep, of the six cultivars were collected from 5-yr-old plots at the Rocky Ford Turfgrass Research Center, Kansas State University, Manhattan, KS. Sod pieces were scraped and washed free of soil before planting in 15 cm diameter by 60 cm deep polyvinylchloride (PVC) tubes filled with a mixture (1:2 v/v) of coarse river sand and Chase silt loam soil (fine, montmorillonitic, mesic, Aquic, Arquidoll) collected from the field. The sand and soil mix was sterilized before planting.

Plants were grown for {approx}3 mo from February to May 1998 in a greenhouse with daily maximum and minimum temperatures of 24 and 15°C and a photoperiod of 14 h. The light regime in the greenhouse was supplemented with 1-kW metal halide lamps placed 1.5 m above the turf canopy. Photosynthetically active radiation (PAR) on a horizontal plane just above the canopy at noon averaged 800 µmol m-2 s-1 during the experimental period. Plants were watered twice weekly to bring soil to near field capacity for {approx}2 mo before the drying treatment began. Controlled-release fertilizer (17-6-10, N-P-K) was topdressed twice prior to dry-down to provide a total of 17 g m-2 N per container. Turf was hand clipped twice weekly at a 6-cm height throughout the experiments.

The experiment consisted of two soil moisture treatments: (i) well-watered control (plants were irrigated every other day until drainage occurred) and (ii) drought stress (irrigation was withheld and soil was allowed to dry for 35 d). Volumetric soil water content (SWC) in the 0- to 20- and 40- to 60-cm soil layers was measured daily using time domain reflectometry (Topp et al., 1980). A set of three-pronged waveguides made of stainless steel, 20 cm long and 3.0 mm in diameter, was buried vertically in each soil layer.Time domain reflectometry (Soil Moisture Equipment, Santa Barbara, CA.) provided a digital readout of volumetric SWC. Hourly water uptake was estimated based on changes in SWC in three cultivars (Rebel Jr., Kentucky-31, and Falcon II). Soil water content in the entire soil profile (0–60 cm) averaged 28% (93% of field capacity) across the experimental period under well-watered conditions. Soil water content in the 0- to 20-cm layer in the dry-down treatment averaged across cultivars declined to {approx}20% (67% of field capacity) by 7 d, 10% (33% of field capacity) by 14 d, and 5% (17% of field capacity) by 18 d and remained at this level during the remainder of the drying period.

During the 35-d treatment period, various shoot physiological characteristics were measured, and the results have been reported in Huang and Gao (unpublished data, 1998). At the end of the 35-d dry-down period, plants were harvested, and various measurements were made on the roots. The soil column was sliced into three soil layers (0–20, 20–40, and 40–60 cm). Roots in each layer were washed free of soil, and then length and dry weight, mortality, and cell membrane stability were determined. Root length was measured with an image analysis system (AgVision, Decagon Device, Pullman, WA). Root dry weight was obtained after roots were dried in an oven at 85°C for 48 h.

Root mortality was quantified by measuring root dehydrogenase activity using triphenyltetrazolium chloride (TTC) reduction technique (Knievel, 1973; Joslin and Henderson, 1984). Dehydrogenase activity is related positively to respiration capacity, which has been used in the study of the viability of different tissues, for example, seeds, leaves, and roots (Joslin and Henderson, 1984; Knievel, 1973; Huang et al., 1997). To determine the amount of live or dead roots in a sample, roots were first washed free of soil, and fresh weight was determined. Roots then were placed into test tubes with 0.6% (w/v) TTC in 0.06 M Na2HPO4–KH2PO4 and 0.05% (w/v) wetting agent (Tween 20), and tubes were incubated in a 30°C water bath for 20 h. Water-insoluble red compound, formazen, formed from the reduction of TTC by dehydrogenase enzymes in living tissues, was extracted in 95% (v/v) ethanol in a 60°C water bath for 4 h. The absorbance of the extractants was recorded at 480 nm with a spectrophotometer (Model U-1100, Hitachi, Tokyo). A regression of absorbance against root weight was determined for live roots and dead roots separately to calculate the percentage of dead root tissue among all roots in dry weight, which is considered the root mortality (Joslin and Henderson, 1984; Knievel, 1973).

