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Divergent Selection for Polyphenol Oxidase and Its Influence on Agronomic, Milling, Bread, and Chinese Raw Noodle Quality Traits

^a Dep. of Plant Sciences and Plant Pathology, Montana State Univ., Bozeman, MT 59717-3140
^b Southern Agric. Res. Center, 748 Railroad Highway, Huntley, MT
^c Western Triangle Agric. Res. Center, PO Box 974, Conrad, MT 59425

^* Corresponding author (jmmartin{at}montana.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Polyphenol oxidase (PPO) (EC.1.14.18.1) activity has been shown to be related to time-dependent discoloration of Asian noodles. Our objective was to determine the impact of divergent selection for PPO activity on agronomic, milling, bread baking, and Chinese raw noodle quality traits in winter wheat (Triticum aestivum L.) populations. Progeny from seven winter wheat populations were selected for high or low PPO activity by the L-3,4-dihydroxyphenylalanine (L-DOPA) assay on whole kernel meal on the basis of a single replication with equal number of high and low lines in each population. The selected lines were evaluated in replicated trials in multiple environments. Agronomic, milling, bread baking, and Chinese raw noodle traits were measured for all entries. The high versus low PPO classification was confirmed from replicated trials with no overlap between low and high PPO lines for any of the populations. Divergent selection had no impact on agronomic traits except for heading date where low PPO lines averaged 1 d later to head than high PPO lines. Flour ash averaged 0.11 g kg^–1 higher for low compared with high PPO lines, but flour protein and flour yield did not differ between groups. The low PPO selection group had weaker dough, as reflected by lower mixograph tolerance, and lower crumb grain scores. Chinese raw noodle texture was altered by divergent selection so that the low PPO group produced noodles that were less elastic, sticky, and chewy than the high PPO group. Chinese raw noodles became darker (lower L*), more yellow (greater b*), and more red (greater a*) with time. Low PPO lines averaged less change in L* (8.9 vs. 9.6) but more change in b* (–7.70 vs. –7.02) and a* (–0.74 vs. –0.62) than high PPO lines over 24 h. Whether these changes in noodle color and texture are desirable may depend on the type of noodle and regional and personal preferences.

Abbreviations: PPO, polyphenol oxidase • QTL, quantitative trait loci • SDSS, sodium dodecyl sulfate sedimentation

	INTRODUCTION

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WINTER WHEAT produced in the U.S. northern Great Plains has historically been used for bread and bread products. Because of changing market conditions and consumer preferences, wheat produced from this region is now being used for Asian noodles. Breeders have only recently begun to place emphasis on Asian noodle quality as a breeding objective. Asian noodles vary markedly throughout the Pacific Rim region. They differ on the basis of ingredients, presence or absence of alkaline salts, and method of preparation (Hou and Kruk, 1998). Hard wheat from the U.S. northern Great Plains is most suitable for use in Chinese raw noodles where consumers prefer firmer texture and bite characteristics. Both the texture and color of noodles are important in determining the quality of the product. While the texture of noodles varies by region and with individual preferences, it is universally accepted that consumers prefer a bright noodle that maintains its color over time (Hou and Kruk, 1998).

A number of studies have shown the concentration of PPO is related to the degree of time-dependent discoloration of noodles (Baik et al., 1995; Kruger et al., 1994; Park et al., 1997). This enzyme is most prevalent in the bran and more specifically in the aleurone layer (Sullivan, 1946). Thus, PPO enzymatic activity increases from bran contamination with increasing flour extraction rate (Hatcher and Kruger, 1993).

Growing environment influenced PPO activity in wheat grain for cultivars grown in the USA and Australia (Baik et al., 1994) and for hard white cultivars grown in the southern Great Plains (Park et al., 1997). These same studies demonstrated PPO activity had a genetic component, as cultivars accounted for a significant proportion of the variation in PPO activity in the grain. Genetic studies have pointed to genes on homeologous group 2 chromosomes as influencing PPO activity. Jimenez and Dubcovsky (1999) using chromosome substitution lines from three cultivars into ‘Chinese Spring’ showed genes affecting PPO activity resided on chromosome 2A. Mapping studies with recombinant inbred populations also detected quantitative trait loci (QTL) for PPO activity on homeologous group 2 chromosomes for M66/‘Opata 85’ and NY18/‘Clark’s Cream' recombinant inbred populations. Individual QTLs accounted for 18 to 23% of the variation in PPO activity. In addition, significant QTLs were located to homeologous group 3 chromosomes for NY18/Clark's Cream and ND2603/‘Butte 86’ populations. Quantitative trait loci expression varied depending on the environment and substrate used (Demeke et al., 2001).

