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Published online 24 January 2006
Published in Crop Sci 46:321-329 (2006)
© 2006 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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GENOMICS, MOLECULAR GENETICS & BIOTECHNOLOGY

Selection for Lodging Resistance in Early Generations of Field Pea by Molecular Markers

Chunzhen Zhanga,*,{dagger}, Bunyamin Tar'anb,{dagger}, Tom Warkentin'b, Abebe Tullub, Kirstin E. Bettb, Bert Vandenbergb and Daryl J. Somersc

a Department of Horticulture, Room 259, Horticulture Hall, Iowa State University, Ames, IA 50011
b Crop Development Centre/Department of Plant Sciences, University of Saskatchewan, 51 Campus Drive, Saskatoon, SK, Canada, S7N 5A8
c Agriculture and Agri-Food Canada, Cereal Research Centre, 195 Dafoe Rd., Winnipeg, MB, Canada, R3T 2M9

* Corresponding author (chz405{at}iastate.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Lodging resistance is a key objective in pea breeding programs. Implementation of marker-assisted selection (MAS) in early generations could significantly enhance the efficiency of the breeding process compared with conventional selection in the F3 or later generations. The objective of this research was to evaluate the effectiveness of MAS for lodging resistance using a combination of a coupling-phase linked marker A001 and a repulsion-phase linked marker A004 in F2 generation field pea (Pisum sativum L.). Eight F2 populations consisting of 680 plants were scored for the markers. A total of 402 F3 families derived from MAS and 187 F3 families from unselected populations were evaluated for lodging reaction under field conditions. The lowest lodging scores for each population were obtained from plants with the combination of A001 marker presence and A004 marker absence. A higher proportion of lodging resistant F3 families was obtained from this marker combination as compared with phenotypic selection in the F3 generation. MAS was less expensive than phenotypic selection in the field. Thus, A001 and A004 are useful for MAS for lodging resistance in early generation pea breeding populations.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
LODGING is usually referred to as a condition in which the stems of crops bend at or near the surface of the ground, which may lead to the collapse of the canopy (Holland, 1990). It is a well-known phenomenon in crops (Lee et al., 1996; Keller et al., 1999). In field pea, lodging is among the major constraints of production. It enhances the canopy microclimate for fungal disease development, reduces photosynthetic ability of the plants, reduces harvest efficiency, and increases harvest cost. For these reasons, lodging can cause up to 74% yield loss in some field pea cultivars (Amelin and Parakhin, 2003). Lodging was correlated with reduced yield in 20 pea cultivars under Saskatchewan conditions (Hashemi et al., 2003).

Lodging resistance in pea is a quantitative trait and is related to morphological traits such as stem stiffness, plant height, and leaf type (Sutcliffe and Pate 1977; Amelin et al., 1991; Tar'an et al., 2003). Direct scoring for lodging resistance in the field can be inconsistent and difficult because lodging is highly affected by environmental conditions such as wind and rain that can occur at different stages of plant development. In general, to effectively score for lodging resistance, breeding lines must be grown in replicated trials in more than one environment to reliably identify genotypes that carry the desirable genes. Thus, it is difficult to select for this trait in early generations in a breeding program because seed supplies are limited and progenies may be segregating for the trait. The use of molecular markers has potential to assist selection in early generation pea breeding lines that carry the genes for lodging resistance. Earlier simulation and experimental studies by Lande (1992), Zehr et al. (1992), Stromberg et al. (1994), and Liu et al. (2004) suggested that MAS would be most effective if it is applied in early generations among progeny from crosses of inbred lines. MAS in early generations allows breeders to discard a large number of lines with inferior genotypes in a inbreeding process and, at the same time, maintain high probability of superior genotypes at homozygosity.

In conventional pea breeding, breeders apply selection for lodging resistance in F3 or later generations since it is difficult to assess on a single plant basis at earlier generations. A large number of families are discarded because of lodging and considerable costs are incurred during the selection procedure. Implementation of selection using molecular markers for lodging resistance in earlier generations would significantly enhance the efficiency of pea breeding process. It could enrich the F3 and subsequent nurseries for lodging resistance and allows more intense selection for other traits of interest.

