The hypothesis of this study was that the concentration and the spatial distribution of elements in potato tubers could be used to predict after-cooking darkening (ACD), one of the most important undesirable potato traits with respect to tuber quality. The objective was to identify the elements (plant nutrients) whose content may relate to the severity of potato ACD. This study used induced coupled argon plasma spectrometry and a variable pressure scanning electron microscope equipped with energy-dispersive x-ray spectrometry to measure these elements. A total of 14 elements were studied [phosphorus (P), calcium, magnesium (Mg), potassium (K), sulfur (S), iron (Fe), copper, sodium (Na), zinc (Zn), boron (B), manganese, aluminum (Al), silicon (Si), chlorine (Cl)] in two potato cultivars with various fertilization regimes. Some elements such as P, Mg and K, S, Zn, and Cl had higher concentrations at the center of the tuber compared with the distal ends, whereas others such as calcium (Ca) showed a decrease from stem end to bud end of the tubers. The concentrations of Na, Fe, Al, and Si were greater near the distal ends of the tuber and decreased toward the center of the tuber. Copper (Cu) and B showed no distribution pattern from the stem to the bud end. When segment element concentrations were compared with their degree of ACD, the elements most strongly correlated with ACD severity were P (r = 0.590), S (r = 0.530), Ca (r = –0.388), Cu (r = 0.382), and Mg (r = 0.338). Predictor models for the severity of ACD had R2 values in the range of 0.398 to 0.480 depending on the data sets used. Phosphorus, Ca, and in one case Cu, were selected as the best element subset to predict the degree of ACD severity. This study elucidates details of tuber element composition and distribution and also provides a useful method to predict the severity of ACD or other tuber qualities. Such predictive models have a strong potential to help the potato processing industry in predicting the occurrence of ACD and in developing agronomic treatments to minimize ACD.
After-cooking darkening (ACD) is an undesirable potato tuber trait, problematic in processed potato products (Wang-Pruski and Nowak, 2004). It is characterized as a change from a tuber's normal flesh color to gray, blue, purple, or black (Hughes, 1962). ACD is most common in boiled or steamed potatoes but is also problematic in processed products such as oil-blanched French fries, dehydrated potatoes, canned potatoes, prepeeled potatoes, and reconstituted dehydrated potatoes (Armstrong, 1963; Dale and Mackay, 1994; Smith, 1987). ACD is caused by the oxidation reaction of the ferrous–chlorogenic acid complex, resulting in a bluish gray compound, ferridichlorogenic acid (Wang-Pruski, 2006). Organic acids such as citric acid are believed to play a role in competing with chlorogenic acid for iron (Fe) and, as a result, decreasing the severity of ACD (Hughes and Swain, 1962a). In general, the degree of ACD is highest at the stem end compared with the bud end of the potato tuber, whereas the center has the lowest degree of ACD (Wang-Pruski, 2006). Previously, it has been suggested that Fe, phosphorus (P), and calcium (Ca) may be linked to the occurrence and severity of ACD (Hughes and Swain, 1962b), but there has been limited investigation to confirm this theory.
Elemental analyses of potato leaves and tubers has been used to provide information on the geographical origin of tubers (Anderson et al., 1999; Di Giacomo et al., 2007) as well as on their sensory (Whittenberger and Nutting, 1950; Zaehringer et al., 1969) and processing quality (Arteca et al., 1980; Hughes and Swain, 1962b, Mohr et al., 1984). However, only a limited amount of research has been done to determine the distribution of elements within each tuber and correlate these to tuber quality. Earlier studies on the distribution of elements in tubers were conducted in the 1960s and early 1970s by Hughes and Swain (1962a), Macklon and DeKock (1967), Johnston et al. (1968), and Bretzloff (1971) and yet detailed information regarding the concentration and distribution of elements in tubers remains incomplete.