PPO activity has been shown to exhibit genetic variation and sufficient heritability (Demeke et al., 2001) to be manipulated through breeding efforts. Selection for reduced PPO activity may induce correlated changes in other traits, with the degree of correlated response depending on heritability and degree of genetic association of the trait (Falconer and Mackay, 1996). Aside from the relationships with noodle discoloration noted above, flour PPO activity has been negatively correlated with kernel weight and flour protein in a set of hard red and hard white winter wheat cultivars (Park et al., 1997). Flour PPO activity was also positively correlated with flour ash only for the hard red winter cultivars (Park et al., 1997). Habernicht et al. (2002) found no correlation between PPO level and grain protein and sodium dodecyl sulfate sedimentation (SDSS) volume, a measure of protein quality, in populations of winter wheat segregating for PPO level. Our objective was to determine the impact of divergent selection for PPO activity on agronomic, milling and bread baking, and Chinese raw noodle textural and color traits in winter wheat populations. This information should prove useful in developing breeding strategies for developing wheat cultivars with desired levels of PPO activity.

	MATERIALS AND METHODS

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Seven winter wheat populations were chosen with the criterion that one or more parents in their pedigree differed in PPO activity. The F₂, F₃, and F₄ segregating populations were advanced in bulk at Bozeman, MT, Williston, ND, and Havre, MT, respectively. Greater than 1000 seeds from each population were planted in each generation. Some selection pressure was placed on winter hardiness during the segregating generations where nonwinter hardy plants did not survive. Single heads were selected in the F₅ primarily at random, with selection against undesirable agronomic traits such as extreme height or very late maturity. Head rows were planted in the F₆ (71–129 depending on the population) at the Arthur H. Post Field Research Laboratory near Bozeman, MT, in 1998. Phenotypically uniform head rows within desirable height and maturity ranges were selected from each population (7–16 depending on the population) and harvested in bulk. Polyphenol oxidase activity was determined for all lines using a modification of the L-DOPA assay reported by Anderson and Morris (2001). Approximately 300 mg of UDY-ground (UDY Corporation, Fort Collins, CO) whole meal flour from each genotype was placed in a single well of a 12-well microtiter plate. Durum wheat (Triticum durum Desf.) was used as a standard check in each microtiter plate. A 1.5-mL aliquot of 5 mM L-DOPA in 50 mM MOPS (3-[N-morpholino] propanesulfonic acid) solution, pH 6.5 was pipetted into each well and stirred. Microtiter plates were rotated for 30 min on a reciprocating shaker at 150 rpm. Samples were visually scored on a 0-to-5 scale with 0 = lightest and 5 = darkest, and durum check = 0. High and low PPO lines were selected within each population with low lines representing 1 and 2 and high lines representing 4 and 5 categories on the visual scale. Fifty-eight lines were selected with the number of lines per population varying from six to 12 with an equal number of high and low PPO lines (Table 1). All except two lines had red seed color. The two white seeded lines occurred in the same population (MT91051//Judith/‘CDC Kestrel’), and one was classified as high and the other was classified as low PPO activity. The cultivars Judith (PI 584526), Neeley (CItr 17860), and Rampart (PI 593889) were included to represent high PPO activity cultivars and Meridian (PI 557013), Nuwest (PI 586806), and Promontory (PI 555458) were included to represent low PPO activity cultivars.