In a population derived from a cross between Carneval and MP1401, two QTL were identified for lodging resistance. One of these QTL was associated with an AFLP-derived SCAR marker (A001) on LG III of the pea map (Tar'an et al., 2003). This locus had a major effect on lodging reaction and accounted for 47% of the total phenotypic variation of mean lodging reaction across 10 environments. The other locus for lodging resistance is linked in repulsion phase with the A004 marker. This marker accounted for 28% of the total phenotypic variation for lodging reaction in the same population. Previous analysis on cultivars with diverse genetic background indicated that the A001 marker was present in 16 cultivars with good lodging resistance, whereas eight of the nine cultivars with poor lodging reaction had the marker absent (Tar'an et al., 2003). The analysis using A004 marker on the same set of cultivars demonstrated that the marker was present in three out of 16 resistant cultivars. These results suggested that the markers have broad applicability for selecting for lodging resistance in pea.

Several reports have described results of MAS in applied plant breeding and the economic benefit of MAS as compared with phenotypic selection procedures for various traits (Ragot and Hoisington, 1993; Schneider et al., 1997; Van Berloo and Stam 1999; Moreau et al., 2000; Yu et al., 2000; Dreher et al., 2003). However, most of these studies were done at advanced generations, and to our knowledge only limited studies were done in earlier generations.

Earlier studies by Haley et al. (1994) and Johnson et al. (1995) demonstrated that the efficiency of MAS for Bean common mosaic virus (BCMV) and rust resistance in common bean could be increased by a marker linked in repulsion with the resistance gene. A repulsion-phase marker is one linked with the susceptibility allele compared with the more traditional marker linked in coupling with the resistance allele. Selection against a repulsion-phase marker provided a greater proportion of homozygous resistant genotypes and a lower proportion of both segregating and homozygous susceptible individuals than did selection for the coupling-phase marker in F2. The current research described the effectiveness of MAS for lodging resistance in early generation of eight pea populations using two markers (A001 and A004) associated with QTL for lodging resistance. The objectives of this research were to examine lodging reaction of F3 families selected on the basis of marker genotypes in the F2 and to examine the effectiveness of marker-based selection for lodging resistance as compared with conventional selection in the F3 families.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Parents and Population Development
Parental lines were selected on the basis of their lodging reaction and other traits of economic importance. Eight lodging resistant cultivars that had previously been confirmed for the presence of A001 and absence of A004 markers were crossed to a lodging susceptible cultivar Carrera, which was confirmed for the presence of A004 and absence of A001 markers (Table 1). The F1 progeny from these crosses were selfed to produce F2 seeds.


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  		Table 1. Treatments, total number of replications for each population and parental lines.

 
Marker-Assisted Selection
The F2 seeds were space-planted in two-row plots in the field at Saskatoon, SK, in 2002. DNA was extracted from leaf tissue of each individual F2 plants and scored for the presence or absence of A001 and A004 markers, respectively. The F2 plants from each cross were grouped into four classes (T1, T2, T3, and T4) on the basis of a combination of A001 and A004 marker scores (Table 1). Seeds from each surviving F2 plant were harvested to generate F 3 families (approximately 40 seeds per family) that were used for lodging evaluation in the field in 2003. The number of F3 families from each marker class and cross for field evaluation are presented in Table 1. Families with fewer than 40 seeds were omitted from lodging evaluation, since plant stands would have been too thin to effectively assess lodging.

Two additional populations from the crosses Carrera/Carneval and Carrera/DS Admiral were also developed without marker assessment to produce F3 families. These families were used as comparison for testing the efficiency of MAS for lodging.

DNA Extraction, PCR Amplification, and Marker Scoring
DNA was extracted with a DNAeasy Extraction kit (Qiagen Inc., Valencia, CA). PCR was done in a total volume of 25 µL containing 1 x PCR buffer, 3.0 mM MgCl2, 200 M of each of dNTPs, 0.4 M of forward and reverse primers, 50 ng of pea genomic DNA and 1 U Taq DNA polymerase (Life Technology, Invitrogen Corp., Carlsbad, CA). The sequences of the primers that were used to generate the markers are following: A001 (F: 5'-cttcaccatccatagtgtcg-3'; R: 5'-cacttgcgttccttgtgtg-3') and A004 (F: 5'-gcgcatgaaatctaggtttg-3'; R: 5'-cacaagaacgaagaacatcg-3'). Amplification was done with a MJ Thermocycler (Bio-Rad Laboratories, Inc., Waltham, MA) which included a 3 min initial denaturation step at 94°C followed by 35 cycles of 94°C for 1 min, 58°C for 30s and 72°C for 1 min. The PCR products were loaded onto a 1.5% (w/v) agarose gel to separate the bands. The bands were visualized and recorded with the Gel-Doc system (SYNGENE USA, Frederick, MD). The presence or absence of the markers was recorded for each individual F2 plant.

