Crop Science 42:1343-1349 (2002)
© 2002 Crop Science Society of America
PLANT GENETIC RESOURCES
Selection for Potato Genotypes from Diverse Progenies that Combine 4°C Chipping with Acceptable Yields, Specific Gravity, and Tuber Appearance
Ryan J. Hayes and
Christian A. Thill*
Dep. of Hortic. Sci., Univ. of Minnesota, 1970 Folwell Ave., St. Paul, MN 55108-6007
* Corresponding author (thill005{at}umn.edu)
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ABSTRACT
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Breeding with a diverse population may improve potato (Solanum tuberosum subsp. tuberosum) breeding efficiency for cold chipping cultivars. The objectives of this research were to determine if breeding populations differing in assumed diversity influence the (i) frequency of cold chipping genotypes, (ii) ability to make gain in other traits subsequent to cold chipping selection, and (iii) frequency of genotypes combining high performance across many traits. Three populations or mating groups were constructed with increasing assumed diversity: Tw, Tuberosum within (intrabreeding program crosses); Tb, Tuberosum between (interbreeding program crosses); and Td, Tuberosum diverse (incorporates wild species). Potato chip color was evaluated in Year 1 on up to 20 genotypes per 4x x 4x cross. Genotypes having acceptable chip color were tested in Year 2 for tuber yield (TY), specific gravity (SG), and general tuber appearance (GTA). More Tw genotypes had acceptable chip color (1.7%) than Td (0.6%) or Tb (0.4%) and resulted from a better chip color mean and increased variation. More Td genotypes had acceptable TY (83.3%) in Year 2 than Tw (52.00%) or Tb (42.86%), and may result from increased heterozygosity. Lower numbers of effective breeding individuals in Td reduced genetic variation for all traits and reduced retention for SG and GTA. Retention after 2 yr of selection was significantly higher in Tw (0.7%) due to superior cold chipping, than Tb (0.2%) or Td (0.3%). Diversity can impact breeding efficiency, but mating groups did not predict diversity. Intermating large numbers of unrelated but adapted parents that incorporate favorable cold chipping alleles from wild species may improve breeding efficiency.
Abbreviations: CIS, cold induced sweetening • EMS, expected mean square • GTA, general tuber appearance • SG, specific gravity • Tb, Tuberosum between • Td, Tuberosum diverse • Tw, Tuberosum within • TY, tuber yield
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INTRODUCTION
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POTATO CHIPS are an important market for potato producers, with 11% of the 1999 harvested U.S. crop devoted to potato chip production (National Potato Council, 2001). Many varietal traits influence chip quality, such as round shaped tubers of sufficient size, lack of internal and external defects, high total solids or SG, and light golden colored chips with good uniformity across these traits. Potato chip color, however, can be considered as market limiting (Thill, 1994). Chips that are dark in color are unattractive to consumers and impart an undesirable flavor (Watada and Kunkel, 1955; Coffin et al., 1987). Consequently, other traits such as high TY and SG have reduced value without acceptable chip color.
Potato chips have a 4- to 6-wk shelf life, requiring a constant supply of fresh chipping potatoes to supply chip processors. Storage of the fall harvested crop is used to meet this demand during the winter and spring months, exposing the crop to losses from spoilage and shrinkage. Storage of potatoes at cold temperatures (4°C) is a management strategy to reduce storage losses (Sowokinos and Preston, 1988). However, 4°C storage can result in dark colored chips through a process known as cold induced sweetening (CIS) (Sowokinos, 2001). Normally, tubers have low reducing sugars and produce light colored chips directly after harvest. But during CIS, high levels of reducing sugars accumulate and participate in a nonenzymatic browning (Malliard-type) reaction with free amino groups during frying that results in dark colored potato chips (Denny and Thornton, 1940, 1941; Habib and Brown, 1957). Resistance to CIS, or cold chipping, is the ability of a genotype to produce light colored potato chips directly after cold temperature storage. Heritable variation for cold chipping has been well documented in 4x and 2x germplasm pools (Hyde and Walkof, 1962; Lauer and Shaw, 1970; Thill and Peloquin, 1994; Hanneman, 1996). Despite the presence of heritable variation for cold chipping, no cultivars exist that chip acceptably direct from 4°C storage.
