Crop Science 40:977-984 (2000)
© 2000 Crop Science Society of America
CROP BREEDING, GENETICS & CYTOLOGY
Broad-Sense Heritability and Stability of Internal Heat Necrosis and Specific Gravity in Tetraploid Potatoes
M.R. Henningera,
S.B. Sterrettb and
K.G. Haynesc
a Rutgers University, New Brunswick, NJ 08903 USA
b Virginia Polytechnic Institute and State University, Painter, VA 23420 USA
c USDA/ARS, Vegetable Laboratory, Beltsville, MD 20705 USA
khaynes{at}asrr.arsusda.gov
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ABSTRACT
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Internal heat necrosis (IHN), a severe physiological disorder of potato (Solanum tuberosum L.) tubers, is characterized by brown spots or blotches that first appear toward the apical end of the tuber parenchyma but in severe cases may involve most of the parenchyma tissue. The purposes of this study were to determine the amount of genetic variation for and the stability of IHN in tetraploid potato clones, and the relationships among IHN, specific gravity, and tuber size distribution. Nineteen potato clones (four cultivars and 15 breeding selections) were grown in Bridgeton, NJ and Painter, VA from 1991 to 1993 in a randomized complete block with four replications of 20 hills per plot. The crop was harvested 2 to 3 wk later than normal harvest time to provide for maximal exposure to heat stress. All tubers were harvested and graded by size. All tubers >64 mm in diameter were quartered longitudinally and rated by size for IHN
. Incidence and severity of IHN were recorded. Broad-sense heritabilities and their 95% confidence intervals were: 0.83 (0.68, 0.93) for incidence of IHN in Size 3 (64–83 mm diameter) tubers; 0.88 (0.71, 0.94) for incidence in Size 4 (>83 mm diameter) tubers; 0.85 (0.72, 0.94) for severity of IHN in Size 3 tubers; 0.90 (0.78, 0.96) for severity of IHN in Size 4 tubers; 0.92 (0.85, 0.97) for specific gravity in Size 3 tubers; 0.77 (0.54, 0.91) for specific gravity in Size 4 tubers; and 0.86 (0.73, 0.94) for total yield. Several of the more IHN-susceptible clones were unstable for IHN both before and after environmental heterogeneity was removed. The correlation between incidence and severity of IHN was very high. There was no correlation between IHN and total yield or specific gravity. The results of this study indicate that breeding high yielding, high specific gravity cultivars for resistance to IHN should be feasible.
Abbreviations: IHN, internal heat necrosis • Size 3, tubers 64 to 83 mm in diameter • Size 4, tubers >83 mm in diameter
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INTRODUCTION
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INTERNAL HEAT NECROSIS is a potentially severe physiological disorder in some potato cultivars. IHN is characterized by round-to-irregular tan-to-reddish brown spots or blotches that appear first toward the apical end of the tuber. Necrotic tissue appears at or near harvest and generally in the larger tubers first (Sterrett and Henninger, 1997).
Potato cultivars differ in their susceptibility to IHN. In regional testing in the mid-Atlantic states from 1988 to 1990, `Atlantic' was very susceptible to IHN, whereas `Norchip' was moderately susceptible, and `Superior' was less susceptible (Porter et al., 1989, 1990; Morrow et al., 1991). Cultivars with high specific gravity, such as Atlantic, are preferred by potato chip processors. Until recently, Atlantic was specified in >80% of the processing contracts in the mid-Atlantic region (Sterrett and Wilson, 1990), although growers were at considerable economic risk from IHN. Between 1986 and 1988, 3.2 to 11.2% of Atlantic acreage remained unharvested because tubers failed to make USDA no. 1 grading standards as a result of IHN (Sterrett and Wilson, 1990). Although Atlantic is the dominant cultivar grown in the mid-Atlantic states, within the last 3 yr the acreage of Atlantic has decreased, whereas the acreage of `Snowden' has increased (Steve Molnar, Wise Foods, Berwick, PA, 1998, personal communication). Snowden is a later-maturing cultivar than Atlantic and, when harvested at the same time as Atlantic, tends to produce smaller tubers that are less susceptible to IHN (Sisson et al., 1998).
The influence of various environments and cultural management practices on the incidence and severity rating of IHN in the cultivar Atlantic in the mid-Atlantic region have been examined previously. Sterrett et al. (1991) reported that the incidence and severity rating of IHN increased with successive harvests, but this varied by year and location. They found that more than one environmental factor influenced both the incidence and severity rating of IHN. In addition, a two-stage model developed to predict IHN using stepwise regression indicated that onset and development of IHN are influenced by environmental stresses during more than one stage of growth (Lee et al., 1992).