Lodging Evaluation
The F3 families from 10 populations were evaluated for lodging reaction at the Canada-Saskatchewan Irrigation Diversification Centre (CSIDC), Outlook, SK, in the summer of 2003. Forty to 45 seeds were planted in three rows in a 0.8-m2 microplot with 30-cm-row spacing and 80 cm between plots. A completely randomized design (CRD) with four treatments was used. Marker classes were considered treatments for this experiment. Since the A001 and A004 markers segregated independently in the F2 generation following a 9:3:3:1 ratio of two gene model, the number of replications for each marker class varied; therefore, an unequal number of replications was used for lodging evaluation of the F3 families (Table 1). Five to 11 microplots of the lodging resistant parent for each corresponding population and four microplots of lodging susceptible parent (Carrera) were included in the field evaluation. Carrera was also planted as a border surrounding the experimental site to reduce edge effects.

Lodging was rated on a microplot basis and was done twice during the growing season. The first scoring was done before physiological maturity on 7 Aug. 2003, and the second was done at physiological maturity (18 Aug. 2003) by a 1-to-9 scale, where 1 = completely upright and 9 = completely lodged (Table 2; Wang 1998). The microplots with lodging score of 4 or less were classified as lodging resistant families on the basis of the traditional selection method (Wang 1998; Tar'an et al., 2003). Since the differences between the lodging scores of the susceptible and resistant parents were greater at the second assessment than at the first assessment, the lodging scores from the second assessment were used for data analysis.


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  		Table 2. Lodging scale used for the assessment of pea micro plots.

 
Plot Maintenance
The seeds were sown on 22 May 2003. Before planting, the seeds were treated with fungicides VitaFlo 280 (Crompton Co., ON, Canada) and Apron FL (Gustafson LLC, Plano, TX) to control seed-borne and soil-borne fungi. The microplots were irrigated with an automatic sprinkler irrigation system to provide favorable environment for lodging expression. The first irrigation (15 mm) was applied on 2 June 2003. Subsequent irrigations of approximately 25 mm each were applied ten times between 17 July and 11 Aug. 2003 at 2- to 3-d intervals. Total water available for the experimental site was 265 mm from irrigation plus 129 mm from rainfall during the growing season. Necessary herbicides, fungicides, and insecticides were applied during the growing season and a hand weeding was done on 2 July 2003. To improve the nitrogen fixation ability and vigor of the plants, a commercial pea granular inoculant supplied by Becker Underwood (Ames, IA) was planted with the seeds at a rate of 6 kg/ha. Results of soil test showed that fertility was sufficient for pea production in this area.

Other Agronomic Traits
Other agronomic traits including vine length, days to flowering, and days to maturity were recorded on a plot basis during the growing season. Vine length was measured from the ground to the tip of the major stem at physiological maturity. Days to flowering were calculated from the day of sowing to the day when 50% of the plants in the microplot had started flowering. Days to maturity were calculated from the day of sowing to the day when 80% of the pods in the microplots had turned yellow.

Data Analysis
Data were analyzed separately for each population by the GLM procedure of the SAS program (SAS Institute, Cary, NC). The effectiveness of using markers for MAS for lodging resistance was determined by t test of the differences in mean lodging scores between pairs of marker classes. The amount of variation (R2) for lodging reaction being accounted for by a single marker or combination of two markers was analyzed by analyzing the coefficient of determination, i.e., the model sum of squares divided by the corrected total sum of squares.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
A total of 680 F2 plants (ranging from 72–101 plants per population) derived from eight crosses were scored for the presence or absence of A001 and A004 markers. The band size of the A001 and A004 markers was 300 and 180 bp, respectively. An example of the segregation of these markers in the F2 populations is shown in Fig. 1 . {chi}2 Analysis showed that the segregation of A001 and A004 markers followed a two independent gene model (9:3:3:1; 0.1 < P < 0.6) in each F2 population.