Inductively coupled argon plasma (ICAP) analysis has traditionally been the technique used to study element concentrations of plant tissue (Anderson et al., 1999; Cubbada and Raggi, 2005). This technique yields, reliably and accurately, simultaneous quantifications of many elements (Andrasi et al., 1998). However, this method is destructive and involves extensive sample preparation before the ICAP measurements. Another method to study surface distribution of chemical elements, energy-dispersive x-ray spectrometry (EDS) coupled to the scanning electron microscope (SEM), has also been used to measure elements in plant tissue (LeRiche et al., 2006; Mohr et al., 1984; Takahashi et al., 2006). Although SEM/EDS has the potential to quantify elements, it has been mainly used to determine the spatial distribution and relative concentrations of elements in plant tissues and is thus useful for measuring gradients of elements within portions of a plant (LeRiche et al., 2006; Takahashi et al., 2006). The current study used both variable pressure (VP)-SEM/EDS and ICAP as complementary analytical techniques to measure the effects of cultivar, fertilizer, and tuber segment on the element distribution of tubers.
The concentration of elements in tubers can be affected by both agronomic treatments (Haase et al., 2007; Mondy and Ponnampalam, 1986; Warman and Havard, 1998; White et al., 2009; Wszelaki et al., 2005) and cultivar (Randhawa et al., 1984). Therefore, the first objective of this study was to present a broad evaluation of the concentration and spatial distribution of most of the essential plant nutrients [P, Ca, magnesium (Mg), potassium (K), sulfur (S), Fe, copper (Cu), sodium (Na), zinc (Zn), boron (B), manganese (Mn), aluminum (Al), silicon (Si), and chlorine (Cl)] in potato tubers of ‘Shepody’ and ‘Russet Burbank’. The second objective was to determine if the concentrations of these elements could be correlated with the ACD.
Materials and Methods
The tubers used in this study were of the common potato processing cultivars Shepody and Russet Burbank. These cultivars were grown for two cropping seasons (2005 and 2006) in three Eastern Canadian locations: Truro, Nova Scotia; Florenceville, New Brunswick; and Summerside, Prince Edward Island. The fertilizer Treatments (1 through 6) used at these locations are given in Table 1. All tubers were harvested in early October of each year and stored according to standard commercial conditions as described previously (LeRiche et al., 2006). Analyses for each treatment were conducted on randomly selected medium size tubers (≈140 to 315 g for ‘Shepody’ and 140 to 270 g for ‘Russet Burbank’). All analyses were done in three replicates. For ICAP, five tubers were pooled to make up every replicate, and for VP-SEM/EDS, three tubers were used and each was considered as a replicate. Tubers were sampled in December and February following both the 2005 and 2006 cropping seasons. Each tuber was cut longitudinally in half along the central pith and half remained raw while the other half was cooked based on the procedures described by Wang-Pruski (2007). The raw half was used for ICAP and VP-SEM/EDS analyses, whereas the cooked half was used for ACD evaluation and ICAP analysis. This sampling procedure assumed radial symmetry along the central axis of the tuber. Once the ACD evaluations were complete, all tubers sampled, as per the procedures described previously, were then preserved by oven-drying in open-lid petri dishes at 65 °C for 48 h before experimental analyses.
Fertilizer treatments for ‘Shepody’ and ‘Russet Burbank’ tubers grown in Summerside, Prince Edward Island, Truro, Nova Scotia, and Florenceville, New Brunswick.
Inductively coupled argon plasma analysis.
Inductively coupled argon plasma analysis was used to determine the concentrations of P, Mg, Ca, K, S, Fe, Na, Al, Zn, Cu, B, and Mn in the tuber segments. The results from Treatments 1 and 2 (Table 1) were used in the element analysis section, whereas the data from Treatments 1 through 6 (Table 1) were used in the regression analysis section. ICAP analyses were performed on both cooked and raw tuber samples. A No. 7 cork borer (8 mm in diameter) was used to take subsamples from the stem end (1), center (2), and bud end (3) of each tuber (Fig. 1A). Before ICAP analysis, each dried sample was ground to pass through a 2-mm screen and decomposed using a nitric/perchloric (HNO3/HClO4) procedure (AOAC, 2000). The concentrations of the elements were measured by an ICAP spectrometer model No. 61 (Thermo Jarell Ash, Franklin, MA) by the Nova Scotia Department of Agriculture soil testing laboratory (Harlow Institute, Truro, Nova Scotia, Canada). Representative standards (five concentration points for each element to build calibration curves) were prepared using pure standards.
Energy-dispersive x-ray spectrometry analysis.