Root cell membrane stability was determined by measuring the amount of UV-absorbing organic substances leaked from roots (Navari-Izzo et al., 1989; Reyes and Jennings, 1994). Roots were rinsed with deionized distilled water, placed in 100-mL flasks with 50 mL of deionized and distilled water, and shaken on a rotary shaker at 22°C for 24 h. A 3-mL aliquot was taken from each flask to measure leakage of organic compounds at 280 nm with a spectrophotometer (Model U-1100, Hitachi, Tokyo). Roots were then retrieved from the solution and frozen in liquid N2 for 20 min. The 3-mL aliquots were returned to the flasks with the original solution. The flasks were shaken for an additional hour, after which 3-mL aliquots again were removed for another absorbance measurement at 280 nm. The ratio between the initial and the final measurements is relative leakage ratio, which estimates cell membrane stability.

Carbon allocation patterns were determined using 14C labeling technique in three cultivars (Rebel Jr., Kentucky-31, and Falcon II) which demonstrate distinct responses in water relations and photosynthetic rates to drought stress (Huang and Gao, unpublished data, 1999). Shoots were enclosed in a transparent plexiglass chamber (15 cm tall and 10 cm in diameter) fitted tightly to the PVC column and exposed for 40 min to pp14.8 x 105 Bq 14CO2 that was released from Na14CO3 (25 x 104 Bq mol-1) by reacting with 1 M HCl. After the 40-min labeling, excessive 14CO2 was absorbed by bubbling the gas through a saturated NaOH solution for 20 min. Three days after labeling, plants were harvested. Leaves, crowns, and roots in the 0- to 20-, 20- to 40-, and 40- to 60-cm layers of soil were dried in an oven at 85°C for 48 h and oxidized with a biological material oxidizer (R. J. Harvey Instrument Corp., Hillsdale, NJ). The 14C activities in leaves, crowns, and roots were measured with a liquid scintillation analyzer (Packard, Deers Grove, IL). Total nonstructural carbohydrates (TNC) in leaves, crowns, and roots at the 0- to 20-, 20- to 40-, and 40- to 60-cm soil depths were measured for only Bonsai, Phoenix, and Houndog V by the method described in Smith et al. (1964). The TNC concentration was not determined in the 14C-labeled Rebel Jr., Kentucky-31, and Falcon II to avoid radioactive contamination.

The experiment involved two factors (six cultivars and two soil moisture treatments) with four replications arranged in a completely randomized design. Treatments were replicated four times in space in the same greenhouse. Treatment effects were determined by analysis of variance according to the general linear models procedure of the Statistical Analysis System (SAS Institute, Cary, NC). Variation was partitioned into cultivar and soil moisture as main effects and their corresponding interactions. To define how each cultivar responded to soil drying, soil moisture treatment analysis was conducted separately within each cultivar. Cultivar comparisons in each parameter were based on the percentage of change of stressed plants from that of well-watered plants for a given cultivar. Differences among treatment means were separated by a protected least significant difference (LSD, P <= 0.05) test.


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
Root Spatial Distribution
Root length in the 0- to 20-cm layer was reduced under soil drying conditions for all six cultivars (Fig. 1) . Root length in the 20- to 40-cm layer was reduced with soil drying for Rebel Jr., Bonsai, and Phoenix, but was not affected for the other cultivars. In the 40- to 60-cm layer, soil drying increased root length by 48% for Houndog V, 110% for Kentucky-31, and 70% for Falcon II; did not affect Bonsai and Phoenix; and reduced root length by 58% for Rebel Jr.



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  		Fig. 1 Root length at different soil depths for six tall fescue cultivars as affected by soil drying. Columns within a given soil depth marked with the same letters are not significantly different based on LSD test (P = 0.05)

 
Responses of root dry weight in different soil depths to soil drying followed the same pattern as root length (data not shown).

