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Crop Science 41:1348-1351 (2001)
© 2001 Crop Science Society of America

NOTES

Isolation distances for minimizing out-crossing in spring wheat

P. Hucl* and M. Matus-Cádiz

Crop Development Centre/Dep. of Plant Sciences, Univ. of Saskatchewan, 51 Campus Dr., Saskatoon, Saskatchewan, Canada S7N 5A8

* Corresponding author (hucl{at}sask.usask.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Currently recommended isolation distances of 3 or 10 m for pedigreed seed production of spring wheat (Triticum aestivum L.) may not be sufficient for cultivars with high out-crossing (OC) rates. The detection of higher than expected OC rates in wheat has directed this research to reassess currently recommended minimum isolation distances. The objective of this study was to determine if increased isolation distances are needed for cultivars that exhibit higher than normal levels of out-crossing. In each of 2 yr, OC rates were determined for four Canadian spring wheat cultivars at each of 15 distances (0–33 m) from a blue aleurone pollen source. Cultivars were grown in rows perpendicular to the pollinator block to the north, south, west, and east. Target rows were replicated four times within each direction. Out-crossing in ‘Katepwa’ and ‘Biggar’ was not detected beyond 3 m. Cultivars ‘Roblin’ and ‘Oslo’ exhibited higher than normal OC at distances of up to 27 m. For Roblin and Oslo, an isolation distance of 30 m is recommended to mitigate OC-derived off-types in the subsequent generation of pedigreed seed.

Abbreviations: CSGA, Canadian Seed Growers' Association • OC, out-crossing


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
THE CANADIAN PEDIGREED SEED PRODUCTION SYSTEM endeavors to provide crop producers with a guaranteed product. Classes of seed including Select, Foundation, and Registered seed are usually involved in the ancestry of certified seed (Anonymous, 1994). Certified seed, the terminal pedigree class, is recommended for use by the crop producer. In Canada, a maximum impurity tolerance of 0.01% (one off-type per 10 000 plants x 100) in Foundation or Registered seed and 0.05% in Certified seed has been deemed acceptable in wheat (Triticum aestivum L.) production. The Canadian Prairie Spring class is the only exception, 0.03% in Registered seed and 0.08% in Certified seed are acceptable in the production of this class of wheat. The appearance of morphological off-types indicates that cultivar purity has been compromised. This can result from intra- or inter-specific out-crossing (OC). Minimum isolation requirements, from another genotype of the same crop species, of 10 m for the production of Select seed and 3 m for the production of Foundation, Registered, or Certified seed are currently recommended in wheat. Several currently registered Canadian spring wheat cultivars have OC rates that are 10 to 15 times higher than the assumed level of approximately 0.3% (Hucl, 1996). For example, highest OC rates were detected for Oslo (6.1%) followed by Roblin (1.4%), Biggar (1.1%), and Katepwa (0.4%). The detection of these higher than expected OC rates in wheat has directed our research to reassess currently recommended minimum isolation distances in the production of pedigreed seed.

Wheat is a self-pollinated species with OC rates that are usually <1%; however, higher OC rates have been reported. For example, OC rates ranging from 0.1 to 4.0% were reported for bread wheat cultivars (Griffin, 1987), 0.1 to 5.6% for 12 winter wheat cultivars (Martin, 1990), and 2.4 to 10.1% for six experimental wheat populations (Enjalbert et al., 1998). Out-crossing rates ranging from 0.3 to 6.1% have been reported for 10 Canadian spring wheat cultivars (Hucl, 1996). High OC rates are of little concern in large commercial fields because the majority of OC will be intra-varietal. In contrast, problems may arise in the case of small plot increases where several genotypes are grown in close proximity. High OC rates can result in obvious off-types if morphologically diverse cultivars are grown in adjacent pedigreed seed plots.

Pollen dispersal also contributes to the appearance of off-types in adjacent pedigreed seed plots. Wheat pollen dispersal distances range from five to a minimum of 48 m (Khan et al., 1973). Several studies have reported seed set on target male sterile lines at 5 (Zeven, 1968), 6 (Miller et al., 1975), 8 (Bitzer and Patterson, 1967), and 48 m (Khan et al., 1973) from their pollen source. Consequently, high OC rates and optimum conditions for pollen dispersal (e.g., strong winds) may lead to an unacceptable level of off-types in the production of pedigreed seed at isolation distances of 3 or 10 m. The objective of this study was to determine if increased isolation distances are required in the production of pedigreed seed for wheat cultivars that exhibit higher than normal levels of OC.