For energy-dispersive x-ray spectrometry analysis, a variable pressure scanning electron microscope coupled with an VP-SEM/EDS was used. The analysis used a strip (2 to 3 mm thick, 10 mm wide) cut longitudinally from the stem end to the bud end of a raw tuber half (Fig. 1B). These analyses were only performed on tubers from Treatments 1 and 2 (Table 1). The strip was further divided into eight equal segments of ≈10 × 10 mm and numbered from the stem end (1) to the bud end (8) (Fig. 1B). The other tuber half was cooked and used to measure the severity of ACD (see evaluation subsequently). VP-SEM/EDS analyses were done as previously reported (LeRiche et al., 2006) using a Hitachi S-3000N VP-SEM (Hitachi, Tokyo, Japan) equipped with and INCAx-sight EDS detector (Oxford Instruments Analytical, Oxford, UK). A randomly selected area (≈2 × 3 mm) at the center of each segment was scanned using VP-SEM/EDS. The samples were viewed at a pressure in the specimen chamber of 50 Pa and a temperature of –20 °C. The working distance was kept constant at 15 mm, the acceleration voltage was set at 20 keV, and a magnification of ×45 was used.
The measured elements using VP-SEM/EDS were: P, Ca, Mg, K, S, Fe, Na, Al, Si, and Cl. Relative quantification was obtained using the analyzer function of INCA software suite Version 4.04 (Oxford Instruments Analytical, Oxford, UK). The data were generated as apparent concentrations in relative values, which were calculated using the following formula:
In 2005, 10 frames per sample were scanned. In 2006, the number of scanned frames per sample was reduced to three frames to increase the sample throughput and to avoid plant tissue damage by the electron beam. A pilot trial was conducted and had confirmed that this change did not affect the accuracy of the readings (data not shown).
After-cooking darkening evaluation.
ACD was evaluated from the surface of the cooked half-tuber segments before the ICAP and VP-SEM/EDS analyses (Fig. 1A–B). The ACD distribution from the cooked tuber half was assumed to be a mirror image of the ACD distribution of the raw tuber half. Tuber halves that were evaluated for ACD were steam-cooked in stainless steel steamers for ≈15 min. Once cooked and cooled, a thin layer (2 to 3 mm) was removed from the cut surface of the tubers to ensure proper exposure of the flesh tissue to air. The tubers were left for 1 h to allow for the full development of the oxidative discoloration. The image of each tuber was captured and digitally evaluated using a UVP Chemi-Imager System and LabWorks™ Imaging Analysis and Acquisition Software (UVP Inc., Upland, CA) in a similar manner as described previously (Wang-Pruski, 2006). In the case of ICAP, five tubers were used as one replicated to determine the ACD. In the case of VP-SEM/EDS, three tubers were evaluated, and each tuber was considered a replication. The cooled CCD camera on the UVP Chemi-Imager System was standardized using a stack of three sheets of white paper (Domtar Inc., Montreal, Canada; 84 brightness) to a mean raw density value of 215 pixels. Once the image was captured, segments were digitally separated into segments by comparing it with the sampled areas of its complementary raw tuber half. The ACD readings generated from the digital photoimaging system were given on a scale from 0.001 to 255.999 pixels in which 0.001 indicates complete black and 255.999 is pure white. Therefore, the higher the reading, the less severe the ACD.
A 3 × 2 × 2 factorial design with tuber segment, cultivar, and fertilizer treatment as factors was used to analyze the ICAP data for elemental analysis. The data were analyzed using the PROC MIXED function in SAS (Version 8; SAS Institute, Cary, NC) with harvest year and sampling date as blocking factors. Segments were treated as a spatially repeated measurement and compound symmetry was chosen as the structure that best characterized the covariance. Multiple means comparisons for main effects and interaction effects were determined using least squares means at α = 0.05. When the P value for an effect was in the range of 0.10 > P > 0.05, it was defined as “marginally significant.” Aside from the comparison between VP-SEM/EDS and ICAP, VP-SEM/EDS data were not statistically analyzed, but instead were used to better visualize and further characterize the distribution across tuber segments as a complement to the ICAP data. Pearson correlation (r) values for the ICAP data were generated using Minitab (Version 14.1; Minitab Incorporated, State College, PA). Values were only considered statistically significant when r > 0.30 and P < 0.05. Regression analyses using ICAP element data with significant correlations to ACD were generated using Minitab. The factors were included in a stepwise fashion (α = 0.15). Residuals generated from the ICAP and ACD data were tested for normality and constant variances and no transformation was needed in the case of ACD, Ca, K, Mg, and P. For S, a square root transformation (
Analysis of the 14 elements. P, Mg, Ca, K, S, Fe, Na, Al, Zn, Cu, B, Mn, Si, and Cl, unless otherwise noted, were performed on raw ‘Shepody’ and ‘Russet Burbank’ from Treatments 1 and 2 (Table 1). ICAP was used to measure the concentration of the 12 elements. P, Mg, Ca, K, S, Fe, Na, Al, Zn, Cu, B, and Mn, in the stem end, center, and bud end segments of tubers. VP-SEM/EDS was used to supplement ICAP results by measuring the element distribution among eight segments of P, Mg, Ca, K, S, Fe, Na, Al as well as Si and Cl (Figs. 2 and 3).