Figure 1
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  		Fig. 1. Examples of the segregation of A001 and A004 markers on the F2 progeny.

 
The mean lodging score of parents and each marker class of F3 families revealed a similar trend in all eight populations. The orders of the mean lodging scores for the groups classed by the single marker were as follows: A001(+) < A001(-), with the mean difference 0.6 across the eight populations; and A004(–) < A004(+) with the mean difference 0.7 across the eight populations (Fig. 2 and Table 3). The order of the mean lodging scores for the combination of the two markers was as follows: susceptible parent > T3 > T4 T1 > T2 > resistant parent (Fig. 3 and Table 3). An exception to this trend occurred in Populations 5 and 6, in which the mean lodging scores of T3 (5.9) were slightly greater than the susceptible parent. The mean lodging score of T2 class varied from 3.4 to 5.1, while the mean lodging score of T3 class ranged from 4.6 to 5.9 across the eight populations. t tests showed significant differences (P ≤ 0.05) between mean lodging scores of the T2 and T3 classes in seven out of eight populations and for the mean across eight populations. Significant differences in lodging score were also observed between T1 and T3 classes in five populations, as well as between classes T1 and T2 in four populations. Mid-parental values were similar to the lodging scores of the T1 and T4 classes.


Figure 2
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  		Fig. 2. Comparison of mean lodging score of marker classes for individual markers, across eight populations.

 

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  		Table 3. Lodging score difference among different classes, t test results between marker classes, and the amount of phenotypic variation accounted for (R2) by each marker in F3 families.

 

Figure 3
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  		Fig. 3. Comparison of mean lodging score of marker combination classes across eight populations. T4: [A001(–) A004(–)] was omitted in Populations 1, 6, 7, and 9 because of a lack of F2:3 families in those populations

 
Significant differences were also observed when comparing mean lodging scores of the families on the basis of two-marker combination versus a single marker (Table 3; Fig. 2 and 3). In all populations, the mean lodging score of T2 class was lower or equal to the lodging scores of the families with either A001(+) or A004(–) alone, except in Population 3, in which the lodging score of the families that had the A004 absent (4.6) was lower than that of T2 (4.8). On the other hand, the mean lodging scores of T3 class were greater or equal to the lodging scores of the corresponding class of A001(–) or A004(+) alone, except in Population 7. The differences of the mean lodging scores between T2 and T3 classes varied from 0.7 to 1.6 with an average 1.0 across the eight populations. On average, the difference (1.0) of the mean lodging scores between T2 and T3 classes of the two markers was greater than the difference of mean lodging scores of the families grouped on the basis of a single marker score only. Thus, using the combination of the two markers was more effective than using the individual markers for MAS.

The amount of variation (R2) for lodging reaction of the F3 families being accounted for by the A001 marker ranged from 0.16 to 0.49 in eight populations, while the A004 marker accounted for 0.14 to 0.55. The average of amount of variation accounted for by each of A001 and A004 marker across the eight populations was 0.29 and 0.28, respectively. The amount of variation for lodging reaction accounted for by the combination of the two markers for each population (19–57%) was generally greater than that of the single markers (Table 3). Out of eight populations, the two-marker combination explained the greatest proportion of lodging variation (57%) in Population 1. In four other populations, this combination explained more than 30% of lodging variation, and only in one population did it explain less than 20% of lodging variation. On average, the combination of the two markers explained 34% of lodging variation across eight populations. These results showed that using the two-marker combination for MAS would be more effective than using either single marker. Also, these markers differed in effectiveness for MAS in the different populations. For example, in Population 1, a high R2 was generated from each of the marker classes; while in Population 6 and 7, a relatively lower R2 was generated by the same marker classes. The effectiveness of these two markers for MAS was also supported by the higher proportion of lodging resistant F3 families in the T2 class as compared with conventional selection at the F3 (Table 4 and Fig. 4 ). From the 8 populations that were evaluated, the proportion of lodging resistant F3 families in the T2 class ranged from 55 to 75%, whereas much lower proportions of resistant F3 families (24 and 33%) were obtained from the two populations used for conventional selection.


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  		Table 4. Relative proportion of lodging resistant F3 families (lodging score 1–4) from T2 class [A001(+) A004(–)] of the eight populations subjected to MAS as compared with two unselected populations.

