Inheritance of Tuber Greening Under Light Exposure in Diploid Potatoes

Jakuczun, Henryka

ABSTRACT

Low tendency to tuber greening under light exposure is a desirable trait for marketing and processing potatoes. Knowledge of the genetic basis of this trait is limited. During two consecutive years tuber greening after 2-week light exposure was evaluated on 17 unselected diploid potato families obtained between parents with varying tuber-greening tendencies. Parents of these families and standard cultivars were evaluated at the same time. External and depth of tuber greening were examined after tubers were exposed to 2 weeks of light in the greenhouse followed by storage at 10 C, 4 C, and after reconditioning at 18 C. Families, storage conditions, and years significantly influenced tuber greening. Distribution of greening indicated it would be possible to select individuals with very low tuber-greening tendency. Generally, external tuber greening was more intense than depth of greening. External tuber greening increased during storage while depth of tuber greening decreased. A significant maternal effect on external tuber greening was found in one of the three sets of reciprocal crosses after storage at 4 C. Both general and specific combining abilities were important in the inheritance of tuber greening. The correlations between external and depth of tuber greening in most families were significant and relatively high. The results indicated that genotypes with low tendency to tuber greening can be selected in diploid families.

RESUMEN

Un carácter deseable de la papa para comercialización y procesamiento, es su baja tendencia al verdeo de los tubércules por exposición a la luz. El conocimiento sobre la base genética de esta característica es limitado. El verdeo fue evaluado durante dos años consecutivos en 17 families de papa diploide no seleccionada, obtenida de progenitores con diferentes tendencias. Se evaluaron al mismo tiempo los progenitores de estas familias y los cultivares estándar. El verdeo externo y profundidad en el tubérculo fue examinado después de que los tubérculos fueron expuestos a dos semanas de luz en el invernadero y luego almacenados a 10 C, 4 C y después que se reacondicionaron a 18 C. Los factores como familias, condiciones de almacenamiento y años influenciaron significativamente el verdeo de los tubérculos. La distribución del verdeo indicó que sería posible seleccionar individuos con tendencia mínima a esta característica. Generalmente el verdeo externo del tubérculo fue más intenso que la profundidad. El verdeo superficial aumentó con el almacenamiento, mientras que la profundidad disminuyó. En uno de los tres grupos de cruzamientos recíprocos se encontró un efecto significativo del progenitor femenino sobre el verdeo externo del tubérculo después del almacenamiento a 4 C. Tanto las aptitudes de combinación general como específica fueron importantes en la herencia del verdeo de los tubérculos. La correlación entre el verdeo externo y profundidad en el tubérculo, en la mayoría de las familias fue significativa y relativamente alta. Los resultados indican que los genotipos con baja tendencia al verdeo del tubérculo pueden ser seleccionados en familias diploides.

Accepted for publication 11 November 2005.

ADDITIONAL KEY WORDS: External and depth of greening, maternal effect, GSA, SCA, tuber treatment

INTRODUCTION

Potato is one of the main crops in the world. In Poland people consume on average approximately 130 kg of potatoes per year. In developing a new cultivar, about 50 resistance and quality traits should be tested in the average 10- to 12-year breeding cycle. Although resistance to tuber greening under light exposure is a desirable trait for marketing, little is known about the genetic basis of this quality trait. Potatoes marketed for table use and for processing are kept in the light, which induces tuber greening. This trait reduces the marketable and processing value of potatoes. Until now cultivars with nongreening tubers have not been bred, although Brune and MeIo (2001) reported that two tetraploid clones had a very low tendency for tuber greening and two other clones were moderate for this trait.

Tuber greening is caused by the formation of chlorophyll upon exposure to light, initially just under the periderm and then in the tuber flesh. Transformation of non-green plastids into chloroplasts is common in the plant world (Thomas 1980; Pyke 1999; Vothknecht and Westhoff 2001). Greened tubers are associated with an accumulation of glycoalkaloids, which are poisonous for people and animals (Edwards et al. 1998; Griffiths et al. 1998; Percival 1999). Many factors can influence tuber greening, such as light (Liljemark and Widoff 1960; Yamaguchi et al. 1960; Akeley et al. 1962; Brown and Riley 1976), maturity of tubers (Buck and Akeley 1967), time and temperature of storage (Griffiths et al. 1998), production treatments (Lewis and Rowberry 1973), or tuber size (Parfitt and Peloquin 1981). The relationship between skin color and type and tuber greening was investigated by Harkett (1975), Brown and Riley (1976), and Reeves (1988).