While the influence of various environments and cultural management practices on the incidence and severity rating of IHN in Atlantic have been studied, very little research has been undertaken to understand resistance to IHN in potato breeding populations. Future breeding strategies to develop resistance to IHN would benefit from determining the amount of genetic variation as a proportion of the total variation (i.e., broad-sense heritability) and estimating the relative importance of clone x environment interactions. Clone x environment interactions are important sources of variation for many polygenic traits in potatoes, such as specific gravity (Haynes et al., 1995). However, for at least one monogenic trait in potatoes, yellow-flesh, clone x environment interactions are relatively unimportant (Haynes et al., 1996). Where clone x environment interactions account for a substantial portion of the observed variability, stability analysis (Shukla, 1972) can be used to partition the observed clone x environment variability among individual clones in the evaluated population. Clones that contribute relatively little to the overall clone x environment interaction are more stable than clones that make a larger contribution.
The continuing emphasis on high specific gravity for chip processing, coupled with the complexity of environmental variables affecting IHN, places potato producers in the mid-Atlantic region at considerable economic risk in growing the cultivar Atlantic. For any new chip-processing cultivar to replace Atlantic in the mid-Atlantic region, it must be high yielding, have high specific gravity, and be resistant to IHN. Currently, no such new cultivars are available. Breeding such a cultivar appears to be the best strategy to restore economic stability to chip growers in this and other similar regions.
The purposes of this study were (i) to estimate broad-sense heritability for resistance to IHN, yield, and specific gravity in 19 clones (4 cultivars and 15 advanced selections from the USDA-ARS potato breeding program in Beltsville, MD); (ii) to determine the importance of clone x environment interactions on IHN, yield, and specific gravity; (ii) to determine the stability of resistance to IHN, yield, and specific gravity in these clones; and (iv) to determine the relationship among IHN, yield, and specific gravity.
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Materials and methods
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Clonal Material
A list of the 19 clones, along with the tuber skin type (red, russet, white) and the parents of each clone used in this study, is given in Table 1
. Clonally propagated plants were grown on a Caribou gravelly loam soil (fine-loamy, mixed, frigid Typic Haplorthod) on Chapman Farm, Presque Isle, ME each year from tubers from virus-tested plants the previous year. Tubers were harvested, stored overwinter at 4°C, 95% relative humidity, and transported to Bridgeton, NJ and Painter, VA in the spring for planting. Prior to planting, seed was cut into 
57-g seed pieces and allowed to suberize for several days at room temperature.
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Table 1 Parentage of the 19 potato clones evaluated in the internal heat necrosis study conducted in Bridgeton, NJ and Painter, VA from 1991 to 1993
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Clones were planted in 1991 and 1993 on a Sassafras sandy loam soil (fine-loamy, siliceous, semiactive, mesic Typic Hapludult), and in 1992 on an Aura loam soil (coarse-loamy, siliceous, semiactive, mesic Typic Fragiudult) at the Rutgers Research and Development Center, Bridgeton, NJ, and clones were also planted in 1991, 1992, and 1993 on a Bojac sandy loam soil (coarse-loamy, mixed, thermic Typic Hapludult) at the Eastern Shore Agricultural Experiment Station, Painter, VA. Planting and harvesting dates varied with growing season and locations (Table 2)
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Table 2 Planting and harvest dates for the 19 potato clones evaluated in the internal heat necrosis study conducted in Bridgeton, NJ and Painter, VA from 1991 to 1993
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For all six environments (New Jersey and Virginia, 1991–1993) clones were planted in a randomized complete block design with four replications of 20 hills per plot. Plants were spaced 0.23 and 0.3 m within the row in New Jersey and Virginia, respectively, and 0.9 m between rows at both locations. Maximum and minimum temperatures were recorded during the growing season and used to compute accumulated heat sums as suggested by the model developed by Lee et al. (1992). These calculated accumulated heat sums were plotted for each environment to compare the different growing seasons.
Plots were mechanically harvested, and tubers were sized into groups <48, 48–64, 64–83 (Size 3), and >83 mm (Size 4) in diameter. Specific gravity was determined by the weight in air and weight in water method (Murphy and Goven, 1959) for the two larger size categories. All tubers in these two size categories were counted, quartered longitudinally, and scored for IHN using the rating scale developed by Sterrett et al. (1991). This scale ranges from 1 = severe IHN to 9 = no IHN. Two measures of IHN were computed for each plot. Incidence of IHN represented the percentage of tubers with IHN; severity of IHN represented the mean IHN rating.
