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Estimating out-crossing rates in spring wheat cultivars using the contact method.

WHEAT IS A SELF-POLLINATED species with OC rates hat are assumed to be <1%, but OC rates > 1% have been reported for wheat plants grown in close proximity. The literature on intraspecific OC in wheat has recently been reviewed in detail by Waines and Hegde (2003). Research on the tendency of western Canadian spring wheat cultivars toward OC is based on a limited number of cultivars (Hucl, 1996; Briggs et al., 1999; Hucl and Matus-Cadiz, 2001). The present research was conducted to assess the effectiveness of estimating OC rates in a larger number of wheat cultivars using greenhouses. There do not appear to be any reports on this approach in the published literature. If OC rates of wheat plants grown in close proximity can be estimated using greenhouse experiments, then estimating the OC rates of a larger cross-section of cultivars should be possible without relying on laborious field-based research. If the tendency of a large number of cultivars to OC can be assessed using only greenhouse experiments, then this information may be useful for developing guidelines for maintaining line purity in breeding programs and pedigreed seed production (Hucl and Matus-Cadiz, 2001). The objective was to estimate the OC rates of 35 Canadian wheat cultivars grown in direct spike contact under greenhouse conditions.

MATERIALS AND METHODS

Seed of Purendo-38, an awnless, spring type blue-aleuroned wheat was obtained from the Crop Development Center, Saskatoon, SK. The blue-grained trait is a dominant gene marker that has been applied within gene flow studies (Hucl and Matus-Cadiz, 2001; Matus-Cadiz et al., 2004). Certified seed was used for all 35 cultivars tested (Table 1). Of the cultivars tested, 34 were Canadian cultivars including 19 of the Canada Western Red Spring (CWRS) class, seven of the Canada Prairie Spring (CPS) wheat class, four of the Canada Western Extra Strong (CWES) class, and four of the Canada Western Amber Durum (CWAD; Triticum turgidum L.) class. Rongotea, a New Zealand cultivar (Griffin, 1987), was grouped with CPS cultivars for presentation purposes because Rongotea has similar agronomic and quality characteristics when compared to CPS cultivars.

For the 2001 and 2002 greenhouse studies, 20 pots of Purendo-38 were seeded weekly for 5 wk starting on 25 May 2001 and 24 Sept. 2002 to ensure sufficient pollen for crossing. Recipient cultivars (one pot per cultivar) were seeded weekly for 4 wk starting on 28 May 2001 and 27 Sept. 2002 to ensure a sufficient number of spikes for crossing. Different greenhouses were used in each year of the study, with artificial light condition being more favorable in 2002 compared to 2001. In 2001, conditions were set at 24/18[degrees]C (day/night) with 18 h light and a photosynthetically active radiation level of 150 [micro]mol [m.sup.-2] [s.sup.-1]. In 2002, conditions were set at 23/18[degrees]C (day/night) with 18 h light and a photosynthetically active radiation level of 250 [micro]mol [m.sup.-2] [s.sup.-1]. Pots (15-cm diameter) were filled with Terra-Lite Redi-Earth (W.R. Grace and Co. of Canada Ltd., Ajax, ON). Six seeds were planted per pot at a depth of 2.5 cm. Seedlings were watered every 2 d and fertilized at Zadoks growth stage 12 (ZGS 12; Zadoks et al., 1974) once using Type 100 Nutricote controlled release granular fertilizer (14-14-14) (Plant Products Co. Ltd., Brampton, ON) at a rate of 0.8 kg [m.sup.-2].

When seedlings reached the two-leaf stage (ZGS 12), up to three seedlings were removed from each pot to allow three plants to establish per pot. Under these growing conditions, plants within pots produced an average of five to six tillers. All tillers were used for crossing. Each recipient cultivar was crossed with Purendo-38 by covering up to three non-emasculated spikes of a given cultivar and one Purendo-38 spike as pollen source with an individual glassine bag (5 by 19 cm). The number of recipient spikes in a bag was based on spike availability. Awns were clipped to 1 cm above their respective glume before crossing to allow glassine bags to slip easily over multiple spikes. The bags were removed 7 d after pollination to allow for normal grain development and ripening. Crossed spikes were harvested at ZGS 92 (seeding dates were maintained separate), air-dried at 24[degrees]C (40% relative humidity) for 7 d, and then individually hand threshed.