Water Uptake
Diurnal changes in SWC, which estimate water depletion rate, were monitored at various times during the treatment for Rebel Jr., Kentucky-31, and Falcon II that has previously shown distinct responses in leaf water relations to soil drying (Huang and Gao, 1999, unpublished data). Changes in SWC in the 0- to 20- and 40- to 60-cm layers at 14 d after soil drying was initiated are presented in Fig. 2 . Water uptake occurred from 0700 to 1700 h for both well-watered and drought-stressed plants, as indicated by rapid decline in SWC. Under well-watered conditions, water depletion rate in the 0- to 20-cm layer was greater than in the 40- to 60-cm layer for Kentucky-31 and Falcon II (Fig. 2A and 2C). Decreases in SWC per hour in the 0- to 20-cm layer from 1 to 23 d under well-watered conditions were greater for Kentucky-31 (0.214% ± 0.03%, means of four replicates ± standard error) and Falcon II (0.193% ± 0.01%) than for Rebel Jr. (0.162% ± 0.02%); decreases in SWC per hour in the 40- to 60-cm layer were 0.161% ± 0.01%, 0.148% ± 0.02%, and 0.170% ± 0.04% for Kentucky-31, Falcon II, and Rebel Jr., respectively, which were not significantly different based on the comparisons of the standard errors between means. When irrigation was withheld, hourly water uptake rates in the 0- to 20-cm layer decreased in all three cultivars and were not different between cultivars (Fig. 2B). However, water uptake rate in the 40- to 60-cm layer increased with soil drying for Kentucky-31 and Falcon II compared with that of well-watered plants. Those cultivars had rates of 0.326% ± 0.03% and 0.264% ± 0.05%, respectively, which were four to five times higher than the rate for Rebel Jr. (0.063% ± 0.01%).



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  		Fig. 2 Water uptake by depth as indicated by diurnal changes in soil water content in the 0- to 20- and 40- to 60-cm soil layer when soil was (A, C) well watered and (B, D) dried at 14 d after drying initiation for Kentucky-31, Falcon II, and Rebel Jr

 
Root Mortality
Soil drying increased root mortality in each soil layer for all cultivars (Fig. 3) . Generally, the greatest increase occurred in the 0- to 20-cm layer, and the effect lessened as depth increased. The magnitude of increase in root mortality in different soil layers under drying conditions relative to that under well-watered conditions also varied with cultivars (Fig. 3). For example, root mortality in the 0- to 20-cm depth under drying conditions increased by 73% for Rebel Jr., 63% for Bonsai, 61% for Phoenix, 72% for Houndog V, 55% for Kentucky-31, and 58% for Falcon II compared with their respective controls.



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  		Fig. 3 Root mortality at different soil depths for six tall fescue cultivars as affected by soil drying for 35 d. Columns within a given soil depth marked with the same letters are not significantly different based on LSD test (P = 0.05)

 
Root Membrane Stability
Soil drying induced leakage of organic solutes from roots at different soil depths for all six cultivars, but to a greater degree for roots in the surface 20-cm layer than in 40- to 60-cm layer (Fig. 4) . Cultivars also varied significantly in the extent of increases in solute leakage under soil drying than well-watered conditions. For example, relative leakage ratios of roots in the 0- to 20-cm layer under drying conditions were 127% higher than that under well-watered conditions for Rebel Jr., 153% for Bonsai, 97% for Phoenix, 108% for Houndog V, 47% for Kentucky-31, and 58% for Falcon II.



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  		Fig. 4 Root electrolyte leakage at different soil depths for six tall fescue cultivars as affected by soil drying for 35 d. Columns within a given soil depth marked with the same letters are not significantly different based on LSD test (P = 0.05)

 
Carbon Allocation
Soil drying reduced the proportion of newly photosynthesized 14C allocated to leaves but increased the proportion allocated to roots for the three cultivars examined (Rebel Jr., Kentucky-31, and Falcon II) (Fig. 5) . The 14C allocated to roots of drought-stressed plants was 17 times that for well-watered plants in Kentucky-31, 20 times in Falcon II, and 12 times in Rebel Jr. The proportion of 14C allocated to crowns was not affected by soil drying in Rebel Jr. and Falcon II but increased in Kentucky-31. The proportion of C allocated to roots in different soil depths also was affected by soil drying (Fig. 6) . Under well-watered conditions, the most 14C was allocated to roots in the 0- to 20-cm layer, and the amount decreased with depth. However, soil drying reversed this effect, with the most 14C allocated to roots in the 40- to 60-cm layer. The response was greater for Falcon II and Kentucky-31 than for Rebel Jr.