Statistical Analysis
Incidence and severity of IHN and specific gravity for tubers in the Size 3 and Size 4 categories, as well as total yield, were analyzed using the mixed procedure in the Statistical Analysis System (Littel et al., 1996). For these analyses, environments, replications, and clones were considered random effects. The estimates of the variance components from these analyses were used to compute broad-sense heritabilities on a mean basis for incidence and severity of IHN and specific gravity for the two larger size categories and total yield (Nyquist, 1991). These same data were also analyzed using the general linear models procedure in the Statistical Analysis System (SAS Institute, 1987). The error mean square from these analyses were used in an approximate t test to compare the difference in incidence and severity of IHN and specific gravity between the two larger size categories (Cochran, 1964). A 95% confidence interval on these estimates of broad-sense heritabilities was also calculated from these later analyses (Knapp et al., 1985). Least square means for clones by environment were computed and the clone x environment interaction was partitioned into stability variance components (
2i) assignable to each clone (Shukla, 1972), using the interactive matrix language procedure in SAS (Kang, 1989). An environmental index was calculated for each variable as the least square mean of all clones across all six environments minus the least square mean of all clones in each environment. Heterogeneity, or nonadditivity, due to this environmental index was removed from the clone x environment interaction, and the remainder of the clone x environment interaction was partitioned into s2i components assignable to each clone (Kang, 1989). The correlations among incidence and severity of IHN and specific gravity in the Size 3 and 4 categories and total yield were calculated for each environment.
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Results and discussion
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Growing conditions in New Jersey and Virginia in 1992 were more favorable for potato production than in 1991 or 1993, as measured by accumulated heat units (Fig. 1)
. The accumulated heat unit model developed by Lee et al. (1992) imposes a penalty when maximum daily temperatures exceed 25°C or minimum daily temperatures exceed 21°C. Accumulated heat units rose early in the growing season, leveled off, and then decreased rapidly in both 1991 and 1993 at both locations because high temperatures prevalent during the later part of the growing season exceeded the optimum minimum and maximum for potatoes. In spite of these vastly different environments, there were no significant differences among environments for incidence or severity of IHN, specific gravity for the two larger tuber size categories, or for total yield (Table 3)
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Table 3 Estimates of the variance components for incidence and severity of internal heat necrosis (IHN) and specific gravity in tubers in the Size 3 and Size 4 categories and total yield, broad-sense heritabilities, and the 95% confidence interval about the broad-sense heritability estimates for 19 potato clones grown in Bridgeton, NJ and Painter, VA from 1991 to 1993
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There were significant differences among the clones for both incidence and severity of IHN in the two larger size categories (Table 3). The incidence of IHN was greater in the Size 4 tubers than in the Size 3 tubers (24 vs. 12%,
). The IHN was more severe in the Size 4 tubers than in the Size 3 tubers (8.49 vs. 8.73,
). This is in agreement with the results reported by Sterrett et al. (1991). As expected, the greatest incidence and severity of IHN was found in Atlantic (Table 4)
. The incidence and severity of IHN were also high in B0184-18, B0233-1, B0243-18, and B0255-9 (Tables 4 and 5)
. The incidence and severity of IHN were low in `Reddale', `Russette', Superior, B9955-11, B0175-21, B0203-21, and B0257-9 in all six environments. In the other remaining clones, the incidence and severity of IHN were generally low, with a few exceptions.
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Table 4 Incidence of internal heat necrosis (IHN) and the stability-variance statistics before (2i) and after (s2i) removing environmental heterogeneity in tubers in the Size 3 and Size 4 categories for 19 potato clones grown in Bridgeton, NJ and Painter, VA from 1991 to 1993
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Table 5 Severity of internal heat necrosis (IHN) and the stability-variance statistics before (2i) and after (s2i) removing environmental heterogeneity in tubers in the Size 3 and Size 4 categories for 19 potato clones grown in Bridgeton, NJ and Painter, VA from 1991 to 1993
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The clone x environment interactions were significant for incidence and severity of IHN in both the larger tuber size categories (Table 3). Atlantic, B0255-9, and B0257-3 were particularly unstable for both incidence and severity of IHN in the Size 3 tubers, although other clones, such as B0184-18, B0233-1, and B0245-15, were also unstable (Tables 4, 5). The two clones (Atlantic and B0255-9) with the greatest incidence and most severe IHN were the most unstable. However, the next most unstable clone for incidence and severity of IHN was B0257-3. There were other clones with a greater incidence and severity of IHN than B0257-3, such as B0184-18, B0233-1, and B0243-18, indicating that instability is not always confined to the most susceptible clones. Broad-sense heritabilities and their 95% confidence intervals for incidence and severity of IHN in the Size 3 tubers were 0.83 (0.68, 0.93) and 0.85 (0.72, 0.94), respectively.
