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Crop Science 43:240-246 (2003)
© 2003 Crop Science Society of America

TURFGRASS SCIENCE

Interspecific Hybridization between Agrostis stolonifera and Related Agrostis Species under Field Conditions

F. C. Belanger*,a, T. R. Meagherc, P. R. Dayb, K. Plumleya and W. A. Meyera

a Dep. of Plant Biology and Pathology, Rutgers Univ., New Brunswick, NJ 08901-8520
b Biotechnology Center for Agriculture and the Environment, Rutgers Univ., New Brunswick, NJ 08901-8520
c Inst. of Environmental and Evolutionary Biology, Univ. of St. Andrews, St. Andrews, Scotland

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


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Creeping bentgrass (Agrostis stolonifera L.) is a commercially important turfgrass used principally on golf courses. Weed control is one of the major problems encountered in golf course maintenance, largely because the major weed, Poa annua L., is another grass species with herbicide responses similar to creeping bentgrass. Development of creeping bentgrass cultivars expressing one of the herbicide resistance genes would provide an effective solution. The prospects of commercialization of transgenic cultivars of creeping bentgrass have raised questions about the potential for pollen-mediated gene flow to related Agrostis spp. In a field study we have measured the frequency of interspecific hybridization between transgenic creeping bentgrass and four related species, A. canina L., A. castellana Boiss. and Reut., A. gigantea Roth, and A. capillaris L. Interspecific transgenic hybrids were recovered between creeping bentgrass and A. capillaris and A. castellana at frequencies of 0.044 and 0.0015%, respectively, which were considerably lower than intraspecific transgenic progeny recovery in the same experimental plots (0.631%). No interspecific transgenic hybrids were recovered with A. gigantea or A. canina.

Abbreviations: kb, kilobase


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
PLANT TRANSFORMATION TECHNOLOGY offers the opportunity to introduce into a plant species genes from unrelated organisms that would not be possible using traditional breeding methods. Genes from organisms as diverse as viruses, bacteria, fungi, and higher plants are currently being identified which confer useful new phenotypes when introduced into particular plant species. Plant transformation is thus a useful complement to conventional breeding in the development of new cultivars having improved agronomic traits.

Transgenic cultivars of a number of agricultural crops, such as corn, soybeans, and canola, are currently commercially available. Herbicide resistance and insect resistance are the transgenic traits most frequently incorporated into commercial cultivars. These transgenic crops have found widespread acceptance in the United States and constitute a large proportion of the cultivated acreage (James, 2001). Although there are clearly agronomic and environmental benefits provided by transgenic crops, there are also concerns, some of them raised recently, concerning environmental impacts and food safety that have limited worldwide acceptance of transgenic crops (McHughen, 2000).

Cultivars of nonfood species, such as turfgrasses, offer the opportunity for evaluation of environmental issues independently of food safety concerns. Herbicide resistance in particular is a potentially very useful objective of genetic engineering, but also one that raises environmental concerns due to potential transfer of herbicide resistance transgenes to related species through interspecific hybridization (Dale, 1992; Ellstrand, 2001). Such hybrids may have the potential to become weeds that cannot be controlled by specific herbicides, thus impacting on the effectiveness of current weed control methods. Interspecific hybridization between a number of crop species and their wild relatives is known to occur (Hancock et al., 1996; Ellstrand et al., 1999).

We are currently investigating the potential of genetic engineering to augment breeding efforts in the development of improved cultivars of creeping bentgrass. Creeping bentgrass is an outstanding cool season turfgrass whose fine leaves, prostrate habit, and tolerance to very low cutting heights have made it the principal species used for putting greens and fairways on temperate climate golf courses around the world. On golf courses, bentgrass is grown as a monoculture. Currently there is no effective method of controlling Poa annua, which is a serious weed problem. Creeping bentgrass is thus an ideal candidate for the genetic modification for weed control using genes that confer resistance to either of the nonselective herbicides, glyphosate or glufosinate. Cultivars developed from such herbicide-resistant plants would allow effective weed control with a herbicide with low environmental impact (Franz, 1985; Kishore and Shah, 1988).