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  		Fig. 5 Carbon allocation to leaves, crown, and roots in three tall fescue cultivars, Rebel Jr., Kentucky-31, and Falcon II, as affected by soil drying. Columns within a given plant part marked with the same letters are not significantly different based on LSD test (P = 0.05)

 


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  		Fig. 6 Carbon allocation to roots at different soil depths in three tall fescue cultivars, Rebel Jr., Kentucky-31, and Falcon II, as affected by soil drying. Columns within a given soil depth or within a treatment marked with the same letters are not significantly different based on LSD test (P = 0.05). The lowercase letters indicate treatment comparisons within a soil depth, and the uppercase letters indicate comparison across soil depths for a given soil moisture treatment

 
Total Nonstructural Carbohydrate
For all three cultivars examined (Bonsai, Houndog V, and Phoenix), concentrations of TNC were higher in leaves of drought-stressed plants than in leaves of well-watered plants (Fig. 7) . Crown TNC concentration increased with soil drying for Houndog V and Phoenix but did not change for Bonsai. Root TNC concentration increased with soil drying for Houndog V but was unchanged for Bonsai and Phoenix.



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  		Fig. 7 Total nonstructural carbohydrate (TNC) contents in leaves, crown, and roots of three tall fescue cultivars, Bonsai, Houndog V, and Phoenix, as affected by soil drying. Columns within a given cultivar marked with the same letters are not significantly different based on LSD test (P = 0.05)

 

   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
Comparisons of shoot drought responses by the six tall fescue cultivars have indicated that Rebel Jr. and Bonsai are relatively drought sensitive, Phoenix and Houndog V are intermediate, and Kentucky-31 and Falcon II are drought resistant (Huang and Gao, 1999). Cultivar variation in shoot responses to drought stress appeared to be associated with the differences in root responses.

Soil drying caused significant decline in root length and dry mass in the upper 20 cm of soil for all six cultivars. However, it enhanced root length and mass in the 40- to 60-cm layer for Houndog V, Falcon II, and Kentucky-31 and reduced root length in the lower soil layer for Rebel Jr. Increases in rooting at deeper soil layers during drought stress have been reported in other species and have been considered an important adaptation mechanism to improve efficiency of plant water uptake (Molyneux and Davies, 1983; Sharp and Davies, 1985; Gallardo et al., 1996; Huang et al., 1997). Enhanced root growth during soil drying could have led to the increased water uptake rates in the deeper soil profile for Falcon II and Kentucky-31 as a compensation for the reduced water uptake in the surface soil layer. Roots that maintain high turgidity could be able to elongate in drying soils and would subsequently grow into the deeper wet soil layers (Sharp and Davies, 1979, 1985). However, soil drying caused a reduction in root growth at all soil depths for Rebel Jr., which could have resulted in low water uptake rates in both the upper and the lower soil layers. The intraspecific variation in the ability of plants to develop an adaptable root system with high water uptake rates in deeper soil depths is important for genetic manipulation of drought resistance (Duncan, 1994).

Drought stress often is suggested as the primary cause of root death in the field, especially in the surface soil (Deans, 1979; Persson, 1979). This is especially true if the majority of the root system is confined to the surface soil horizon, which commonly occurs in turfgrass species (Hays et al., 1991; Marcum et al., 1995; Carrow, 1996b; Huang et al., 1997). Considerable root death from drought stress has been reported in various plant species (Klepper et al., 1973; Hayes and Seastedt, 1987; Jupp and Newman, 1987; Smucker et al., 1991; Stasovski and Peterson, 1991; Huang and Nobel, 1992). However, some species exhibit much more tolerance of dry soils with little root death (Portas and Taylor, 1976; Marshall, 1986; Kosola and Eissenstat, 1994). The wide variation among species in root mortality in response to drought stress reported in the literature suggests high genetic variability among species.