Although tuber greening is caused by environmental factors, it has a genetic component. Variability in tuber greening intensity among cultivars and stocks has been observed (Akeley et al. 1962; Buck and Akeley 1967; Brown and Riley 1976; Parfitt and Peloquin 1981; De Maine et al. 1988; Reeves 1988; Dale et al. 1992; Jakuczun 1993; Griffiths et al. 1994). Reeves (1988) reported polygenic inheritance for this trait. Parfitt and Peloquin (1981) reported that additive and epistatic effects were important and found no maternal effects for this trait. Reeves (1988) distinguished three elements of tuber greening: external, internal, and depth of greening, which were found to be independently inherited.

Generally intensity of tuber greening was evaluated visually on a 1-5 (Akeley et al. 1962; Parfitt and Peloquin 1988), 0-5 (Reeves 1988; Jakuczun 1993) or 1-9 scale (Griffiths et al. 1994; Brune and MeIo 2001). The correlation between visual estimation of greening and chlorophyll accumulation has been high (Akeley et al. 1962; Harkett 1975; Dale et al. 1992; Griffiths et al. 1994).

In the Plant Breeding and Acclimatization Institute in Ml[bar]ochów, as part of a program involved in long-term recombinant breeding of diploid potatoes, genotypes expressing resistance to tuber greening upon exposure to light under greenhouse conditions were selected (Jakuczun 1993). They were composite interspecific hybrids with several wild Solanum species and dihaploids of S. tuberosum in their pedigree.

The aim of this experiment was to study the inheritance of external and depth of tuber greening due to chlorophyll accumulation under light exposure in unselected diploid families.

MATERIAL AND METHODS

The same plant material and experiment conditions used to evaluate tuber greening in this experiment were simultaneously used to examine the inheritance of glucose content in tubers (Jakuczun and Zimnoch-Guzowska 2004).

The material included 17 unselected families, their eight parents, and standard cultivars. In two parental clones, DG 88-63 and DG 88-89, a weak external tuber greening (EWG) and small depth of tuber greening (DWG) were found. External (EMG) and depth of tuber greening (DMG) were moderate in DG 88-1616. DG 88-214 and DW 88-4556 were characterized by strong external tuber greening (ESG) and moderate depth of greening (DMG). The remaining three clones, DG 86-965, DG 88-632 and DG 88-1591, expressed strong external tuber greening (ESG) and depth of greening (DSG). These diploid parents were interspecific Solanum hybrids, the origin of which has been described previously by Jakuczun and Zimnoch-Guzowska (2004). Considering external tuber greening, six families were obtained in matings between parents susceptible and resistant to greening (ESG x EWG), another six families were from crosses between parents moderate and susceptible to greening (EMG x ESG), and five families were from crosses between parents susceptible to greening (ESG x ESG) (Table 1). For depth of tuber greening two families were obtained in crosses between parents resistant and susceptible to tuber greening (DSG x DWG), nine families came from matings between parents moderate and susceptible to greening (DMG x DSG), two families were obtained between parents with moderate tendency to tuber greening (DMG x DMG), and the remaining four families were from matings between parents with moderate and weak depth of greening (DMG x DWG) (Table 1). Generally, females were strongly greened in external and strongly to moderately in depth of tuber greening. Males ranged from having a weak to strong tendency to both types of greening and were important source of variation of the trait in this experiment. Three seed parents were crossed to four pollinators in a North Carolina (NC) design II resulting in 10 families; no progenies were obtained in the other two families. Reciprocal crosses represented another three sets of families. The number of genotypes in the families varied from 35 to 158. In total, 1,229 genotypes were evaluated each year.

The Polish cultivars Irys and MiIa and Dutch cultivars Bintje and Saturna were included in the study as standards. Bintje expressed weak to moderate external and depth of tuber greening. Irys and MiIa expressed strong external and depth of tuber greening. Tubers of Saturna greened strongly on the surface and moderately in depth.

The experiment was performed during 1995 and 1996: in the first year on seedlings grown from true seeds and in the second year on tuber progeny. In 1995 the seedlings were grown in the field in one-lull plots. In 1990 the tuber progeny were grown in four-hill plots without replications. Harvest was by hand in the beginning of October in 1995, and in the beginning of September in 1996. In both seasons tubers were preconditioned in regular storage for two weeks at 10 C. Then, three samples of tubers from each genotype were put in the following storage conditions: 10 C for 3 months (treatment A), 4 C for 5 months (treatment B), and 4 C for 5 months with reconditioning at 18 C for 2 weeks (treatment C).