Some of the specific issues relating to the release of transgenic turfgrasses have been reviewed (Johnson and Riordan, 1999). One of the issues is the possibility of hybridization between creeping bentgrass and related Agrostis spp. Creeping bentgrass is a self-sterile, wind pollinated, outcrossing species and thus may have the potential for introgression of transgenes into related species.

There is limited information available on the potential extent of interspecific hybridization of creeping bentgrass. The existence of presumed hybrids between creeping bentgrass and related species, based on intermediate morphological characteristics, has been reported (Bradshaw 1958; Stuckey and Banfield, 1946; Edgar and Forde, 1991). Davies (1953) reported on hybrids resulting from bagged crosses between creeping bentgrass and A. capillaris (referred to A. tenuis Sibth.) In a series of cytological examinations, hybrids between creeping bentgrass and A. vinealis Schreber (referred to as A. canina subsp. montana Hartm.), A. capillaris (referred to as A. tenuis), and A. gigantea were examined for chromosome pairing at metaphase I of meiosis (Jones, 1956a,b,c). From the cytological results, Jones determined the chromosome numbers and proposed a model for genome organization of these Agrostis spp., which is summarized in Table 1. He concluded that creeping bentgrass was an allotetraploid with one ancestral diploid genome in common with A. capillaris (Jones, 1956b).


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  		Table 1. Chromosome number and genome organization (from Jones, 1956a,b,c; Nelson, 1985) of the Agrostis spp. used in this study.

 
From this previous work, it appears hybridization between creeping bentgrass and related species can occur. Little information is available, however, on the frequency of its occurrence in cultivation or in nature. We have therefore investigated the frequency of interspecific hybridization between transgenic creeping bentgrass and four related Agrostis spp. under field conditions. The herbicide resistance trait provides an easily screenable marker for identification of any hybrids formed. Transgenic creeping bentgrass plants have been produced which contain the bar gene which confers resistance to the herbicide glufosinate (Hartman et al., 1994; Lee et al., 1996). The effectiveness of the bar gene in the transgenic creeping bentgrass plants was confirmed by field testing where the herbicide was applied at commercially recommended rates (Lee et al., 1997). In our study, these plants were used as pollen parents and glufosinate resistance was used as a marker for identification of interspecific hybrids in the progeny.

The Agrostis spp. chosen as the pollen recipients for our investigation were A. canina, A. capillaris, A. gigantea, and A. castellana (Table 1). These four species were chosen since they constitute the most widespread related Agrostis spp. that are likely to hybridize with creeping bentgrass. These four species are all cultivated species that have also become naturalized in various regions of the United States (Hitchcock, 1971, p. 338–352.). Native Agrostis spp. also occur across the USA, but these were not included in this study. The objective of this study was to determine the frequency of interspecific hybridization between creeping bentgrass and the four related Agrostis spp. under field conditions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Plant Materials
Two lines of transgenic creeping bentgrass plants previously generated by particle bombardment (Line 4475) and protoplast transformation (Line 5061) were used as pollen parents (Hartman et al., 1994; Lee et al., 1996). Both lines had previously been shown to be fertile, with the herbicide resistance trait segregating approximately 1:1 in the progeny (Lee et al., 1997; Belanger, 1996, unpublished data). The plants were clonally propagated in the greenhouse.

Nontransgenic plants of A. canina, A. capillaris, A. castellana, A. gigantea, and A. stolonifera were used as maternal parents. Nontransgenic seed of the creeping bentgrass cultivar Cobra was obtained from International Seeds, Inc., Halsey, OR. A. canina. cv. SR 7200 and A. capillaris cv. SR 7100 seed were from Seed Research, Corvallis, OR. A. castellana cv. Highland and A. gigantea cv. Streaker were from Great Western Seed Company, Albany, OR. Plants of the four related Agrostis spp. and the nontransgenic creeping bentgrass were established from seed in the greenhouse during the winter of 1997. Each plant originated from a single seed.

Glufosinate is a nonselective herbicide with activity against most plants. We are not aware of any reports of natural resistance to this herbicide. To ensure that glufosinate was effective against the Agrostis spp. used in this study, it was tested on {cong}500 seedlings of each species. All of the plants were killed by the herbicide.