Cross-pollination events from Purendo-38 to recipient cultivars were identified by the expression of a light-blue pigment in the aleurone layer of putative [F.sub.1] seed. Seeds possessing a light-blue aleurone were visually identified using a fluorescent light box source and separated from the remaining seed lot. Putative light-blue [F.sub.1] seeds were sown in the greenhouse and resulting plants were selfed and grown to maturity to confirm the light-blue [F.sub.1] seed was the result of an out-crossing event. Before planting, the seeds were surface sterilized for 8 min using a 2.5% sodium hypochlorite and 0.1% (v/v) Tween 20 solution, rinsed for 5 min with water, followed by a rinse with 70% ethanol, and then air-dried at room temperature. Seeds were pregerminated in the dark at 15[degrees]C for 10 d in a petri dish (each containing a Whatman No. 1 filter paper) and subsequently transferred to soil. The pregerminated seeds were planted (2.5-cm depth) in 15-cm-diameter pots (three plants per pot) as described above and grown to maturity using the greenhouse conditions described for 2002. Spikes from individual plants were harvested and hand threshed separately. The [F.sub.1]-derived [F.sub.2] seed was classified as segregating (3:1 blue/nonblue seed ratio) or nonsegregating (all nonblue seeds) for the blue-aleurone trait.

Out-crossing rates were calculated for each cultivar as follows: OC (%) = 100(total number of confirmed light-blue seeds observed/total number of seeds collected) where confirmed light-blue seeds included only light-blue [F.sub.1] seeds that segregated for the blue-aleurone trait in greenhouse grow outs. In 2001 and 2002, OC rates were calculated by pooling values across the four seeding dates. Means were tested for significance (P [less than or equal to] 0.05) using one-tailed t tests (Minitab Version 13; Minitab Inc., State College, PA).

RESULTS AND DISCUSSION

This 2-yr greenhouse-based study indicates that OC rates in spring wheat cultivars grown in direct spike contact under greenhouse conditions are generally at levels below 2.8% but can exceed 10% (Table 1). Generally, OC rates were less than 2.8% in 2001 and 2002, with the exception of CPS cv. Genesis (3.5% in 2002), CWES cv. Wildcat (6.3% in 2001; 4.2% in 2002), and CWES cv. Glenlea (10.6% in 2001; 8.6% in 2002). In 2001, OC rates ranged from 0 to 2.8% for CWRS cultivars (mean = 0.6), 0 to 1.5% for CPS cultivars (mean = 0.4), 0 to 10.6% for CWES cultivars (mean = 4.3), and 0 to 0.4% for CWAD cultivars (mean = 0.2). In 2002, OC rates ranged from 0 to 2.0% for CWRS cultivars (mean = 0.7), 0 to 3.5% for CPS cultivars (mean = 1.7), 1.5 to 8.6% for CWES cultivars (mean = 4.1), and 0.2 to 1.4% for CWAD cultivars (mean = 0.5).

Eighteen out of the 35 cultivars tested were prone to OC levels of [greater than or equal to] 1% in at least 1 yr of the study while Rongotea, Glenlea, and Wildcat were prone to OC, with rates of [greater than or equal to] 1% in both years. Out-crossing levels in the CPS class were significantly lower in 2001 (mean = 0.4) compared with 2002 (mean = 1.7) while OC levels in the three other wheat classes were similar in both years. These results suggest that OC rates in wheat are cultivar and greenhouse environment dependent. Twenty of the cultivars tested possessed awns (Table 1). Research in hybrid wheat production has shown that the free-falling movement of pollen is impeded by the awns of male-sterile plants (de Vries, 1971), suggesting that their removal before crossing in our study may have promoted higher OC rates than would have been expected had the awns been retained. In contrast, interspecific barriers to hybridization were likely of greater importance in impeding OC rates in the CWAD class.