 

Figure 4
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  		Fig. 4. Distribution of F3 families from two unselected populations on the basis of their lodging score in the 2003 field trial at Outlook, SK. Lodging score of 4 (vertical line) was used as the threshold value for phenotypic selection for lodging resistance.

 
Correlation analysis showed that lodging score was significantly negatively correlated with vine length in two (Populations 2 and 8) out of eight populations. A significant negative correlation was also observed between lodging and days to flowering in two populations (Populations 1 and 2), and between lodging and days to maturity in two populations (Populations 1 and 8). However, a significant positive correlation between days to flowering and lodging was observed in Population 5. Significant correlation (positive) between maturity and vine length was observed only in Population 8.

The means of vine length, days to flowering, and days to maturity for each parental line are shown in Table 5. The vine length of the lodging resistant parents ranged from 50 to 59 cm, whereas the vine length of Carrera was 48 cm. Days to flowering among the lodging resistant parents ranged from 47 to 52 d. The days to flowering and days to maturity for Carrera were 48 and 78 d, respectively. The days to maturity for the lodging resistant parents ranged from 76 to 83 d.


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  		Table 5. Mean vine length, days to flowering and days to maturity of each parent.

 
The mean vine length varied from 48.4 to 57.4 cm across the eight populations (Table 6). The population mean for vine length was generally within the range of the two corresponding parents. The mean vine length difference between A001(+) and A001(–) marker classes varied from 0.9 cm to 9.7 cm with an average 5.2 cm across the eight populations, while A004(+) and A004(–) marker classes varied from 0 cm to 7.2 cm with an average 2.9 cm across the eight populations. Significant differences between the A001(+) and A001(–) marker classes were observed in six populations. However, significant differences between the A004(+) and A004(–) classes were observed only in two populations. On the basis of the combination of the two markers, significant differences in mean vine length between T2 and T3 classes were observed in five populations (Populations 2, 4, 6, 7, and 8). The amount of variation (R2) for vine length accounted for by A001, A004 and the combination of the two markers ware ranged from 0.04 to 0.39, 0.04 to 0.19, and 0.04 to 0.43, respectively. On average the A001, A004, and the combination of the two markers explained 19%, 12% and 22% of vine length, respectively, across the eight populations (Table 7).


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  		Table 6. Mean vine length, days to flowering and days to maturity for each marker classes of F3 families and population.

 

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  		Table 7. Vine length difference among different classes, t test results between marker classes and the amount of phenotypic variation accounted for (R2) by each marker in F3 families.

 
The mean number of days to flowering varied from 46.5 to 49.5 d across the eight populations. On average across the eight populations the plants matured in 76.4 to 80.0 d (Table 6). Similar to vine length, the trends in mean days to flowering and days to maturity were generally within the range of the two corresponding parents. Significant differences in days to flower between the A001(+) and A001(–) marker classes were observed only in Populations 3 and 5, whereas significant difference in days to mature between the A004(+) and A004(–) classes were observed only in Populations 5 and 7. Similar results were observed when analyzing the combination of the two markers, i.e., significant differences in days to flowering between T2 and T3 classes were observed in Population 3 and 5, whereas, for days to maturity significant differences between T2 and T3 classes were observed on Population 5 and 7 (data not shown).


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The marker analysis across eight F2 populations confirmed the association of A001 and A004 markers with lodging resistance. The A001 and A004 markers segregated following a two independent gene model in all populations. The A001 marker was associated with allele for lodging resistance and A004 associated with allele for lodging susceptibility. Evaluation of lodging reaction and marker scores of parents are necessary before using these markers for MAS in segregating populations.