Experimental Plot Design
During the summer of 1998, two experimental plots were established at the Rutgers Research Facility in Upper Deerfield, NJ. The two plots were separated from each other by {cong}140 m. At the field location, the prevailing winds are generally from the southwest.

The design of each plot was identical and consisted of a hexagonal array including 90 sample points for pollen reception and a central point for pollen dispersal. The distance between each sample point was 3 m. The maximum distance from the central point was 15 m. A hexagonal design was used to incorporate a spatial geometry to maximize the number of equidistant plants as well as to provide a complete coverage of the area surrounding the focal transgenic plants (Boffey and Veevers, 1977). At each sample point there were five pollen recipients, one plant of each of the four related Agrostis spp. and one nontransgenic creeping bentgrass. A nontransgenic creeping bentgrass plant was included at each point since the recovery of transgenic progeny from these plants provides an indication of where in the plot transgenic pollen was available to the related Agrostis spp. The plants at each point were separated from each other by plastic garden edging to ensure that they did not intergrow over the course of the project. These plants each originated from a single seed and so represent distinct genotypes. We were thus monitoring the potential for hybrid formation from 90 individual genotypes of each species in each plot. At the center of the array were five plants of one of the transgenic glufosinate-resistant bentgrass lines. Transgenic creeping bentgrass Line 4475 was in Plot 1 and transgenic Line 5061 was in Plot 2. A diagram of the experimental plot design and a photograph of one of the sample points are shown in Fig. 1.



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  		Fig. 1. (A) Diagram of experimental plot design. At the center point were five transgenic creeping bentgrass plants. At each of the 90 surrounding points were one plant of nontransgenic creeping bentgrass and one plant of each of the four related species. The distance between each sample point was 3 m. (B) Photograph of one of the sample points. The plants are (1) creeping bentgrass, (2) A. capillaris, (3) A. gigantea, (4) A. canina, and (5) A. castellana.

 
Screening for Transgenic Progeny
Agrostis spp. require a field vernalization period for flower induction so the earliest seed harvests were made after a full year in the field. In the summers of 1999 and 2000, seed from each plant was harvested individually and thrashed using a seed sieve. For screening, all the seed obtained from each plant was sown into 28- by 54-cm flats of commercial potting mix (Pro-Mix, Premier Brands Inc., Red Hill, PA). After {cong}3 to 4 wk of growth, the seedlings were sprayed with the glufosinate-based commercial herbicide Finale (AgrEvo Environmental Health, Montvale, NJ) at the rate recommended by the manufacturer. The final concentration of glufosinate was 0.35%. Approximately 2 wk after the spray, all surviving seedlings were counted and transplanted and a representative sample of the dead plants counted. The surviving plants were later sprayed again to confirm that they were herbicide resistant.

DNA Gel Blot Analysis
To confirm that herbicide resistance in seedlings was due to the presence of the bar gene, a selected sample of resistant seedlings was subjected to DNA gel blot analysis. DNA was isolated from leaf blades as previously described (Reddy et al., 1996). Twenty micrograms of DNA were digested with the restriction enzymes BamHI and EcoRI. DNA gel blot analysis was as previously described (Chu et al., 1994). The 32P-labeled 1-kilobase (kb) BamHI/HindIII fragment from the plasmid pBarGus (Fromm et al., 1990) was used as the probe. This fragment contains the 0.6-kb bar coding sequence, the 0.26-kb Nos 3' end and {cong}0.2 kb of additional vector sequence.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Transgene Flow to Nontransgenic Creeping Bentgrass
The plants were harvested in both 1999 and 2000. In each year, we initially screened for herbicide resistance among progeny harvested from creeping bentgrass to confirm that transgenic pollen was dispersed. The pollen production from the transgenics was lower in 1999 than in 2000, as determined by the recovery of glufosinate-resistant progeny from the nontransgenic creeping bentgrass (Table 2). In 2000, after an additional year of growth, there was more than a four-fold increase in the frequency of transgenic creeping bentgrass progeny from the nontransgenic plants. Because rates of recovery of transgenic creeping bentgrass progeny were so much higher in 2000, our screening for interspecific transgenic hybrids was limited to that year.