In wheat, successful OC depends on the receptivity of the stigma, the viability of the pollen, and availability of pollen during the receptive period (Johnson and Schmidt, 1968). These factors vary with genotype and environment (de Vries, 1971, 1972, 1974). Pollen dispersal during flowering varies with environmental factors including prevailing winds, wind speed, temperature, humidity, and precipitation (de Vries, 1971, 1972, 1974). Higher OC rates have been associated with higher wind speeds and prevailing wind directions during the flowering period of a wheat crop (Hucl and Matus-Cadiz, 2001). De Vries (1972) reported that the highest concentration of pollen dispersal occurred at temperatures between 16 and 20[degrees]C and 70 to 75% relative humidity. Daily humidity and temperature, during anthesis were not recorded in the present study and may have been useful in explaining, in part, the higher OC rates in the CPS class in 2002 relative to 2001 (Table 1). Seed production in 2002 was fourfold higher than in 2001. The higher grain production is evidence of more vigorous plant growth, likely a result of the 67% higher photosynthetically active radiation levels and cooler temperatures experienced with the planting in the fall of 2002.

Direct spike contact combined with the four seeding dates used in our current study, unlike the one (Hucl and Matus-Cadiz, 2001) and three (Hucl, 1996) seeding dates in our previous field studies, maximized the level of flowering synchrony between the donor blue-aleuroned pollen source and recipient cultivars. Hucl (1996) reported maximum OC rates for 11 wheat cultivars ranging from 5.2% (Oslo) to 0.2% (Columbus and CDC Makwa) in a field study. In the present greenhouse study, Glenlea (10.6% in 2001; 8.6% in 2002), Wildcat (6.3% in 2001; 4.2% in 2002), and Genesis (0.2% in 2001; 3.5% in 2002) tended to be prone to OC while Oslo tended toward low levels of OC (0.7% in 2001; 1.2% in 2002). In contrast, Hucl (1996) reported that under field conditions Glenlea (0.6%), Wildcat (1.1%), and Genesis (0.3%) tended to be less prone to OC relative to Oslo (5.2%). Similarly, Hucl and Matus-Cadiz (2001) also reported higher OC rates for Oslo under field conditions (3.2%) relative to the rate reported in our current study. Other research has reported that grain yield in Oslo responds differently to moisture stress under controlled environment (Baker, 1996) versus field conditions (Hucl and Graf, 1990), with Oslo showing changes in genotype rank from one environment to another only under field conditions.

Based on this initial research, some cultivars appear to be consistently low out-crossers regardless of environment while others appear to be prone to OC depending on the environment. In both years, the CWES class had the highest mean OC rates of all classes studied (4.3% in 2001; 4.1% in 2002), in large part due to the cultivars Glenlea and Wildcat. Spike laxness appears to result in a greater degree of floret opening in wheat (de Vries, 1971). The CWES cultivars Glenlea and Wildcat have laxer, fusiform spikes and lower pollen stainability relative to other spring wheat cultivars (Hucl, 1996). Hucl (1996) suggested that these morphological traits may, at least in part, explain the proneness of the two cultivars toward OC under field conditions. Out-crossing rates estimated under greenhouse conditions appear to be poor predictors of OC rates under field conditions. Further research is needed to quantify genetic variability for OC and assess the nature of the G X E interaction in determining OC in wheat.

ACKNOWLEDGMENTS

Appreciation is expressed to L. Ehman, M. Grieman, W. Schatz, K. Jackle, A. Overlid, and S. Campbell for their technical assistance.

Abbreviations: CPS, Canada Prairie Spring; CWAD, Canada Western Amber Durum; CWES, Canada Western Extra Strong: CWRS, Canada Western Red Spring; OC, out-crossing.

REFERENCES

Baker, R.J. 1996. Oslo and Biggar spring wheats respond differently to controlled temperature and moisture stress. Can. J. Plant Sci. 76:413-416.