Milling and bread-baking tests were performed by approved methods (American Association of Cereal Chemists, AACC, 1995). Flour protein was determined with a Technicon Infra-Alyzer 400 NIR system (Technicon Industrial Systems, Tarrytown, NY) and expressed at a 140 g H₂O kg^–1 flour moisture basis. The combustion method was the reference with a LECO FP-528 (LECO Corp., St. Joseph, MO) nitrogen analyzer (AACC Method 46-30). Wheat was milled on a Brabender Automat Mill (South Hackensack, NJ) after a two-stage temper to 150 g kg^–1 moisture. Mixograph dough properties were evaluated by AACC method 54-40. Tolerance to mixing was scored on a 1-to-8 scale by visually comparing mixographs to standard reference mixograph charts, adjusted for protein content (Pomeranz, 1987). Standard bake tests were conducted by AACC method 10-10B. Bake absorption was determined as the amount of water required to bring dough to proper consistency for bread baking. Bake mixing time was recorded as time to bring dough to minimum mobility. Loaf volume was determined by the volume of canola seeds displaced. Crumb grain was scored on a visual 0-to-5 scale (5 = best) by an experienced baker. Color and texture of Chinese raw noodles were determined from the same flour as used for bread analyses. Chinese raw noodles were prepared from 100 g and 29.2 mL salt (NaCl) water solution (4.29% w/v) added to the flour during a 30-s time period. Mixing continued for 5 min on a National pin mixer (National Mfg. Lincoln, NE). After mixing, dough was rested in a plastic bag for 30 min. at room temperature, then folded and passed through an Ohtake laboratory noodle machine (Ohtake Manufacturing Co., Ltd., Tokyo) three times at a gap size of 5 mm and a temperature of 30°C. After a second 30 min rest at room temperature, dough was successively passed through five progressively narrower roller settings to achieve a final noodle sheet thickness of 1.2 mm. Dough sheets were stored in plastic bags at room temperature and evaluated for color with a Minolta CR 310 Chroma meter (Minolta, Ramsey, NJ) using the Commission Internationale de l'Eclairage (CIE) L* (brightness) a* (red-green) b* (yellow-blue) color system 0 and 24 h after sheeting. More positive values of L*, a*, and b* indicate increasing white, red, and yellow, respectively. Texture profile analysis (springiness, cohesiveness, hardness, and chewiness) was performed on five strands of rinsed noodles 0 and 5 min after cooking with a TA-XT2 Texture Analyzer (Texture Technologies Corp., Scarsdale, NY) assembled with a 6-mm flat lexan probe.

Flour PPO activity was determined by extracting flour with 50 mM MOPS pH 6.5, containing 50 mM SDS in a 1.5-mL Eppendorf tube with a motor unit and pestle fitting the conical bottom of the tube for 30 s. Tubes were shaken for 30 min on a MS1 minishaker (IKA-Works, Wilmington, NC) before centrifugation for 10 min. at 20000 x g (Jukanti et al., 2003). Supernatants (20–100 µL, depending on activity) were transferred to 96 well microplates, and extraction buffer (50 mM MOPS pH 6.5) was added to a total volume of 100 µL. Enzyme assays were started by adding 100 µL of 10 mM L-DOPA substrate solution in 50 mM MOPS buffer pH 6.5. Optical density at 475 mm was assayed kinetically during 20 min with a SPECTRAmax PLUS³⁸⁴ spectrophotometer (Molecular Devices, Sunnyvale, CA). Activities were calculated from the increase in optical density during the linear phase of the reaction (Jukanti et al., 2003).

Because of extreme climatic conditions, which limited seed from each plot, and resource limitations, not all traits were measured in each environment. Heading date and plant height were measured at Bozeman in 2000, 2001, 2002 and Huntley in 2001. Grain yield, test weight, kernel weight, and grain hardness were measured at Bozeman in 2000, 2001, and 2002. Because extreme drought limited grain yield to 886 kg ha^–1 at Huntley and 611 kg ha^–1 at Conrad, seed from the three replications was bulked for each entry at these locations to obtain sufficient seed for subsequent quality evaluations. All flour, bread baking, and noodle quality analyses were performed on two replications from the 2000 and 2001 Bozeman environments, and the bulked seed from the Conrad and Huntley environments where these environments were treated as two replications of a third environment. The flour PPO analyses were performed only for the Bozeman 2000 and 2001 environments from two replications for each environment. The consequences of treating the Huntley and Conrad environments as single replications of a third environment may be that error is partially confounded with environment interactions.

Data for each trait were analyzed via mixed effects analysis of variance by PROC MIXED in SAS (SAS Institute, 1997) using a model for randomized block split plot design combined over environments. Entries within a population (subplots) were partitioned into PPO class and entries within PPO class. Environments, populations, and PPO class and their interactions were considered fixed effects, while blocks within locations and the interaction with populations, entries within PPO class–population combinations and interactions with environments were considered random effects. Tests of significance of fixed effects were accomplished by combining the appropriate linear combination of mean squares utilizing Satterthwaite's (Satterthwaite, 1946) approximation using the DDFM = SATTERTH option with the MODEL statement in PROC MIXED. Difference between high versus low PPO class means within a population were tested by slicing the population x PPO class interaction by population using the SLICE = population option with the LSMEANS statement. Check cultivars were analyzed separately, where all factors except blocks were considered fixed by PROC MIXED in SAS (SAS Institute, 1997).