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  		Table 2. Recovery of transgenic progeny from nontransgenic creeping bentgrass parent plants.

 
Progeny from the nontransgenic creeping bentgrass plants was screened for the presence of glufosinate-resistant progeny as an indication of where in the experimental plot transgenic pollen was available to the related Agrostis spp. (Fig. 2). Clearly in 2000, transgenic pollen was available throughout both experimental plots. Progeny seedlings from the center transgenic creeping bentgrass plants from both experimental plots segregated approximately 1:1 for glufosinate resistance. On the basis of the 1:1 segregation of the bar gene from the transgenic creeping bentgrass parents, theoretically twice the number of herbicide-resistant creeping bentgrass seedlings recovered were progeny from the transgenic pollen parents.



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  		Fig. 2. Number of transgenic creeping bentgrass progeny recovered from nontransgenic creeping bentgrass parents at each sampling point from Plots 1 and 2 in 2000. Arrow indicates the direction of the prevailing winds.

 
There were also readily apparent effects of wind direction and distance on pollen dispersal. The effect of prevailing winds from the SW was apparent since 62% (Plot 2) to 66% (Plot 1) of the transgenic creeping bentgrass progeny were recovered on the half of the plots in the downwind direction. For this analysis, the plots were divided into halves diagonally. The half considered in the direction of the prevailing winds included the upper left sample point and the lower right sample point (Fig. 2). The occurrence of transgenic pollen in portions of the plots upwind from prevailing winds presumably reflects effects of short-term fluctuation in wind direction.

Pollen concentration is generally found to rapidly decline at increasing distances from the source (Bateman, 1947; Copeland and Hardin, 1970; Giddings et al., 1997; Griffiths, 1950; Paterniani and Stort; 1974). Similar results were obtained here for creeping bentgrass. The percentage of transgenic progeny recovered from both fields at each distance from the center is plotted in Fig. 3. The data for the entire fields and for each half of the fields, relative to the prevailing winds, are each plotted separately. Similar results were obtained for all three analyses. Between the 3- and 12-m points, the frequency of recovery of transgenic creeping bentgrass progeny dropped off rapidly with increasing distance from the source plants. Between the 12- and 15-m points, there was only a small change in percentage recovered, typical of the leptokurtic nature of pollen dispersal in wind pollinated plants (Ellstrand, 1992). The greatest distance of pollen dispersal could not be determined from this study since transgenic progeny were recovered at the 15-m points in each plot.



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  		Fig. 3. Percentage of transgenic creeping bentgrass progeny recovered from nontransgenic creeping bentgrass parents at each sampling distance from the central transgenic creeping bentgrass pollen source.

 
Transgene Flow to Related Agrostis Species
The results presented above on recovery of transgenic creeping bentgrass progeny from the nontransgenic creeping bentgrass plants indicate that transgenic pollen was available to the related Agrostis spp. throughout both plots. Table 3 summarizes the recovery of transgenic hybrid progeny from the four related nontransgenic Agrostis spp. in the plots. Transgenic hybrid progeny between creeping bentgrass and A. capillaris and A. castellana were recovered in both plots. Since the transgenic creeping bentgrass plants segregate 1:1, theoretically twice as many interspecific hybrids were actually formed. No hybrid progeny were recovered from the A. gigantea and A. canina plants in either plot. For the A. capillaris plants, all of the samples from both plots were screened as were the A. castellana plants from Plot 1. For the A. castellana plants from Plot 2 and for the A. gigantea and A. canina plants, the samples from the downwind half of the field were screened. For each sample screened, all the progeny harvested from that plant were screened. The differences in number of progeny screened among the different species reflects the differences in relative fertility among the species. Many of the A. canina plants did not survive to the second year or were weak, and so the total number of progeny screened is low.


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  		Table 3. Recovery of interspecific transgenic Agrostis hybrid progeny form nontransgenic Agrostis parent plants in 2000.