Historically, selection for chip color occurs 3 to 4 yr after making the cross (Thill and Peloquin, 1995) with advancement in Years 1 and 2 based upon visual evaluation of tuber traits (Tai, 1975). Consequently, superior chipping genotypes may not be advanced past Years 1 and 2. Selection for chip color in early generations has been proposed as a strategy to increase the efficiency of breeding for cold chipping cultivars (Neele and Louwes, 1989; Thill and Peloquin, 1995; Hayes and Thill, 2000). Selection the first year new genotypes are in the field could save up to 4 yr in the breeding cycle by reducing population size while retaining only cold chipping genotypes, delaying selection pressure for traits with low heritabilities until larger sample sizes can be evaluated, and rapidly identifying superior chipping parents and families (Thill and Peloquin, 1995).
Cultivated potato has been reported to have a narrow genetic base (Mendoza and Haynes, 1975; Love, 1999). A narrow genetic base may limit breeding efficiency by reducing the number of acceptably chipping genotypes retained for further evaluation and limiting variation for other economically important traits needed for genetic gain in subsequent generations. Working with cold chipping, Thill and Peloquin (1995) proposed crossing unrelated parents and use of populations improved by incorporation of wild potato species to increase genetic variation. This may increase the number of acceptably chipping genotypes retained from early generations and allow for gain in other traits subsequent to a previous selection pressure.
The objectives of this research were to determine if 4x x 4x breeding populations with different levels of assumed parental diversity influence (i) the frequency of cold chipping genotypes, (ii) the ability to select for additional traits within previously selected cold chipping genotypes, and (iii) the frequency of genotypes that combine acceptable performance across multiple traits.
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MATERIALS AND METHODS
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Breeding Population Development and Handling
Tetraploid families used in this research were classified into three mating groups based upon the relationship between the parents crossed. When a family was the result of crossing parents that were developed from the same breeding program (intrabreeding program crosses), it was placed into a mating group termed Tw. When a family was the result of crossing parents that were developed from different breeding programs (interbreeding program crosses), it was placed into a mating group termed Tb. A final and third mating group was comprised of families where one or both of the parents crossed was a 4x interspecific hybrid between cultivated potato and wild potato species. This mating group is termed Td. For this research, assumptions were made about the amount of genetic variation present within each mating group (Table 1). The assumptions were based upon the idea that advanced selections and cultivars developed within a common breeding program often have common ancestry and are more related to each other than genotypes from a different breeding program. This would arise through breeders using crosses between elite breeding lines developed from within their own program across multiple breeding cycles. Genetic variation would be higher when parents developed in different breeding programs are crossed than if parents developed in the same breeding program are crossed. Genetic variation might further increase when parents that incorporate wild potato species are used. Therefore, the assumed levels of diversity for the mating groups from least to most were Tw, Tb, and Td (Table 1).
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Table 1. Mating designs for progeny populations for selection of cold chipping ability using parents from the USDA, Aberdeen, ID (ID), Michigan State University (MI), North Dakota State University (ND), Cornell University (NY), and University of Wisconsin-Madison (WI) potato breeding programs.
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Forty-five families were classified as intrabreeding program crosses, or Tw. Botanical seeds for this mating group were obtained by requesting seed samples of cold chipping families from the USDA in Aberdeen, ID, Michigan State University, North Dakota State University, Cornell University, and the University of Wisconsin-Madison breeding programs. The Tb mating group had 62 families from interbreeding program crosses. The parents used to generate these families were current cultivars and breeding lines developed in different potato breeding programs. The Td mating group was comprised of 66 families that were crosses among 4x interspecific hybrids, and between 4x interspecific hybrids and the 4x parents used in the Tb mating group. The interspecific hybrids incorporate the wild potato species S. tarijense Hawkes and the cultivated South American species S. phureja Juz. and Bukasov, and S. tuberosum L. subsp. andigena (Juz. and Bukasov) Hawkes. The families for the Tb and Td mating groups were generated by crosses made at the University of Minnesota, St. Paul, MN, during the winter of 1998.
Botanical seed from the 173 families was treated with gibberellic acid (1 g gibberellic acid L-1 water) for 24 h to break dormancy. Following germination, 72 seedlings per family were transplanted to 8- by 6-cm pots. Plants were grown in a greenhouse under artificial 16-h daylengths in St. Paul, MN, during the fall of 1998 to produce greenhouse grown seedling tubers. The largest tuber from each of 50 randomly selected genotypes per family was used as seed for field planting of single hill progeny (1 plant genotype-1) at Morris, MN, on 24 and 25 May of the 1999 growing season.