Tubers were exposed to 2 weeks of daylight in the greenhouse after these three storage treatments. Two elements of tuber greening (external and depth) were evaluated visually according to the 0-5 scale developed by Reeves (1988), where O indicates lack of tuber greening, and 5 indicates tubers were very green. Intensity of tuber greening in the unselected families was compared to the parents and standards.

Data were statistically analysed by Statistica, MSTAT-c, DGH (Kala et al. 1996) and SERGEN 3 (Calinski et al. 1998). Family means and standard deviations were calculated. Repeatability of the trait was estimated by Pearson's correlation coefficients (r). Family means were compared with mid-parent (M-P) values by U'Mann-Whitney's test (Lomnicki 1995). Analyses of variance were performed for parents and standard cultivars and for families. Effects of years were considered random and effects of treatments and families were considered fixed. Thus expected mean squares for the analyses of variance of families were estimated as follow: year (Y) = σ^sup 2^^sub e^ + sty σ^sup 2^^sub f^; treatment (T) = σ^sup 2^^sub e^ + s σ^sup 2^^sub fyt^ + sf σ^sup 2^^sub yt^ + sfy σ^sup 2^^sub t^; family (F) = σ^sup 2^^sub e^ + s σ^sup 2^^sub fyt^ + st σ^sup 2^^sub fy^ + sty τ^sup 2^^sub f^; Y x T = σ^sup 2^^sub e^ + sf σ^sup 2^^sub yt^; F x Y = σ^sup 2^^sub e^ + st σ^sup 2^^sub fy^; F x T = σ^sup 2^^sub e^ + s σ^sup 2^^sub yt^ + sy τ^sup 2^^sub ft^; F x Y x T = σ^sup 2^^sub e^ + s σ^sup 2^^sub fyt^; error = σ^sup 2^^sub e^; where s = number of tested tubers, f = number of families, t = number of treatments, y = number of years. Significances of treatments' and families' effects were estimated by denominator degrees of freedom (Satterwaite's approximation).

Analysis of variance based on 2-year means of three reciprocal sets of families was done to study maternal effects on tuber greening. The general (GCA) and specific (SCA) combining abilities of parents were estimated from the 10 families obtained in a NC design II mating scheme based on 2-year means. Two of the 12 families were missing for a complete NC design II, and values for them were estimated by the MISVALENTS function of the MSTAT program. General analysis of combining ability and estimation of GCA and SCA effects were made using SERGEN 3 (Calinski et al. 1998). Maternal effects and combining abilities were calculated on 2-year means of families represented by 60 individuals. In families 95-9 and 95-17 with lower number of individuals, the number was completed with missing values and mean values of the traits in families.

RESULTS

Influence of Years, Families and Tuber Treatments on Tuber Greening

Analysis of variance on the parents and standards cultivars showed there was no significant year, tuber treatment, or year x tuber treatment interaction effects for either external or depth of tuber greening (Table 2), although for some genotypes significant variability of greening between years was observed. Correlation coefficients (r) between years for parents and standard cultivars were highly significant for both external and depth of tuber greening under all three storage conditions. Correlation coefficients were 0.93**, 0.55**, and 1.0** for external greening and 0.75**, 0.68**, and 0.68** for depth of greening following storage at 3 months at 10 C, 5 months at 4 C, and after reconditioning after 5 months at 4 C, respectively (Table 3).

Analysis of variance indicated a significant effect of families, years, and the interactions of families, treatments, and years, for external tuber greening in the 17 unselected families, while all the effects and interactions (except families x treatments) were significant for depth of greening (Table 4). Generally, both external and depth of tuber greening were greater in 1995, the first year, than in 1996, the second year (Tables 5, 6). For external tuber greening such significant differences were observed in eight families after treatment A, 11 families after treatment B, and two families after treatment C. In none of the families was external tuber greening significantly greater in the second year than in the first year. Significantly greater tuber-greening depths were found in the first year than in second year of evaluation in eight families after treatment A, three families after treatment B, and six families after treatment C. In treatment C tuber-greening depth was significantly greater in the second year of testing in four families. In general, correlation coefficients between the two years of testing within families for both elements of greening were significant but rather low to moderate (Table 3).

External tuber greening ranged from O to 5 in all families except one in 1995. Family mean of external tuber greening did not differ significantly from the mid-parent value in any of the families under treatments A and C, and was significant in only one family under treatment B. Two-year mean external greening in the ESG x EWG families was significantly lower than in the ESG x ESG families for all the treatments (Table 7). In addition, ESG x EWG families had significantly less external greening than EMG x ESG families under treatment A. EMG x ESG families had significantly less greening than ESG x ESG families under treatment C.