	RESULTS

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Growing environments were diverse because of extreme climatic conditions. Grain yield averaged 6600, 6506, and 5213 kg ha^–1, respectively for 2000, 2001, and 2002 at Bozeman. Flour protein for the three environments where flour, bread and noodle quality traits were measured averaged 111 g kg^–1 for Bozeman 2000, 114 g kg^–1 for Bozeman 2001 and 136 g kg^–1 for the composite location (grain from Huntley and Conrad). The single replications from Huntley and Conrad averaged 145 g kg^–1 and 126 g kg^–1, respectively.

Agronomic Traits
Interaction of PPO class with genetic background was not important for agronomic traits except for grain yield (Table 2). High vs. low lines did not differ in grain yield when averaged over populations (Table 3). However, one of the seven populations (AC Readymade/Jules) showed high PPO lines yielded greater than low PPO lines (P < 0.01) resulting in the PPO class x genetic background interaction. On average, low PPO lines headed 1 d later than high PPO lines (P < 0.01) There were no differences between high and low groups for plant height, or test weight.

Chinese Raw Noodle Texture and Color Traits
Changes in PPO activity altered Chinese raw noodle texture. Flour from the high PPO selection group produced noodles with higher values for springiness, cohesiveness, and chewiness (Table 6). High PPO lines tended to produce harder noodles than low PPO lines, although that trend was not significant (P = 0.12). Responses for noodle textural traits were consistent across populations and environments (Table 2)

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Our results demonstrate that selection for PPO activity on the basis of the L-DOPA assay proposed by Anderson and Morris (2001) with a qualitative visual scale was able to separate lines into high and low groups in subsequent generations. This demonstrates that PPO activity is highly heritable in these populations. Previous studies estimated heritability of PPO activity ranging between 0.44 and 0.84 depending on the substrate and population (Demeke et al., 2001).

Among agronomic traits, heading date showed the most consistent correlated response to divergent selection for PPO. The correlated response in heading date points to a possible genetic relationship between heading date and PPO. Genetic studies have localized genes affecting both traits on homeologous group 2 chromosomes. Law et al. (1978), Scarth and Law (1983), and Worland and Sayers (1996) have mapped genes controlling photoperiod response, which influences heading date to chromosomes 2B and 2D. Mapping studies using recombinant inbred lines have also placed QTLs for heading date on homeologous group 2 chromosomes (Sourdille et al., 2000; Li et al., 2002). Similarly, Jimenez and Dubcovsky (1999) and Demeke et al. (2001) have mapped genes affecting PPO activity on homeologous group 2 chromosomes.

For the flour and bread quality traits, selection for low PPO, when averaged over populations, resulted in increased flour ash, lower mixograph tolerance score, and lower crumb grain score compared to the high PPO selection group. Mixograph tolerance score is a subjective measurement that incorporates protein quantity and quality as well as dough rheological properties, and is believed to be a measure of dough strength. Since flour protein was not altered consistently, weaker dough in the low PPO group may have resulted from changes in protein quality, which was not measured. We did observe instances where the magnitude and direction of the correlated response was specific to individual populations with no average effect, with bake absorption and loaf volume being examples. Chinese raw noodles were less springy, cohesive and chewy for the low compared to the high PPO selection group. Noodle springiness is an attempt to mimic elasticity, while noodle cohesiveness measures the extent to which a material sticks to itself (Epstein et al., 2002). Chewiness is the product of springiness, cohesiveness, and hardness, and estimates energy needed to disintegrate a substance before swallowing. These components are derived from each other and/or area and length measurements that are not independent. Thus, some components would be expected to change together. Collectively, the textural components attempt to simulate bite and mouth feel. The change toward less elastic, less sticky, and less chewy noodles with selection for low PPO activity may reflect the trend toward weaker dough.