Additional progeny were established in 1999 with seedling transplants from botanical seed from 97 families (27 in Tw, 34 in Tb, and 36 in Td) of the original 173 used to generate the greenhouse grown seedling tubers. Ninety-six seeds per family were sown directly into 210-cell (2 cm x 2 cm x 5-cm-deep) plug trays, and after sufficient growth, fifty seedlings per family were transplanted to the field at Morris, MN, on 24 and 25 June 1999.
Families propagated with seedling tubers were planted with all the genotypes from a family in a single plot with plots in random order. Families propagated with seedling transplants were planted in an adjacent field with all the genotypes from a family in a single plot also in random order. Field spacing was 4.5 m between plots, 1.1 m between genotypes within plots, and 1.1 m between rows. Standard cultural practices were used for management of the crop. Supplemental irrigation was not provided. All genotypes were harvested on 12 to 15 Oct. 1999. At harvest, 5196 progeny (1476 from Tw, 1778 from Tb, and 1942 from Td) were sampled by taking a four-tuber maximum sample from each of up to 20 randomly selected genotypes per family. Clonal integrity was maintained by bagging and labeling tubers from each genotype separately.
Cold Chipping Evaluation and Analysis
Genotypes sampled at harvest were stored at 4°C and evaluated for cold chipping ability after 3 and 6 mo of storage. Potato chips were made at the USDA Potato Research Worksite (USDA-PRW) in East Grand Forks, MN. One half of one tuber per genotype was sliced into chips of 1-mm thickness and fried at 185°C for 
90 s in a continuous feed fryer having 1/20 the scale of a commercial operation. Tuber samples were chipped in random order and completed within a week. Chip color was scored for each genotype on a scale of 1 (light) to 10 (dark) with a score of 4 or less being acceptable by industry standards (Thill, 1994).
Frequencies of acceptably chipping genotypes were tabulated for each mating group. Chi square with 2 df was used to test for dependence of frequencies with mating groups from each storage period (Steele and Torrie, 1980). Chip color means after 3 and 6 mo storage were calculated for each mating group, and significant differences among means was determined with pooled two-sample t-tests (Moore and McCabe, 1999). Phenotypic variances for chip color among mating groups were compared with an F-test for equality of spread (Moore and McCabe, 1999).
Agronomic Trials
Progeny from the Tw, Tb, and Td mating groups that chipped acceptably after 6 mo of storage at 4°C were selected for further evaluation of agronomic traits during the 2000 growing season at Morris, MN. The number of genotypes evaluated were 25 from Tw, 7 from Tb, and 12 from Td, along with the commercial cultivars Atlantic, NorValley, and Snowden. Genotypes were planted two plants per plot in a completely randomized design with 27 genotypes having two replications and 19 genotypes having one replicate due to insufficient seed. Spacing of genotypes was 4 m between plots, 0.3 m between plants within plots, and 1.1 m between rows. Planting was accomplished on 16 May 2000 and standard cultural management was practiced during the growing season. Supplemental irrigation was not provided.
Plots were harvested on 19 Sept. 2000. The yield data were collected on a per-plot basis and converted to yield per plant (g plant-1). Specific gravity and GTA data were collected on a per-plot basis. Specific gravity was determined with the equation SG = air weight/(air weight - water weight), and GTA was scored as 1 desirable to 5 undesirable. The GTA scale incorporates several tuber characteristics including eye depth, smoothness, shape, color, and size distribution of harvested tubers.
Analysis of variance was conducted with the GTA, SG, and TY data with the sources of variation in the ANOVA due to mating groups (includes commercial cultivars as a factor level) and genotypes within mating groups. Least significant difference was used to compare means between mating groups and commercial cultivars when the ANOVA indicated mating groups as a significant source of variation.
Genetic variation (Vg) among genotypes within mating groups was estimated from the mean squares of a one-way ANOVA for each mating group separately with genotypes as the only source of variation. The expected mean square (EMS) for genotypes in these ANOVAs is EMS = 
2Error + r2Genetic, with representing variance. Estimates of genetic variation were calculated by setting the genotype mean square from the ANOVA equal to the EMS and solving for the Vg term. Differences among mating group variances were compared using chi square with 2 df as described in Steele and Torrie (1980).