Likewise, depth of tuber greening ranged from O to 5 in all families except one in 1995. Family mean of depth of greening did not differ from mid-parent value in any of the families under any treatment. Two-year means of depth of greening was similar under all treatments for the DMG x DWG, DSG x DWG and DMG x DMG families (Table 7). In DMG x DWG families, depth of greening was significantly less than in DMG x DSG families under all treatments. In addition, in DSG x DWG and DMG x DMG families, depth of greening was less than in DMG x DSG families under treatment B and C.

In this experiment storage time and temperature have to be considered together as environmental factors. In general, external tuber greening slowly increased with prolonged tuber storage whereas depth of tuber greening decreased, and greening was greater for external in comparison to depth of tuber greening in all three treatments. External tuber greening differed significantly in ESG x EWG families between treatments A and C (Table 7). Depth of tuber greening differed significantly in all groups of families between treatments A and C, in DSG x DWG and DMG x DMG families between treatments A and B, and in DMG x DSG families between treatments B and C (Table 7).

Some of the progeny greened significantly less than their parents. For external greening, such progeny were noted in EMG x ESG and ESG x ESG families in all treatments. For depth of tuber greening such progeny were found in DMG x DSG, DMG x DMG and DMG x DWG families under all treatments. For external greening overall percentage of progeny which scored O was 3%, 7%, and 7% in treatments A, B, and C, respectively. For depth of greening overall percentage of progeny that scored O was 10%, 21%, and 25% in treatments A, B, and C, respectively. Progeny that scored O for external greening were observed in 14 out of 17 families and for depth of tuber greening in 16 out of 17 families.

Most of the correlation coefficients between external and depth of tuber greening for unselected families were significant; however, they varied considerably (Table 8). In few cases the correlation coefficients were insignificant. For all 1,229 progeny r ranged from 0.46 to 0.68 in 1995 and from 0.50 to 0.72 in 1996. Analogous correlation coefficients for the parents and standard cultivars were significant and varied from 0.75 to 0.80 in 1995 and from 0.77 to 0.88 in 1996 (Table 8).

Maternal Effect

Analyses of variance on three sets of reciprocal families revealed significant maternal effects on external tuber greening in only one set of reciprocal families under treatment B. The parents of these reciprocal families had strong and moderate tendency to greening. No maternal effects were found for depth of greening under any treatment.

General and Specific Combining Ability

Mean squares from the analysis of variance of 12 families in the NC design II indicated significant GCA and SCA effects for both external and depth of tuber greening (Table 9). There were significant differences between years for both types of greening under the three tested storage conditions with the exception of depth of tuber greening after treatment C. GCA of the males was significant for both types of greening under all treatments. GCA of the females for external tuber greening was significant under two storage treatments, but for depth of tuber greening GCA was only significant under treatment A. SCA was significant for both types of greening under all treatments. However, significant SCA effects for respective crosses were found sporadically. For external tuber greening, it was documented in three families under treatment A. For depth of tuber greening, it was found in one family under treatment A and B and in a second different family under treatment B. GCA x year interactions were significant for both types of tuber greening for females and for depth of greening for males, excluding treatment C. SCA x year interactions were significant in external and depth of tuber greening except for external greening under treatment C.

Significant positive and negative GCA values for external and depth of tuber greening were identified in the seven tested parents (Tables 10, 11). For female DG 88-214, classified as ESG and DMG, significant positive GCA was found for external tuber greening in 1996 in two of the storage treatments (A and C); GCA for depth of greening was negative in seedling progeny (1995), but positive in tuber progeny (1996) in two of the treatment combinations. For female DW 88-4556, also classified as ESG and DMG, significant negative GCA was found for external tuber greening in 1996 in all three storage treatments and for depth of tuber greening in 1996 in two storage treatments (A and C). However, a significant positive GCA and negative SCA were found in the seedling progeny (1995) under all three storage treatments. In the strongly greened female DG 88-1591, significant positive GCA was found for external tuber greening in 1996 under two storage treatments (A and C) and for depth of tuber greening in 1995 under storage treatment A. Most estimates of GCA for the male parent DG 88-63, classified as EWG and DWG, were significantly positive while for strongly greened male DG 86-965 they were negative. Estimates of GCA for male DG 88-89, classified as EWG and DWG, tended to be negative while for moderately greened parent DG 88-1616, they tended to be positive.