The concentration of P in tubers was affected by cultivar, segment, and fertilizer (Table 2). The distribution of P from stem to bud, as determined using ICAP, shows that the stem end segments contained significantly less P than the center and bud end segments, which were not significantly different from one another. This was true for both cultivars and fertilizer regimes (Table 3). The concentration of P was greater in ‘Shepody’ tubers compared with ‘Russet Burbank’ and was also greater in tubers from fertilized plots compared with those from nonfertilized plots (Table 3). In the case of ‘Shepody’, the difference in P concentration between the two fertilizer regimes was significant for the center and bud end segments but not for the stem end. For ‘Russet Burbank’, the difference between the two fertilizer regimes was only significant for the bud end segment (Table 3).
Test for effects (P values) of cultivar (Cv), segment (Seg), fertilizer regime (Fert), and their interaction (x) on the concentration of elements in potato tubers.
Tuber element concentrations of raw ‘Shepody’ and ‘Russet Burbank’ tubers from fertilized and nonfertilized plots as affected by cultivar, fertilizer regime, and tuber segment.
VP-SEM/EDS analysis showed that Segment 1 (stem end) had a much lower P concentration than the other segments and the concentration of P increased gradually from Segment 1 to Segment 4 and then decreased toward the bud end (Fig. 2). The concentration of P within tubers was correlated with S, Mg, K, and, to a lesser extent, Zn (Table 4).
Linear correlation coefficients (r) among 12 elements from raw ‘Shepody’ and ‘Russet Burbank’ tuber segments from the stem, center, and bud segment of fertilized and nonfertilized plots analyzed using inductively coupled argon plasma.
The concentration of Mg in tubers was affected by a cultivar–segment interaction as well as fertilizer (Table 2). In ‘Shepody’ tubers from both fertility regimes, the distribution pattern of Mg was similar to that of P with a lower concentration in the stem segment compared with the two other segments, which showed little difference in concentration (Fig. 2). In nonfertilized ‘Russet Burbank’, an increased Mg concentration from stem to bud end was observed, but in fertilized ‘Russet Burbank’ tubers, there was no difference among any of the segments (Table 3). Overall, a significantly higher concentration of Mg in ‘Shepody’ tubers compared with ‘Russet Burbank’ tubers was observed (Table 3). At the stem end of nonfertilized ‘Shepody’ tubers, however, the concentration of Mg compared with that of ‘Russet Burbank’ tubers was not statistically significant. Interestingly, Mg was the only element that was more abundant in nonfertilized plots compared with fertilized plots. The differences in Mg between the two fertility regimes were much more evident in ‘Russet Burbank’ tubers than in ‘Shepody’ (Table 3). The VP-SEM/EDS analysis of Mg indicated that the concentration of Mg increased from Segment 1 to Segment 3 and then decreased again, this time more gradually, toward the bud end of the tuber (Fig. 2). The concentration of Mg was correlated to the same elements as P, namely S, P, K, and Zn (Table 4).
The concentration of Ca in tubers is best characterized as a three-way interaction among cultivar, fertilizer, and segment (Table 2). In the case of ‘Shepody’ from fertilized plots, the concentration of Ca was not different between the stem and center and had a marginally lower concentration at the bud end compared with the stem end. In nonfertilized ‘Shepody’ tubers, there were no significant differences in Ca concentration among any of the segments. In ‘Russet Burbank’ tubers, there was a significantly higher concentration of Ca at the stem end compared with the center segment from fertilized plots and marginally higher concentration in stem end segments from nonfertilized plots (P = 0.0965) (Table 3). Higher Ca concentrations in fertilized plots compared with nonfertilized plots was the general trend observed, but differences among the two fertilizer regimes were not always significant (Table 3). In all segments, there was a lower concentration of Ca detected in ‘Shepody’ tubers compared with ‘Russet Burbank’ tubers (Table 3).