   Materials and Methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Breeder seed of four spring wheat cultivars, Katepwa (Canada Western Red Spring [CWRS]), Roblin (CWRS), Biggar (Canada Prairie Spring [CPS]), and Oslo (CPS), possessing different OC levels (Hucl, 1996) were used in this study. Out-crossing rates for these cultivars were determined at distances ranging from 0 to 33 m from the pollen source. Blue aleurone wheat (Abdel-Aal and Hucl, 1999) was used as the pollen source in each of 2 yr (1995 and 1996) at Saskatoon to quantify OC rates. A field protocol similar to that described by Khan et al. (1973) was used to determine desirable isolation distances. The blue-seeded pollinator was seeded in a 5 by 5 m block on 3 May 1995 and 3 May 1996 at a low rate (60 seeds/m2) to promote tillering and thus extend the period of pollen shedding. The pollinator block was at the one leaf stage when the target rows were seeded. Fertilizer (11–51–0, N–P–K) was drilled in with the seed at a rate of 50 kg ha-1. The experiments were established on fallow land. The soil type was a Vertic Haploborall silty clay.

In 1995 (10 May) and 1996 (24 May), four target rows of each cultivar were seeded perpendicular to the sides of the pollinator block to the north, south, west, and east. Target rows were 35 m in length (250 seeds/m2) and spaced 30 cm apart. The four cultivars were randomized within four blocks (replications) in each direction. The experiment was bordered by 12 rows of ‘Sceptre’, a Canada Western Amber Durum wheat (T. turgidum L.) cultivar. The area surrounding the experiment was filled with spring wheat increase plots, none of which carried the blue aleurone gene.

At maturity, within each target row, 10-cm lengths of row were cut and threshed from the following distances from the pollen source: 0, 0.5, 1, 2, 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, and 33 m. Total seeds per sample were counted using an ESC-1 electronic seed counter (Agriculex Inc.). The number of seeds counted per sample averaged 538 (standard error ±35) in 1995 and 644 (±9) in 1996. The blue aleurone trait is expressed in the hybrid seed (F1). Seeds with light-blue aleurone were visually identified using a fluorescent light box source. Light blue seeds were grown out the following year to confirm visual identifications. Wind speed and direction data were collected within 5 km of the trail site by Environment Canada. Out-crossing rates were calculated as follows:

where ci is the ith cultivar and dj is the jth distance. For each year, the averaged OC rate of a cultivar at any particular distance from the pollen source is based on four replications. Average percent OC values were tested to be significantly different from zero (P = 0.05) based on one-tailed t-tests.


   Results and Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Average OC rates were higher in 1995 than in 1996 (Table 1). The magnitude of OC was greatest for the cultivar Oslo followed by Roblin, Biggar, and Katepwa. The highest OC rates detected in this present study (3.8%, Oslo; 2.6%, Roblin; 0.4%, Biggar; 0.2%, Katepwa) were generally not as high as in our previous study (6.1%, Oslo; 1.4%, Roblin; 1.1%, Biggar; 0.4%, Katepwa; Hucl, 1996); however, the tendencies of these cultivars towards high/low OC rates are consistent across studies. The single seeding date for target rows in our current study, unlike the three seeding dates in our previous study, probably reduced the level of "nicking" between the pollen source and target cultivars. The estimated pollination period in both years was approximately 11 d. In 1995, averaged heading dates were as follows: pollen source (06-28), Roblin (06-22), Oslo (06-23), Biggar (06-27), and Katepwa (06-28). High levels of nicking were expected between the pollen source and Biggar/Katepwa. Low levels of nicking, if any, were expected between the pollen source and Roblin/Oslo. Detectable levels of OC for Roblin and Oslo suggest that nicking did in fact occur between the pollen source and Roblin/Oslo (Table 1). This successful level of nicking is probably explained by the observation that stigmas of male-fertile plants are known to be receptive for a period of 4 to 13 d (DeVries, 1971). In 1996, averaged heading dates were as follows: pollen source (07-06), Roblin (07-08), Oslo (07-10), Biggar (07-15), and Katepwa (07-14). Thus, higher levels of nicking were expected between the pollen source and Roblin/Olso compared to the pollen source and Biggar/Katepwa. The higher level of nicking between the pollinator source and Roblin/Oslo in 1996, compared with 1995, did not translate into increased OC rates in 1996. Thus, the differential heading of target cultivars does not appear to be associated with differential cultivar OC rates.