Briggs, K.G., O.K. Kiplagat, and A.M. Johnson-Flanagan. 1999. Floret sterility and outcrossing in two spring wheat cultivars. Can. J. Plant Sci. 79:321-328.

de Vries, A. p. 1971. Flowering biology of wheat particularly in view of hybrid seed production: A review. Euphytica 20:152-170.

de Vries, A.P. 1972. Some aspects of cross pollination in wheat (Triticure aestivum L). 1. Pollen concentration in the field as influenced by variety, diurnal pattern, weather conditions, and level as compared to the height of the pollen donor. Euphytica 21:185-203.

de Vries, A.P. 1974. Some aspects of cross pollination in wheat (Triticure aestivum L). 4. Seed set on male sterile plants as influenced by distance from pollen source, pollinator: Male sterile ratio, and width of the male sterile strip. Euphytica 23:601-622.

Griffin, W.B. 1987. Out-crossing in New Zealand wheats measured by occurrence of purple grain. N. Z. J. Agric. Res. 30:287-290.

Hucl, P. 1996. Out-crossing rates for 10 Canadian spring wheat cultivars. Can. J. Plant Sci. 76:423-427.

Hucl, R, and R.J. Graf. 1990. Evidence of cross-over interaction involving four spring wheat cultivars, p. 135-142. In New Frontiers in Prairie Agriculture. Proc. Soils and Crops Workshop. 22-23 Feb. 1990. Univ. of Saskatchewan Ext. Press, Saskatoon, SK.

Hucl, R, and M.A. Matus-Cadiz. 2001. Isolation distances for minimizing out-crossing in spring wheat. Crop Sci. 41:1348-1351.

Johnson, V.A., and J.W. Schmidt. 1968. Hybrid wheat. Adv. Agron. 20:199-232.

Matus-Cadiz, M.A, R Hucl, M.J. Horak, and K. Blomquist. 2004. Gene flow in wheat at the field scale Crop Sci. 44:718-727.

Waines, J.G., and S.G. Hegde. 2003. Intra-specific gene flow in bread wheat as affected by reproductive biology and pollination ecology of wheat flowers. Crop Sci. 43:451-463.

Zadoks, J.C., T.T. Chang, and C.F. Konzak. 1974. A decimal code for the growth stages of cereals. Weed Res. 14:415-421.

R. G. Lawrie, M. A. Matus-Cadiz, and P. Hucl *

Dep. of Plant Sciences and Crop Development Centre, Univ. of Saskatchewan, 51 Campus Dr., Saskatoon, SK, Canada S7N 5A8. Received 20 Apr. 2005. * Corresponding author (hucl@sask.usask.ca).

doi:10.2135/cropsci2005.04-0021
Table 1. Out-crossing rates (%) for cultivars crossed in 2001
and 2002 using the direct spike contact method.

                                                          Out-
                                                        crossing

Class ([dagger])      Cultivar           Awn type         2001

CWRS                  AC Abbey           Awned         0
                      Laura              Awned         0.23
                      McKenzie           Awned         0
                      Prodigy            Awned         0
                      Superb             Awned         0.42
                      AC Cadillac        Awnless       0
                      AC Cora            Awnless       0
                      AC Domain          Awnless       1.25
                      AC Eatonia         Awnless       0
                      AC Elsa            Awnless       0.81
                      AC Intrepid        Awnless       0
                      AC Majestic        Awnless       1.52
                      AC Splendor        Awnless       0
                      CDC Bounty         Awnless       0
                      CDC Makwa          Awnless       2.76
                      CDC Teal           Awnless       0
                      5600HR             Awnless       1.47
                      Katepwa            Awnless       0.94
                      Roblin             Awnless       1.26
Mean ([+ or -] SE)                                     0.6a (0.2)
                                                       ([double
                                                       dagger])
CPS                   AC Crystal         Awned         0
                      AC Foremost        Awned         0.27
                      AC Karma           Awned         0
                      AC Taber           Awned         0.67
                      Biggar             Awned         0
                      Genesis            Awned         0.23
                      Oslo               Awned         0.72
                      Rongotea           Awned         1.48
Mean ([+ or -] SE)                                     0.4b (0.2)
CWES                  Bluesky            Awned         0.45
                      CDC Rama           Awned         0
                      Wildcat            Awned         6.28
                      Glenlea            Awnless      10.63
Mean ([+ or -] SE)                                     4.3a (2.5)
CWAD                  AC Avonlea         Awned         0
                      AC Navigator       Awned         0.42
                      Plenty             Awned         0
                      Sceptre            Awned         0.31
Mean ([+ or -] SE)                                     0.2a (0.1)
Statistics
                      Grand mean                       0.9a
                      Standard error
                        ([+ or -] SE)                  0.3
                      Minimum                          0
                      Maximum                         10.6