Using VP-SEM/EDS, the distribution of Ca was found to remain constant for the first half of the tuber (Segments 1 through 4) and then decreased gradually toward the bud end (Fig. 2). The concentration of Ca was correlated to that of Mn, Na, S, Al, and Cu (Table 4).
The concentration of K in tubers was unaffected by cultivar or by fertilizer, but was affected by segment (Table 2). The concentration of K was lower at the stem end compared with the center and bud end segments, where the concentrations of K were no different from one another (Table 3). These observations were confirmed by VP-SEM/EDS analysis, which showed an increase in the concentration of K from Segment 1 (stem end) to Segment 4 before the concentration stabilized and then decreased slightly in the final two segments (Fig. 2). The concentration of K was correlated to that of P, Mg, and S (Table 4).
The concentration of S in tubers was affected by cultivar and segment, but not fertilizer (Table 2). The concentration of S in tubers was significantly higher in ‘Shepody’ compared with ‘Russet Burbank’ tubers (Tables 4). In fertilized tubers of both cultivars, a higher S concentration was found at the center of tubers compared with the stem and bud; these two segments were similar in S concentration. A similar trend was observed in nonfertilized ‘Shepody’, but in this case, the difference between the stem and center was only marginally significant (P = 0.0818). In nonfertilized ‘Russet Burbank’, there was still less S at the bud end compared with the stem end, but, in this case, the center segment was not significantly different from either the stem end or the bud end (Table 3).
The detailed results obtained using VP-SEM/EDS showed the distribution of S to be parabolic with a lower concentration of S at the distal ends of the tuber and increased concentration toward the center (Fig. 2). The concentration of S was correlated to that of Mg, P, and, to a lesser extent, Zn, Fe, Ca, and K (Table 4).
The concentration of Fe was affected by a cultivar–fertilizer interaction, segment, and marginally by a segment–fertilizer interaction (P = 0.08) (Table 2). In fertilized ‘Shepody’ tubers, the bud end segment contained a lower Fe concentration than the stem end and center segments, which were not different from one another (Table 3). In nonfertilized ‘Shepody’ tubers, there was no difference among any of the segments (Table 3). In ‘Russet Burbank’ tubers, on the other hand, differences among segments were only observed in the nonfertilized tubers. Here, the stem end had a higher concentration of Fe than that of the two other segments, which were not different from one another (Table 3). Higher Fe concentrations were found in fertilized ‘Shepody’ tubers compared with their corresponding segments in nonfertilized ‘Shepody’ tubers and ‘Russet Burbank’ tubers from either treatment, which were not different from one another (Table 3). The VP-SEM/EDS analysis showed that Fe concentration was higher near the distal tuber ends and was lower near the center of the tubers (Fig. 3). The concentration of Fe was correlated to that of Zn and S (Table 4).
The distribution of Na in tubers was affected by a cultivar–segment interaction, a segment–fertilizer interaction, and marginally by a cultivar–fertilizer interaction (P = 0.0615) (Table 2). ‘Shepody’ tubers from fertilized plots accumulated more Na in the stem end and center compared with nonfertilized tubers. A similar trend was observed when comparing fertilized and nonfertilized ‘Russet Burbank’ tubers, but in this case, the accumulation was not significant and only observed in the stem end segment (Table 3). In both ‘Shepody’ and ‘Russet Burbank’ tubers from fertilized plots, the general trend was for a lower concentration of Na in the center segment of tubers compared with the stem end and bud end segments. In tubers of both cultivars from fertilized plots, the center segment contained the least Na and the stem end segment contained the most, whereas the bud end segment had an intermediate concentration (Table 3). In nonfertilized tubers of ‘Shepody’, there was no difference among the stem and center segments and there was a higher Na concentration at the bud end. In nonfertilized ‘Russet Burbank’ tubers, on the other hand, there was an equal amount of Na detected at the stem and bud ends and less detected in the center segment (Table 3). A lower concentration of Na was measured in all segments of ‘Shepody’ tubers compared with their corresponding segments in ‘Russet Burbank’ tubers in nonfertilized plots and only at the stem end in fertilized plots (Table 3). VP-SEM/EDS showed little difference in Na concentration between segments and no clear pattern of distribution was found (Fig. 3). The concentration of Na was only correlated to the concentration of Ca (Table 4).