View this table:
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  		Table 1. Average percent out-crossing for four wheat cultivars at various distances to the north, east, south, and west of the pollen source. Data are the average of four replications.

 
The highest OC rates were obtained to the west of the pollen source in 1995 and 1996 (Table 1). In 1995, averaged over distances and cultivars, OC rates were highest to the west of the pollen source (0.36%) followed by the north (0.17%), south (0.14%), and east (0.12%). In 1996, the highest OC rate was obtained to the west (0.11%) of the pollen source followed by the north (0.07%), east (0.05%), and south (0.04%). Contamination was increased in years with winds favorable to OC. Average wind speed during the estimated pollination period (11 d) was very similar in 1995 (13.9 km h-1) and 1996 (12.7 km h-1). The prevailing wind direction differed during the pollination period in 1995 and 1996. In 1995, the prevailing winds during the pollination period were northeastward or southeastward. Thus, the prevailing wind direction was associated with elevated OC rates in the westward direction of the experiment in 1995. In 1996, the prevailing winds during the same period were northeastward on 4 d with the remaining days being divided evenly among the other three directions. Thus, the prevailing wind direction did not appear to be associated with OC distribution in 1996. Differences in prevailing wind direction over years suggest that isolation distances should not be based on experiments oriented in only one direction from the pollen source.

The distances at which OC was detected for a given cultivar x direction x year combination are presented in Table 1. The OC rate decreased with distance from the pollen source. For Katepwa and Biggar the maximum distance at which OC was detected was 3 m. Thus, current isolation distances are satisfactory for cultivars Katepwa and Biggar, both low OC cultivars. The maximum distance at which OC was detected for both Roblin and Oslo was 27 m. No OC was detected for any of the four cultivars at a distance of 30 or 33 m to the north, east, south, or west of the pollen source in either year. The average distance at which 95% of the cumulative OC occurred for a given cultivar x direction combination is presented in Table 2. Roblin exhibited higher than normal OC at distances in the range from 0 to 12 m followed by Oslo, ranging from 0 to 6 m. Oslo had the highest OC rate; however, OC generally occurred within 7 m of the pollen source. Averaged over years, OC at 6 m to the west of the pollen source decreased by a factor of 10 for Roblin and five Oslo. This decrease in OC, for a self-fertile wheat crop, is substantially greater than that reported in the literature for male-sterile wheat (Khan et al., 1973). It appears that the high OC cultivars, Roblin and Oslo, are subject to contamination at isolation distances beyond those currently recommended by the Canadian Seed Growers' Association (CSGA). Contamination using 27 m of isolation suggests that a requirement of 30 m should become the minimum isolation requirement for Roblin and Oslo to minimize OC off-types in the subsequent generation of seed. This distance is approximately 10 times the current isolation requirement for Foundation, Registered, or Certified seed production and three times the requirement for Select seed production.


View this table:
[in this window]
[in a new window]
  		Table 2. Average distance (m) from pollinator source (± SE) at which 95% of total out-crossing (OC) was observed within a target row.

 

   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
In wheat, the CSGA currently recommends an isolation distance, from another genotype of the same crop species, of 10 m for the production of Select seed and 3 m for the production of Foundation, Registered, and Certified seed. This study indicates that isolation requirements are satisfactory for Katepwa and Biggar, cultivars with low OC rates. In contrast, cultivars Roblin and Oslo, with high OC rates, exhibited contamination at distances of up to 27 m. In the absence of other studies, we suggest that 30 m be used as the minimum isolation requirement for the production of pedigreed seed of cultivars suspected to exhibit higher than normal levels of OC.


   ACKNOWLEDGMENTS
 
Appreciation is expressed to G. Trowell and P. Lynn for their technical assistance. Thanks are extended to Mr. D. Gehl (Agriculture and Agri-Food Canada) for providing breeder seed of Katepwa, Biggar, and Roblin and to Dr. R. Graf (Saskatchewan Wheat Pool) for providing breeder seed of Oslo. This research was funded by a grant from the Canadian Seed Growers' Association.

Received for publication May 2, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 




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