Retention and discard decisions were made among 4°C chipping genotypes for each trait individually (TY, SG, and GTA) and then for all three traits combined. Genotypes were discarded that performed significantly worse than the best performing commercial cultivar. Therefore, retained genotypes were not significantly different, or were significantly better than the best performing commercial cultivar. The best performing commercial cultivars were Atlantic for yield and SG, and NorValley for GTA. Genotypes that were significantly worse than the best performing check were identified by paired t-tests that used a pooled standard deviation calculated with only the genotypes having two replications.
Retention frequencies were recorded for each mating group for TY, SG, and GTA individually and then for all three traits combined. Each 4°C chipping genotype was indicated by 1 = retained, and 0 = discarded for use in one-way ANOVA to test mating groups as a significant source of variation for the percentage retained. Least significant difference was used to compare mating groups for the percentage of genotypes retained when the ANOVA indicated mating groups as a significant source of variation. Z-tests were used to detect differences between mating groups for the proportion of genotypes retained from the initial segregating population after 2 yr of selection for cold chipping and agronomic traits (Moore and McCabe, 1999).
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RESULTS
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Mating groups were significantly different for mean chip color with Tw having the lowest chip color means after 3 and 6 mo of storage followed by Td and then Tb progenies (Table 2). Mating group phenotypic variances were also significantly different. All variances were significantly different after 3 mo of storage (Tw > Td > Tb), and the Tw variance was significantly greater than Tb and Td after 6 mo of storage (Table 2). The Tw mating group had 22 and 25 acceptably chipping genotypes after 3 and 6 mo of storage, respectively, followed by Td with 19 and 12, and Tb with 5 and 7 acceptably chipping genotypes (Table 3). The significant chi square statistics (
2, 2 df = 15.3 - 3 mo and 2, 2 df = 21.5 - 6 mo) indicates that these frequencies show dependence with the three mating groups (Table 3).
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Table 2. Chip color means and variances after 3 and 6 mo of storage at 4°C for the mating groups Tuberosum with (Tw), Tuberosum between (Tb), and Tuberosum diverse (Td) from plants grown at Morris, MN, in 1999.
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Table 3. Frequency and percentage of acceptably chipping genotypes in the mating groups Tuberosum with (Tw), Tuberosum between (Tb), and Tuberosum diverse (Td) after 3 and 6 mo of storage at 4°C from plants grown at Morris, MN, in 1999.
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Agronomic trials of selected 4°C chipping genotypes indicated that mating groups were a significant source of variation for yield (Table 4). Mean yield of Tw was significantly lower than Tb, Td, and the commercial cultivars with no significant difference among the latter three (Table 5). Mating groups did not differ significantly for mean SG or GTA; however, a broad range of means was observed among mating groups for these traits (Tables 4 and 5).
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Table 4. Analysis of variance for tuber yield (TY), specific gravity (SG), and general tuber appearance (GTA) for selected 4°C chipping genotypes within the mating groups Tuberosum within (Tw), Tuberosum between (Tb), Tuberosum diverse (Td) and commercial cultivars grown at Morris, MN, in 2000.
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Table 5. Mating group mean tuber yield (TY), specific gravity (SG) and general tuber appearance (GTA) and ranges of genotype means for selected 4°C chipping genotypes within the mating groups Tuberosum within (Tw), Tuberosum between (Tb), Tuberosum diverse (Td) and 3 commercial cultivars grown at Morris, MN, in 2000.
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Genetic variances were not equal (2, 2 df = 6.6 P-value < 0.05) between mating groups for yield (Table 6). Tuberosum between had the largest amount of genetic variation for TY, followed by Tw and then Td with the least amount of genetic variation. The Td mating group had the least amount of genetic variation for SG and GTA, although the differences were not statistically significant (Table 6).
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Table 6. Estimates of genetic variation among 4°C chipping genotypes within the mating groups Tuberosum within (Tw), Tuberosum between (Tb), and Tuberosum diverse (Td) for tuber yield (TY), specific gravity (SG), and general tuber appearance (GTA) grown at Morris, MN, in 2000.
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Selections were made among 4°C chipping progeny within each mating group for genotypes that were not significantly different or significantly better than the best performing commercial cultivar for the traits TY, SG, and GTA individually and then for all three traits combined (Table 7). The percentage of genotypes retained based upon yield comparison to Atlantic (highest yielding commercial cultivar) was significantly higher within the Td than in Tb or Tw. The percentage retained for GTA (comparisons made with NorValley) and SG (comparisons made with Atlantic) were not significantly different among the mating groups. However, the percentage retained from Td for these traits was noticeably lower than Tw and Tb. Out of all three traits measured, more genotypes were rejected due to poor yields than either SG or GTA. The percentage of genotypes having acceptable performance across all three traits from each mating group was not significantly different.