 
The overall percentage of transgenic A. capillaris and A. castellana hybrids recovered was 0.044 and 0.0015%, respectively, of the total number of progeny screened from these species. The percentage recovery of the hybrids at each distance from the center is plotted in Fig. 4. The transgenic hybrids recovered were found both in downwind and upwind positions in the plots. The highest percentage of the hybrids recovered from both A. capillaris and A. castellana was from plants at the closest sample distance, 3 m, from the center transgenic creeping bentgrass plants. The percentage recovery of hybrids dropped dramatically between the 3- and 6-m distances. For the hybrids with A. capillaris, the recovery dropped to zero at the 15-m distance. Some hybrids with A. castellana were recovered at the 12- and 15-m distances.



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  		Fig. 4. Percentage of transgenic interspecific hybrid progeny recovered from nontransgenic A. capillaris and A. castellana at each sampling distance from the central transgenic creeping bentgrass pollen source.

 
Transgenic hybrid progeny were identified as plants that survived two rounds of spraying with glufosinate. To confirm the presence of the bar gene, DNA was isolated from randomly selected hybrid individuals and subjected to DNA gel blot analysis. The herbicide-resistant phenotype was correlated with the presence of the bar gene for all samples tested (Fig. 5). No hybridizing bands were detected in the nontransgenic plants (Lanes 1, 6, and 9).



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  		Fig. 5. Gel blot of DNA isolated from transgenic and nontransgenic Agrostis plants. The DNA was digested with EcoRI and BamHI and probed with the bar coding region. Lane 1, nontransgenic creeping bentgrass; Lane 2, transgenic creeping bentgrass Line 4475; Lane 3, transgenic creeping bentgrass progeny from Plot 1; Lane 4, transgenic creeping bentgrass Line 5061; Lane 5, transgenic creeping bentgrass progeny from Plot 2; Lane 6, nontransgenic A. capillaris; Lane 7, transgenic A. capillaris hybrid progeny from Plot 1; Lane 8, transgenic A. capillaris hybrid progeny from Plot 2; Lane 9, nontransgsenic A. castellana; Lane 10, transgenic A. castellana hybrid progeny from Plot 1; Lane 11, transgenic A. castellana hybrid progeny from Plot 2. The positions of size markers, in kb, are indicated.

 
As would be expected, the two independent transgenic creeping bentgrass lines used as pollen parents each had a different pattern of transgene integration. Transgenic Line 4475, used as the pollen source in Plot 1, had two prominent hybridizing bands at {cong}6 kb and 1 kb (Lane 2), indicating two integration events. There was also a faint hybridizing band at {cong}1.3 kb, which resulted from incomplete digestion. The transgenic creeping bentgrass progeny plant from a nontransgenic creeping bentgrass plant in Plot 1 (Lane 3) and the transgenic A. castellana hybrid progeny from Plot 1 (Lane 10) had two prominent bands at the same size as those in 4475. The transgenic creeping bentgrass progeny plant also had the faint band. The transgenic A. capillaris hybrid (Lane 7) had the same three bands, but the band at 1.3 kb was more intense than that at 1 kb, indicating incomplete digestion. In bagged crosses between Line 4475 and nontransgenic creeping bentgrass, the herbicide resistance trait segregated 1:1 (Belanger, 1996, unpublished data) as if only one transgene copy were functional or both copies were tightly linked. Progeny seed harvested from Line 4475 in this field study also segregated 1:1. That the transgenic creeping bentgrass and hybrids recovered from Field 1 had the same hybridizing bands as Line 4475 suggests that the two transgene copies were linked.

The transgenic creeping bentgrass Line 5061, which was used as the pollen parent in Plot 2, had a single hybridizing band at {cong}7 kb (Lane 4). In previous DNA gel blots of Line 5061, the single band was at {cong}2 kb, indicating that there was incomplete digestion in this sample. The transgenic creeping bentgrass progeny plant (Lane 5) and the transgenic A. castellana hybrid progeny from Plot 2 (Lane 11) had a single band at 2 kb. The transgenic A. capillaris hybrid from Plot 2 had two bands at 7 kb and 2 kb (Lane 8) resulting from partial digestion.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The commercial release of transgenic crops has been accompanied by ongoing scientific debate of the long-term effects and potential risks of release into the environment of genetically engineered organisms. Concerns regarding the potential risks of release of transgenic organisms are focused both on the transgenic organisms themselves (i.e., development of insect resistance due to release of BT transgenics) and on the long-term impacts of introgressive hybridization of modified genes into wild relatives of the transgenics (Dale, 1994; Ellstrand and Hoffman, 1990). There is concern that such introgressive hybridization could possibly result in production of new weed species that may be difficult to control. Thus, there has been a critical need for scientific data that can be used to quantify rates of hybridization and consequent gene flow from transgenic cultivars to nontransgenic wild relatives.