Three of the clones (B9955-11, B0186-3, B0203-1) failed to produce any tubers in the Size 4 category in at least one environment. In the tables (Tables 4, 5, and 6)
these are indicated by NT where no tubers were produced in that particular environment, with the overall mean being nonestimable, indicated by NE. These three clones were subsequently dropped prior to the stability analyses run on the Size 4 tuber traits. For the Size 4 tubers, B0045-6 and B0255- 9 were unstable for both incidence and severity of IHN. However, for Size 4 tubers, Atlantic still had the greatest incidence and severity of IHN of all clones, even if it was no longer one of the most unstable clones. Broad-sense heritabilities and their 95% confidence intervals for incidence and severity of IHN in the Size 4 tubers were 0.88 (0.71, 0.94) and 0.90 (0.78, 0.96), respectively.
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Table 6 Specific gravity (1.0 omitted) and the stability-variance statistics before (2i) and after (s2i) removing environmental heterogeneity in tubers in the Size 3 and Size 4 categories for 19 potato clones grown in Bridgeton, NJ and Painter, VA from 1991 to 1993
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There were significant differences among clones for specific gravity within each size category (Table 3). However, there was no difference in specific gravity between the Size 4 and 3 tubers (Table 6). B0175-21, Atlantic, B0177-20, B0175-20, B0255-9 and B0257-3 had the highest specific gravities. Reddale, Superior, B0184-18, B0233-1, B0243-18, and B0203-21 had the lowest specific gravities. B0179-18 and B0186-3 were the most unstable for specific gravity in Size 3 tubers. B0186-3 was one of the clones that did not produce any Size 4 tubers in three of the six environments. Many clones failed to produce more than a few tubers in the Size 4 category. Specific gravity determined by the weight in air and weight in water method can be difficult to ascertain with accuracy when less than 1500 g of potatoes are weighed. Extreme values of specific gravity for Size 4 tubers in Table 6, such as for Superior in Virginia during 1993 and B0177-20 in New Jersey during 1991, are the result of too few tubers for very accurate determinations to be made. Broad-sense heritabilities and their 95% confidence intervals for specific gravity in Size 3 and 4 tubers were 0.92 (0.85, 0.97) and 0.77 (0.54, 0.91).
There were significant differences among clones for total yield (Table 3). B0184-18, Atlantic, B0233-1, and Reddale were the highest yielding clones (Table 7)
. B9955-11, B0255-9 and B0257-9 were the lowest yielding clones. Russette and B0045-6 were the most unstable clones for total yield. Broad-sense heritability and its 95% confidence interval for total yield was 0.86 (0.73, 0.94).
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Table 7 Average yield of all size tubers and the stability-variance statistics before (2i) and after (s2i) removing environmental heterogeneity for 19 potato clones grown in Painter, VA and Bridgeton, NJ from 1991 to 1993**
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The correlations between incidence and severity of IHN for either size category were very high for all six environments (Table 8) , indicating that either incidence or severity could be used to effectively measure IHN in the clones. For either size category, there was no correlation between either incidence or severity of IHN and either specific gravity or total yield in all six environments.
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Table 8 Correlations among incidence and severity of internal heat necrosis (IHN) and specific gravity for tubers in the Size 3 and Size 4 categories and average yield of all size tubers
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Previous studies have suggested that nonadditive genetic variance is important in yielding ability (Killick, 1977; Plaisted et al., 1962; Tai, 1976), while both additive and nonadditive genetic variance are important in specific gravity (Haynes et al., 1995; Killick, 1977; Tai, 1976). The very high estimates of broad-sense heritability obtained for all of the traits measured in this study, coupled with the lack of correlation between measures of IHN and either specific gravity or total yield indicate that breeding a high yielding, high specific gravity clone without IHN should be feasible. Still unknown is the relative importance of additive and nonadditive variation for IHN. However, since portions of both additive and nonadditive genetic variation can be transferred in tetraploid crosses (Kempthorne, 1957), the possibility of developing a high yielding, high specific gravity clone to replace Atlantic exists. The most difficult part of the breeding effort may be to include chipping ability in this future clone. Chip color has been reported to be highly heritable when evaluated under a given storage condition (Accatino, 1973; Stevenson and Cunningham, 1961; Loiselle et al.,1990; Lynch et al., 1992). However, all of these estimates were obtained for potatoes grown under cool North American conditions. To our knowledge, no heritability estimates of chip color immediately following harvest under high temperature growing conditions have been made. With potato harvests beginning in June in North Carolina and proceeding north through Virginia in July and New Jersey in August, the mid-Atlantic states supply much of the chip stock to east coast chip plants until fall harvest across the northern tier states. The development of new potato cultivars with resistance to internal heat necrosis, either for fresh market or chip stock, would help to restore economic stability to commercial potato producers in this region.
Received for publication June 4, 1999.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