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Table 7. Number and percentage of 4°C chipping genotypes retained based on tuber yield (TY), specific gravity (SG), general tuber appearance (GTA), and the combination of all three traits from the mating groups Tuberosum within (Tw), Tuberosum between (Tb), Tuberosum diverse (Td) evaluated at Morris, MN, in 2000.
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Considering the initial segregating single hill populations, 19 genotypes were retained after 2 yr of selection. Ninety-nine percent or 5152 genotypes were eliminated after the first round of selection based upon 4°C chipping, while 43% of 4°C chipping genotypes were eliminated the second year. The percentage retained out of the initial seedling generation from each mating group was 0.7% from Tw, 0.2% from Tb, and 0.3% from Td. The proportion retained from Tw was significantly greater than Tb (Z = 2.5) and Td (Z = 2.0). The proportion of genotypes retained from Tb and Td were not significantly different.
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DISCUSSION
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Progeny that chipped acceptably direct from 4°C storage were identified from single hill plants propagated in the field for the first year (Table 3). Sufficient variation for chipping was present in all mating groups after both 3 and 6 mo of storage to select good chipping progeny for future evaluation. Consequently, we were not limited in mating structure for early generation selection of cold chipping genotypes. Early clonal generation selection identified 44 cold chipping progeny 1 yr after making the cross (Table 3) as well as good chipping parents and families, potentially saving up to 4 yr in the breeding cycle.
Since families were not replicated, it was not possible to separate field environment effects from genetic effects. While replication would allow estimation of these two components, the lack of formal replication does not detract from these results since high heritabilities for chip color have been reported: 0.81 and 0.87 (Stevenson and Cunningham, 1961), and 0.77 and 0.86 (Accatino, 1973). There were no consistent trends within the field between chip color mean, phenotypic variance, and frequency of acceptably chipping genotypes, suggesting that the field was largely homogenous for chip color and the environment likely made only a small contribution to the observed variation. Some replication was present in the form of seedling transplants. While differences between progenies propagated with seedling transplants and greenhouse grown tubers were present, the percentage of each mating group propagated as seedling transplants and greenhouse grown tubers were roughly the same (Tw, 28%:26%; Tb, 35%:36%; and Td, 37%:38% transplants:tubers) and would not bias the results concerning mating groups. Furthermore, the propagation method did not cause a rank order change between mating groups, or, large rank order changes between families (data not shown). These factors support the assertion that family and mating group differences were largely genetic, and less environmental.
Mating groups were not equal in the frequency of good chipping genotypes selected from them (Table 3). The dependence between the frequency of acceptably chipping progeny and mating groups seem to result from differences in mating group chip color mean and variance due to the association of these statistics with the frequency of acceptably chipping progeny. These results support the assertion that variation is important in determining the frequency of acceptably chipping genotypes. However, classifying crosses into mating groups did not predict the amount of variation for cold chipping, since the Tw mating group had the greatest variation and frequency of acceptably chipping genotypes. To determine the cause of this result, the pedigree records for Tw families were examined to identify potential sources of variation. While pedigree records were limited in most cases, exotic germplasm was present in all the 4°C chipping genotypes from Tw. In 23 of 25 of the pedigrees, the estimated amount of exotic germplasm was >12.5%. This is in contrast to Tw families, were all progeny were discarded for unacceptable chip color. Here, only 52% of the families incorporated >12.5% exotic germplasm. The exotic germplasm present in the pedigrees of selected 4°C chipping genotypes was likely used specifically to improve cold chipping since it includes, but is not limited to, published sources of cold chipping such as ND860-2 (Ehlenfeldt et al., 1990), NDA2031-2 (Love et al., 1998), and S440 and S438 (Thill and Peloquin, 1999) incorporating S. phureja, S. tuberosum subsp. andigena, and S. tarijense, respectively.
Favorable specific combining ability effects may also explain the cold chipping observed in the Tw mating group. It is possible that breeders contributed families that not only used their best parents, but were also the best combinations of these parents. The Tb and Td mating groups used parents and families that were, at the time of crossing, unselected for cold chipping. Therefore, it is noteworthy that the Td mating group had better cold chipping than Tb and supports the results of Thill and Peloquin (1995). These authors found that crossing unrelated parents may increase the frequency of cold chipping genotypes.