The study reported here was designed to evaluate, under field conditions, the potential for interspecific hybridization of creeping bentgrass with four related Agrostis spp.: A. canina, A. capillaris, A. castellana, and A. gigantea. Our experimental design was intended to mimic, on a small scale, the conditions likely to exist in nature. In a natural setting, species of Agrostis that may hybridize with creeping bentgrass are likely to exist in groups rather than as isolated plants. Therefore, interspecific transgenic pollen will be competing with any intraspecific nontransgenic pollen. In this study there were 90 individual genotypes of each species in each experimental plot, thus providing competing pollen. Under these conditions, interspecific transgenic hybrids were recovered at much lower levels than the level of intraspecific transgenic creeping bentgrass progeny. Although we were sampling a total of 180 genotypes for each species, other genotypes may result in higher or lower cross-compatibility. Distance from the transgenic pollen source had a substantial impact on hybrid recovery. The percentage of hybrid progeny recovered was highest at the sample points closest to the center transgenic pollen source and dropped off dramatically at further distances.

In a separate greenhouse study designed to maximize the possibility of interspecific hybridization, we have recovered hybrids between creeping bentgrass and all four of the Agrostis spp. used here (Belanger, Plumley, Day, and Meyer, 1998, unpublished data). From the greenhouse study, it is clear that interspecific hybridization between creeping bentgrass and A. gigantea and A. canina is biologically possible, although in the field study reported here no hybrids were recovered with these two species. In order for interspecific hybridization to occur, there must be overlap in flowering between the two parental plants. In the greenhouse study, the flowering of the plants was artificially manipulated to ensure that plants of the different species were at similar stages of flower development. In the field study reported here, the A. castellana, A. gigantea, and A. canina plants flowered earlier than the creeping bentgrass plants and were essentially finished flowering before the creeping bentgrass plants were shedding pollen. The hybrids recovered with A. castellana must have originated from a few late receptive ovules. An individual Agrostis plant can have multiple panicles at different stages of flower development. There can therefore be variation in time of anthesis of individual florets on one plant. There was substantial overlap in flowering time of the creeping bentgrass and A. capillaris plants and concomitantly a higher percentage of hybrids with A. capillaris were recovered. Time of day of flowering and availability of other compatible pollen sources will also influence the amount of interspecific hybridization. In different regions of the country, the overlap in pollination times may be different than those reported here.

Hybridization between nontransgenic crop species and wild relatives has been reported for a number of species (Ellstrand et al., 1999; Hancock et al., 1996). With increasing interest in transgenic technology as a means of cultivar improvement, questions have been raised regarding the potential agricultural and ecological consequences of such hybridization involving transgenic crops. When hybridization is biologically possible, it can be expected to occur in nature at some level. That level will depend on the degree of compatibility of the two species, the proximity of the wild species to the transgenic crop, and on the degree of overlap in flowering between the two species (Hancock et al., 1996). In this field study we found low levels of interspecific hybridization between creeping bentgrass and the related species A. capillaris and A. castellana. Thus, hybrids can and do form under field conditions, albeit at low levels in the present study.

In addition to evaluations of the likelihood that hybridization will occur, Hancock et al. (1996) have suggested that assessment of the risks of such hybridization focus more on the nature of the transgenes themselves and whether they will confer any selective advantage on wild populations. In the absence of herbicide application, it is unlikely that a herbicide resistance transgene would confer any advantage on wild unmanaged Agrostis populations.


   ACKNOWLEDGMENTS
 
We thank Polina Kogan, Cindy Laramore, Jessica Parezo, and Harry Hodge for excellent assistance. We thank Albert Ayeni for help and guidance at the field site. This work was supported by grant 97-39210-5008 from the USDA.

Received for publication October 22, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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