Polyphenol oxidase activity was reported to be negatively related to flour protein among hard red and hard white wheat cultivars (Park et al., 1997). On the other hand, Baik et al. (1994) found a positive association between PPO activity and flour protein in a diverse set of U.S. and Australian cultivars. The small decrease in flour protein for the high PPO group compared to the low PPO group for two of the seven populations and no difference for the remaining populations would indicate the relation between PPO activity and flour protein would not hinder selection of genotypes with low PPO activity and high flour protein. Hatcher and Kruger (1993) and Baik et al. (1994) showed PPO activity was concentrated in the bran fraction of flour, and that PPO activity increased with increased flour extraction rate within a cultivar, and flour ash was linearly related with PPO activity as well. Park et al. (1997) also found flour PPO was positively correlated with flour ash among hard red wheat cultivars. The significant average increase in flour ash for low compared to high PPO selection groups was surprising given that no difference between groups was detected for flour yield. This average difference between groups for flour ash may not be biologically meaningful.

Associations between PPO activity and bread quality and Chinese noodle textural traits have not been extensively reported. If one assumes that PPO activity is inversely related to noodle brightness (L*) at 24 h (brighter noodles having lower PPO activity), then L* at 24 h might be used as a surrogate for PPO activity. Habernicht et al. (2002) found L* at 24 h was inversely related to loaf volume, and Lang et al. (1998) noted a similar relationship but only in one of three environments, implying that loaf volume decreased with increased L* and presumably decreased PPO activity. Our results showed no consistent trend in correlated response for loaf volume with selection for PPO activity. Both studies cited above found L* at 24 h was not statistically related to bake absorption.

Correlated response to selection for PPO was most pronounced and consistent across populations for noodle color traits where divergent selection for PPO altered the noodle color profile so that low PPO lines produced noodles that were brighter but acquired more red and yellow color over time. PPO activity has previously been shown to be positively associated with change in L* and negatively associated with change in a* and b* (Baik et al., 1995). The changes in noodle color profile associated with divergent selection for PPO activity confirm these relationships. It has been widely reported that noodle brightness (L*) declines with increasing grain protein when the relationship was examined among genotypes (Habernicht et al. (2002) and Davies and Berzonsky, 2003) and within a genotype where protein differences were attributed to environments (Habernicht et al., 2002; Souza et al., 2004). It is noteworthy that our results showed noodle brightness was enhanced with selection for low PPO while flour protein was not changed. This supports the hypothesis that there is not a strong genetic relationship between PPO activity and flour protein. Baik et al. (1995) reasoned that change in noodle L* among genotypes that differed in protein concentration was influenced more by genetic differences in PPO activity than by protein concentration, while change in L* within a genotype was influenced more by protein concentration than by PPO activity.

Consumers prefer noodles that are bright in color and maintain that bright color with time. Our results show that selection against high PPO activity can produce changes in the noodle color profile to lessen time dependent discoloration. The mean difference between high and low PPO lines for change in L* was about 0.7 units. It is not known whether consumers would be able to discern such a difference. It is noteworthy that L* after 24 h and change in L* with time were altered, but initial L* was not changed with divergent selection for PPO activity. Environment and genotype play a key role in influencing noodle brightness (Habernicht et al., 2002; Souza et al., 2004) and the environmental variation may over shadow changes in brightness controlled by genetically manipulating PPO activity. The increased yellowness imparted from lowering PPO activity may contribute to increased consumer satisfaction for noodle types where alkaline salts are added, because consumers prefer these noodles with a bright, yellow color. With Chinese raw noodles, consumers prefer a bright, but creamy appearance, and the increased yellow color associated with low PPO activity may be detrimental to product quality.

Our results show that a breeder could genetically manipulate PPO activity. Divergent selection for PPO activity had greatest impact on noodle color profile. Small, but significant changes in noodle textural characteristics were also observed. Whether these changes in noodle color and texture are desirable may depend on the type of noodle and regional and personal preferences. Agronomic traits were not affected by divergent selection for PPO activity, except that low PPO lines headed later than high PPPO lines. This may be a concern if later heading translates into later maturity.

	ACKNOWLEDGMENTS

 
The authors gratefully acknowledge financial support from the Montana Wheat and Barley Committee for this project.

Received for publication December 24, 2003.

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This Article Abstract Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Martin, J. M. Articles by Bruckner, P. L. Search for Related Content PubMed Articles by Martin, J. M. Articles by Bruckner, P. L. Agricola Articles by Martin, J. M. Articles by Bruckner, P. L. Related Collections Wheat Crop Genetics