Our study demonstrated that root mortality responses to drought stress varied with soil depths and cultivars. Drought stress increased root mortality in all three soil layers (0–20, 20–40, and 40–60 cm), but the increase was most dramatic in the top layer. Increases in root mortality in each soil layer were the most severe for Rebel Jr. and least severe for Kentucky-31 and Falcon II. Variation in root mortality among cultivars appeared to be associated with variations in water uptake and photosynthetic capacity during soil drying (Huang and Gao, 1998, unpublished data). Shoot growth depression of several warm-season turfgrasses during drought stress was attributed more to root mortality than to total root length or mass (Huang et al., 1997), suggesting that persistent root growth could be important in drought resistance.

The physiological factors influencing root mortality under soil drying conditions are not yet understood. In many grass species lacking a suberized hypodermis, most of the cortex readily dies when exposed to short periods of drought (Shone and Flood, 1983; Jupp and Newman, 1987). In a previous study with two tall fescue cultivars (Kentucky-31 and MIC18), Huang and Fry (1998) found severe root dessication during soil dry-down, as evidenced by the collapse of cortical cells and loss of root turgor. Although dehydration tolerance involves many different factors, cell membrane stability is a basic requirement for the maintenance of physiological functions in plants (Bewley, 1979). Stress-induced loss of cell membrane integrity is associated with an efflux of solutes (Levitt, 1980). Relative electrolyte leakage from cells or tissues during water stress can be used as a measure of dehydration tolerance (Blum and Ebercon, 1981; Martin et al., 1987). In our study, soil drying induced leakage of organic solutes from roots at different soil depths for all cultivars, especially Rebel Jr. and Bonsai. However, less leakage occurred from roots in the deepest soil layer. Severe leakage of solutes from roots could be closely related to root death.

Several researchers working with woody plants have suggested that root death is related to carbohydrate availability (Marshall, 1986; Kosola and Eissenstat, 1994). Marshall (1986) found insignificant root mortality in Douglas-fir [Pseudotsuga menziesii (Mirbel) Franco] exposed to drought alone, but when drought was coupled with shoot shading, 30 to 40% of roots were killed. He suggested that starch and sugar depletions (carbohydrate imbalance) were the primary causes of root death. Carbon allocation to surface fine roots of citrus was reduced 80% when roots were exposed to localized soil drying (Kosola and Eissenstat, 1994). However, Hallgren et al. (1991) found that drought had no effect on carbohydrate depletion related to root mortality in loblolly pine (Pinus taeda L.) seedlings. Our study indicated that carbohydrate supply to roots was not a contributor to root death during drought stress in tall fescue cultivars. The TNC contents in both leaves and roots either increased or were unaffected under soil drying conditions. Also, allocation of newly photosynthesized C to roots increased during drying, with greater increases in Kentucky-31 and Falcon II than in Rebel Jr., and a greater proportion of newly fixed C was allocated to roots in the deepest soil layer. Nicholas et al. (1985) found that biomass allocation to roots of drought-stressed plants was maintained in a drought-intolerant genotype and increased in a drought-tolerant genotype of wheat (Triticum aestivum L.). Increases in C investment in roots when water is limited in the soil may enhance survival of roots and plants during prolonged periods of drought stress.

In summary, root physiological characteristics associated with drought resistance in tall fescue cultivars included enhanced root growth and water uptake in the lower soil depths and the maintenance of root activity. Root death of tall fescue cultivars during soil drying could be related directly to root desiccation, as evidenced by severe leakage of organic solutes from roots in drying soil. Carbohydrate supply to roots was not a contributor to root death during drought stress. This was supported by the increased or unaffected contents of total nonstructural carbohydrates in both shoots and roots and the increased C allocation to roots under drying conditions.Nicolas Lambers Simpson Dalling 1985


   ACKNOWLEDGMENTS
 
The authors thank Dr. Mary Beth Kirkham and Dr. Robert Carrow for their critical review of the manuscript. Thanks also go to Mr. Xiaozhong Liu for the assistance in root mortality measurement. Partial support for this research came from State and Hatch funds and the Kansas Turfgrass Foundation.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Contribution no. 99-97-J of the Kansas Agricultural Experiment Station.

Received for publication November 9, 1998.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 





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