The effectiveness of the A001 and A004 markers for MAS for lodging resistance in F2 was evidenced by the significant differences in lodging score among marker classes in the F3 families. The F3 families with A001(+) had improved lodging resistance in seven out of eight populations, whereas the F3 families with the A004(–) had improved lodging resistance in six out of eight populations (Table 3). Both A001 and A004 markers explained an intermediate to a relatively large amount of lodging variation for each population, which indicated that the markers were effective for selection for lodging resistance. Although the individual A001 or A004 marker was effective in most populations, using a combination of the two markers resulted in a higher proportion of the lodging variation being accounted for by the markers. By using the two markers simultaneously for selection, the most lodging resistant class was T2 for which both alleles for lodging resistance were present. The most lodged families were obtained from T3 class for which both alleles for lodging resistance were absent. The mean lodging score for T2 class was significantly lower than that for T3 class in all populations except Population 7. Since the expected frequency of T2 in F2 population is 3/16, up to 13/16 of the F2 plants in the population without the desired marker combination could be discarded. The high proportion of lodging resistant F3 families in the T2 class also supported the effectiveness of MAS. From the eight populations evaluated, the proportion of lodging resistant F3 families in the T2 class ranged from 55 to 75%, whereas much lower proportions of resistant F3 families (23 and 31%) were obtained from conventional selection in two populations.

The mean lodging differences between the A001(+) and A001(–) classes, between A004(+) and A004(–) classes, as well as between the T2 and T3 classes were –0.6, 0.7, and 1.0, respectively, across the eight populations. Compared with the mean of all the populations, T2 class had a 0.5 lower lodging score across the eight populations (Fig. 3 and Table 3). The 1-to-9 lodging scale has been used for more than 20 yr in pea breeding assessments in Canada and Europe. However, no important cultivars remain in production which have mean lodging scores at physiological maturity of 8 or 9. In addition, no cultivars have been developed thus far that have mean lodging scores as good as 1 or 2. Therefore, in a practical plant breeding sense, the 1–91-to-9 scale is reduced to a 3-to-7 scale, i.e., a five point ordinal scale. Plant breeding efforts, with or without the use of markers, which reduce the long-term mean lodging score by one unit on the de facto five point ordinal scale (equivalent to a 20% improvement), are valuable contributions indeed (Zhang, 2004). Even a 0.5-unit improvement will be economically beneficial.

The total estimated consumable cost of marker analysis for MAS in the current study is approximately $2.00 CAD per sample. Approximately 200 samples could be analyzed in 1 d by one technician at a total labor cost of approximately $160.00 CAD. Thus, the total cost of the two-marker analysis per sample is $2.80 CAD. The total cost of evaluating one F2:3 family in a microplot is approximately $5.00 CAD. Thus, if MAS allows for the discarding of undesirable marker classes in F2, an overall saving to breeding programs should be realized.

The effectiveness of the A001 and A004 markers for MAS was affected by the lodging performance of the parental cultivars. The greater the difference in lodging scores between the two parental cultivars the more effective the markers were for MAS. For instance, in the eight populations assessed, the lodging differences between the two parents were greater than 1.5 in Populations 1, 2, 3, 4, and 8. For Populations 1, 2, and 3, the lodging differences between single marker classes were highly significant. In Populations 6 and 7, in which the differences between parental lodging scores were 1.5 and 1.3, respectively, the lodging differences between single marker classes were generally insignificant. Population 5 (Carrera x Integra) was an exception to the above trend. In this population, the parental lodging difference was relatively small (1.3); however, the lodging differences between single marker classes were highly significant. Thus, the effectiveness of MAS was greater in populations in which the parents had large differences in lodging scores.

In the current research, a similar trend in the lodging performance of the parental cultivars and classes of marker combinations across populations was observed. This trend was susceptible parent > T3 > T4 T1 > T2 > resistant parent. The T2 class that had the lowest mean lodging score had both alleles for lodging resistance present at the two loci. These results supported the initial finding that there are at least two loci affecting lodging reaction in pea (Tar'an et al., 2003). However, the mean lodging score of the T2 class was not superior to the corresponding lodging resistant parent. There could be two reasons for this result. First, the amount of variability (R2) for lodging reaction that was accounted for by the markers was limited. For example, the highest R2 across the eight populations was 0.57. In the original population from which the markers were identified, the R2 was 0.59 (Tar'an et al., 2003). These results suggested that there could be other factors related to lodging resistance, which these markers could not explain. Second, some F2 plants were heterozygous for the A001 locus in the T2 marker class. This may have reduced the effectiveness of MAS. Additional marker analyses of individual F3 plants randomly selected from the T2 class of Populations 2, 5, and 6 showed that the A001(+) class segregated into A001(+) and A001(–), whereas the A004(–) did not segregate (data not shown).