Overall, more cold chipping genotypes were discarded from Tw and Tb due to low yields than any other trait. Alternatively, high frequencies of acceptably yielding genotypes were retained from the Td mating group. Other researchers have found high frequencies of high yielding genotypes when breeding with tuberosum wild potato species hybrid parents (Concilio and Peloquin, 1991). These results suggest that the frequency of high yielding genotypes is an important factor that can influence the efficiency of a potato breeding program.
Tuber yield in potato is reported to be controlled by nonadditive genetic effects with high yields resulting from creation of tri- and tetraallelic loci in progeny (Mendoza and Haynes, 1974). In support of this, heterosis for yield has been reported from use of unrelated 4x and 2x tuberosum wild potato species hybrid parents that increase allelic diversity (Mok and Peloquin, 1975; Cubillos and Plaisted, 1976; De Jong and Tai, 1977, 1991; Tarn and Tai, 1977; Hutten et al., 1994). Given the large amount of literature demonstrating high yields when using tuberosum-wild potato species hybrids, we feel that it is safe to conclude that increased allelic diversity is responsible for the observed yields of Td genotypes. However, the presence of exotic germplasm in the 4°C chipping genotypes from Tw did not result in high yields. While it is beyond the ability of our data to identify the reason for this, it could be due to negative effects of the particular parents used on yield, or repeated crossing of related parents containing the same exotic germplasm as might be done to improve cold chipping.
The Td mating group had considerably less genetic variation for TY, SG, and GTA than either Tw or Tb. This resulted in fewer genotypes retained from Td with acceptable SG and GTA. The amount of genetic variation observed between 4°C chipping genotypes was influenced by the previous selection for cold chipping ability. Consequently, mating groups did not predict the amount of genetic variation within a previously selected population. The reduced amount of genetic variation in Td was likely due to a low number of effective breeding individuals. A factor that can reduce the number of effective breeding individuals is an unequal contribution of families to the next generation (Falconer and Mackay, 1996). Out of the 12 cold chipping genotypes within Td, seven have pedigrees that use six related females incorporating S. phureja that were crosses to only five different males. The other five Td 4°C chipping genotypes use two full-sib males with 25% S. tarijense crossed to four different females. Consequently, the group of 4°C chipping genotypes within Td is a relatively homogenous population of what are possibly quite heterozygous individuals.
Considering the initial segregating population, the Tw mating group resulted in 0.7% of its progeny remaining after 2 yr of evaluation for 4°C chipping, TY, SG, and GTA, which was significantly higher than either Tb or Td. Three main factors emerged from this research that impacted these frequencies. The first factor was the frequency of genotypes chipping acceptably from 4°C storage. The ability to identify 4°C chipping genotypes was most limiting in this research due the low frequency of progeny possessing this trait and supports its initial selection in segregating progeny. Priority needs to be given to breeding schemes and germplasm that increase the frequency of cold chipping genotypes. Secondly, the majority of 4°C chipping genotypes in Tw and Tb were eliminated due to low yields. It would appear that attention to factors that impact heterozygosity is needed to maintain a high number of high yielding genotypes. Lastly, maintaining a heterogeneous population after selection for 4°C chipping also seems beneficial for further gain in SG and GTA. On the basis of these factors, we feel that breeding efficiency at the 4x level for cold chipping cultivars may by improved by intermating a large number of unrelated but adapted parents that are improved for 4°C chipping through introgression of favorable cold chipping alleles from wild species. The consequence would be to increase the frequency of 4°C chipping genotypes and the frequencies of high yielding genotypes while maintaining variation for other traits.
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ACKNOWLEDGMENTS
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Sincere thanks goes to Mr. Martin Glynn and staff at the USDA Potato Research Worksite, East Grand Forks, MN, for assistance in processing of potato chips; to the West Central Research and Outreach Center for cultural management of field trails; and to the potato breeding programs which contributed botanical seed for this research. This research has been supported in part by the Minnesota Agricultural Experiment Station, Red River Valley Potato Growers Association (RRVPGA), Minnesota Area II Research and Promotion Council, and The Snack Food Association (SFA).
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NOTES
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This manuscript is Scientific Journal Series no. 011210080 of the Dep. of Horticultural Science, Univ. of Minnesota, St. Paul, MN.
Received for publication July 11, 2001.
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ABSTRACT
INTRODUCTION