DISCUSSION

Tuber greening is a commonly occurring phenomenon that is difficult to breed against because it is influenced by environmental conditions and can increase in the field, in storage, or at the market place. Akeley et al. (1962) found a genetic component to tuber greening and suggested the possibility of selection against tuber greening. Tuber greening under light exposure has been of minor importance in several breeding programs. However, in descriptions of some new potato cultivars created in the U.S.A., tuber greening has been evaluated along with other traits (Haynes et al. 1992; Holm et al. 1992; Reeves at al. 1990, 1994a, 1994b, 1995, 1996, 1997). Until now, a potato cultivar that resists tuber greening has been not bred but, considering the fact that both tetraploid (Brune and MeIo 2001) and diploid (Jakuczun 1993) non-greened or weakly greened clones have been selected, it seems that the trait could be introduced into the available breeding gene pool.

Influence of Year, Families, and Tuber Treatments on Tuber Greening

External and depth of tuber greening in the standard cultivars and parents were not influenced by year (Table 2), but there was a significant year effect for both traits in the segregating families (Table 4). Correlation coefficients between 1995 and 1996 for the parents and standard cultivars for both types of tuber greening were highly significant (Table 3). Analogous correlation coefficients in the segregating families in most cases were positive and significant, but they were lower in comparison to those for the standard cultivars and parents (Table 3). In general, external and depth of tuber greening were greater in the seedlings than in the subsequently tuber-propagated generation (Tables 5 and 6). Brown and Riley (1976) found differences between years in tuber greening in eight out of 10 cultivars. Reeves (1988) examined 144 cultivars exposed to light and reported a significant year effect on internal tuber greening and depth of greening, but no year effect on external greening. Our data do not conform to previously published data, but this may be due to physiological differences in the plant material. In 1995 the segregating families were propagated from true seeds, while in 1996 they were tuber propagated. Also, different crop management practices for seedlings and tuber progenies could be responsible for these differences.

Storage treatments influenced depth of tuber greening more than external greening (Table 7). Significant differences in external tuber greening under A and C treatments were found in only one family (ESG ? EWG). Depth of greening was significantly less in treatment C than in treatment A. Buck and Akeley (1967) found that intensity of tuber greening in six varieties was stronger after storage at 4.4 C than at 12.8 C or at 21.1 C, but greening was stronger after 2 months of storage than after 4 months. Griffiths et al. (1998) found that chlorophyll accumulation was little affected by storage temperature, but was slightly, although significantly, higher in tubers stored for 6 weeks in comparison to those stored for 14 weeks.

The correlations between external and depth of tuber greening we observed in this study indicate that these two types of tuber greening may be partially independently inherited (Table 8). Reeves (1988) also suggested that the three types of tuber greening he investigated might also be inherited independently.

Maternal Effect

There was little evidence that maternal effects were important for either external or depth of tuber greening. In only one storage regime for one set of reciprocal crosses was the effect significant. This is in agreement with the results of Parfitt and Peloquin (1981) who found no evidence for cytoplasmic inheritance of tuber greening.

General and Specific Combining Ability

Both GCA and SCA effects were important in tuber greening in diploid potatoes (Table 9). The male parents used in this study affected both external and depth of tuber greening much more than the female parents (Tables 10 and 11). They represented a wider range of variability of tuber greening from weak to strong, while there was little variation in the female parents. GCA was significant for the males in all storage environments for both external and depth of tuber greening, but significant for the females in only three cases over years. Significant GCA x year interactions of females for external and depth of greening in all treatments can suggest that they were probably much more influenced by environments than males (Table 9).

Few genetic studies on tuber greening have been reported and those were from many years ago. Akeley et al. (1962) concluded that in tetraploid potatoes tuber greening was inherited in a quantitative manner and that dominance was incomplete. They stated that genotypes with a weak tendency to tuber greening could be effectively selected. Parfitt and Peloquin (1981) found that inheritance of tuber greening in diploid potatoes was due to additive and epistatic effects. They estimated narrow sense heritability as 0.27 and broad sense heritability as 0.66, which indicated that the trait could be effectively selected in breeding programs.

Our studies found that both external and depth of tuber greening are inherited. The occurrences of no greened genotypes will allow the chance to transfer this trait into tetraploid level. We intend to transfer a low tendency of tuber greening into tetraploid breeding material via 4x-2x crosses.

ACKNOWLEDGMENTS

The authors thank Dr. Kazimiera Zgórska for comments in realization of research and Dr. Leszek Domanski for statistical guidance.

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Henryka Jakuczun* and Ewa Zimnoch-Guzowska

Plant Breeding and Acclimatization Institute, Ml[bar]ochów Research Centre, Platanowa 19 Str., 05-831 Ml[bar]ochów, Poland

* Corresponding author: Tel: (+4822)7299248; Fax: (+4822)7299247; Email: hjakuczun@ihar.edu.pl

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