The lodging trial was conducted at Outlook, SK, under a sprinkler irrigation to provide favorable conditions for the expression of lodging. However, the lodging differences between the susceptible and the resistant parents were not as large as expected. Potentially the plots were lacking nitrogen at the pod-filling stage of the season. Although the soil test had indicated that N was sufficient at the time of planting, it may have been leached out during 11 applications of irrigation, and no top-dressed N was added during the growing season. As a result, the pea plots were not as vigorous as they could have been, and lodging was not expressed as obviously as expected under ideal conditions. For instance, in the 2001 growing season, the average vine length of Carrera was 65 to 70 cm and its lodging score was 6 to 7 (Canada-Saskatchewan Irrigation Diversification Centre, 2001a), but in the current trial, the average vine length of Carrera was 48 cm and the average lodging score was 5.7. The high temperature during the growing stage between flowering to pod filling periods may have also affect the lodging expression. Samarin (1975) and Obraztsov and Amelin (1990) demonstrated that the growing conditions at the pod-filling stage are critical to lodging expression. Lodging is usually correlated with vine length and the weight of pods at physiological maturity. Because of suboptimal physiological growth, the seed yield (1540 kg/ha) was also lower than average pea seed yield (4000 kg/ha) under irrigated conditions in the research trial area (Canada-Saskatchewan Irrigation Diversification Centre, 2001b).

In the current study, a significant negative correlation between lodging score and vine length was observed in two out of eight populations. These results were in contrast from the general understanding that taller plants tend to lodge more. However, this result was consistent with the population derived from MP1401 x Carneval, in which lodging score was negatively correlated with vine length (r = –0.59; P < 0.001; Tar'an et al., 2003). Knyaz'kova (1987) also found that some taller lines had better lodging resistance than some shorter lines in a pea inbred population. There might be a balance among the factors such as vine length, stem stiffness, and lodging performance in pea. Some studies suggest the existence of this balance; for example, Obraztsov and Amelin (1990) indicated that the optimum height for lodging resistant pea plants was 60 to 90 cm. Taller or shorter plants were inferior for lodging resistance and yield. McPhee and Muehlbauer (1999) reported that stem strength was positively correlated with internode length (r = 0.36, P < 0.001) among 418 Pisum accessions. In the current study, the lodging susceptible parent was shorter than most lodging resistant parents. In Populations 2 and 8, in which a significant correlation was observed between lodging and vine length, the mean vine length of the lodging susceptible parent (Carrera) was 44 cm, while the mean vine length of the lodging resistant parents (Carneval and MP1101) was 55 cm and 50 cm, respectively. In the current study, A001 marker also explained vine length variation in most of the eight populations (Table 7); these results are consistent with the population in which the A001 marker was identified (Tar'an et al., 2003). Lodging was also correlated with days to flowering and days to maturity in a few populations, but clear trends were not evident.

Lodging resistance is a quantitative trait and many genes might be involved in the expression of this trait. Markers associated with the QTL for this trait have different efficiency in different breeding populations and may not be efficient in some populations. For instance, the difference in lodging scores between A001(+) and A001(–) classes was only significant in seven out of eight populations, and that between A004(+) and A004(–) classes was only significant in six out of eight populations. One QTL can only partially explain lodging variation in any breeding population. Thus, combining two or more markers for lodging resistance for MAS is important to obtain maximal response. Moreover, phenotypic selection should be used when QTL only explain a low percentage of phenotypic variation. Keller et al. (1999) indicated that the most efficient way to improve lodging resistance would be by a combination of indirect selection on vine length and stem stiffness, together with MAS using the markers associated with QTL for lodging resistance that did not coincide with QTL for other morphological traits. With advances in pea genomic research, gene(s) controlling properties associated with lodging resistance could be identified. Future MAS may use the gene itself as a genetic marker.


   ACKNOWLEDGMENTS
 
Thanks to Brent Barlow, Carmen Breitkreutz, Scott Ife and Terry Hogg for their technical assistance with the field trials. Financial assistance from the Agricultural Development Fund of Saskatchewan Agriculture, Food and Rural Revitalization, and from the Saskatchewan Pulse Growers is greatly appreciated.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Submitted as partial fulfillment of the M.Sc. degree by the senior author at the University of Saskatchewan.

{dagger} These authors contributed equally to this work. Back

Received for publication February 6, 2005.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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