                                                         Total
                                            Out-       number of
                                          crossing       seeds

Class ([dagger])      Cultivar              2002          2001

CWRS                  AC Abbey           0.22          210
                      Laura              1.24          437
                      McKenzie           1.27          281
                      Prodigy            1.22          278
                      Superb             1.95          237
                      AC Cadillac        0             271
                      AC Cora            0.33          271
                      AC Domain          0.43          319
                      AC Eatonia         0.71          284
                      AC Elsa            0.41          371
                      AC Intrepid        0.33          149
                      AC Majestic        0.25          197
                      AC Splendor        0.19          233
                      CDC Bounty         0.26          310
                      CDC Makwa          0.32          326
                      CDC Teal           1.25          261
                      5600HR             0.69          614
                      Katepwa            0.50          213
                      Roblin             0.98          317
Mean ([+ or -] SE)                       0.7a (0.1)    294 (23)

CPS                   AC Crystal         1.51          423
                      AC Foremost        0.71          369
                      AC Karma           2.31          283
                      AC Taber           1.34          299
                      Biggar             0.68          342
                      Genesis            3.47          427
                      Oslo               1.22          279
                      Rongotea           2.09          338
Mean ([+ or -] SE)                       1.7a (0.3)    345 (21)
CWES                  Bluesky            2.11          442
                      CDC Rama           1.52          212
                      Wildcat            4.18          223
                      Glenlea            8.55          348
Mean ([+ or -] SE)                       4.1a (1.6)    306 (55)
CWAD                  AC Avonlea         0.26           81
                      AC Navigator       1.43          238
                      Plenty             0.33          242
                      Sceptre            0.15          318
Mean ([+ or -] SE)                       0.5a (0.3)    220 (50)
Statistics
                      Grand mean         1.3a          298
                      Standard error
                        ([+ or -] SE)    0.3            16
                      Minimum            0              81
                      Maximum            8.6           614

                                           Total
                                         number of
                                           seeds

Class ([dagger])      Cultivar              2002

CWRS                  AC Abbey           1372
                      Laura              1527
                      McKenzie           1577
                      Prodigy            1316
                      Superb             1178
                      AC Cadillac        1103
                      AC Cora            1215
                      AC Domain          1163
                      AC Eatonia         1696
                      AC Elsa            1224
                      AC Intrepid         906
                      AC Majestic        1201
                      AC Splendor        1034
                      CDC Bounty         1173
                      CDC Makwa          1586
                      CDC Teal           1357
                      5600HR             1167
                      Katepwa            1410
                      Roblin             1325
Mean ([+ or -] SE)                       1291 (47)

CPS                   AC Crystal         1523
                      AC Foremost         987
                      AC Karma            951
                      AC Taber            821
                      Biggar             1326
                      Genesis            1210
                      Oslo               1478
                      Rongotea            764
Mean ([+ or -] SE)                       1133 (104)
CWES                  Bluesky            1328
                      CDC Rama            988
                      Wildcat            1005
                      Glenlea            1193
Mean ([+ or -] SE)                       1129 (81)
CWAD                  AC Avonlea         1170
                      AC Navigator        630
                      Plenty             1204
                      Sceptre            1291
Mean ([+ or -] SE)                       1074 (150)
Statistics
                      Grand mean         1211
                      Standard error
                        ([+ or -] SE)      41
                      Minimum             630
                      Maximum            1696

([dagger]) CWRS, Canada Western Red Spring; CPS, Canada Prairie
Spring; CWES, Canada Western Extra Strong; CWAD, Canada Western
Amber Durum.

([double dagger]) Means followed by different letters within a
row are different at P [less than or equal to] 0.05 using
one-tailed t tests.
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Author:Lawrie, R.G.; Matus-Cadiz, M.A.; Hucl, P.
Publication:Crop Science
Geographic Code:1USA
Date:Jan 1, 2006
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