For Al concentrations, the only significant observation was a segment–fertilizer interaction (Table 2). In both cultivars from the nonfertilized plots, there was a higher concentration of Al at the bud end compared with the stem end segment. This increase, however, was only marginally significant between the stem end and bud end segment in ‘Russet Burbank’ tubers (P = 0.0911) (Table 3). In fertilized tubers of ‘Shepody’, only a marginally significant difference between the stem and bud was observed (P = 0.0579) and no difference among any of the segments was observed in ‘Russet Burbank’ (Table 3). The VP-SEM/EDS analysis showed that the concentration of Al was highly variable (large error bars) and that there was little change from the stem end to the bud end of the tubers (Fig. 3). The concentration of Al was correlated to that of Cu and Ca (Table 4).
In the case of Zn, a significant segment effect as well as a marginal fertilizer effect (P = 0.083) was observed (Table 2). For ‘Shepody’ tubers, there was a higher concentration of Zn in fertilized tubers compared with nonfertilized tubers in some segments. This was also observed in ‘Russet Burbank’, but in this case, the differences were not found to be significant (Table 3). In ‘Shepody’ tubers, a lower concentration of Zn was observed at the stem end compared with the center and bud end segments in most cases (Table 3). In the case of ‘Russet Burbank’ tubers from both fertility regimes, there was no difference in the concentration of Zn among any of the segments (Table 3). The concentration of Zn was correlated to that of Fe, P, S, and Mg (Table 4). As a result of its low concentration in the tuber tissues, Zn was not detectable using VP-SEM/EDS.
The concentration of Cu in tubers was affected by cultivar, fertilizer, marginally by segment, and marginally by a cultivar–fertilizer interaction (P = 0.0925) (Table 2). In the fertilized trial, there was a higher Cu concentration detected in segments of ‘Shepody’ tubers compared with their corresponding segments in ‘Russet Burbank’ tubers, except between the bud end segments (Table 3). In ‘Shepody’ tubers, more Cu was detected in fertilized plots when compared with nonfertilized plots for the stem and bud end segments. Fertilized ‘Shepody’ tubers contained a significantly lower Cu concentration in the bud end segment and the concentration was higher toward the stem end of the tuber (Table 3). The concentration of Cu was only correlated to the concentration of B (Table 4). As a result of its low concentration in the tuber tissues, Cu was not detectable using VP-SEM/EDS.
For B, there was some variability among segments and fertilizer treatments (Table 3). One noticeable trend was the numerically higher B concentration from the stem end to the bud end of ‘Shepody’ tubers; however, all variability was deemed nonsignificant (Table 2). The concentration of B was only correlated to that of Cu (r = –0.441) (Table 4). B was not detectable using VP-SEM/EDS.
The concentration of Mn was affected by a cultivar–segment interaction as well as a segment–fertilizer interaction (Table 2). With the exception of the bud end segments, significantly more Mn was detected in fertilized plots compared with nonfertilized plots for both cultivars (Table 3). ‘Russet Burbank’ tubers contained a higher concentration of Mn compared with ‘Shepody’ tubers in the center and bud end segments from fertilized plots, whereas a higher Mn concentration was only observed in the center segment in tubers from nonfertilized plots (Table 3). Manganese showed completely different patterns of distribution between fertilized and nonfertilized tubers. In fertilized tubers, its concentration was progressively lower from stem to bud end, whereas in nonfertilized tubers, it was progressively higher from the stem end to the center and remained stable toward the bud end (Table 3). The concentration of Mn was only correlated to that of Ca (Table 4). Mn was not detectable using VP-SEM/EDS.
The concentration of Si was not detectable using ICAP. The pattern of Si distribution in tubers from stem to bud, as determined using VP-SEM/EDS, was similar to that of Fe (Fig. 3). There appeared to be higher, more variable concentrations of Si near distal ends of the tuber (Segments 1 and 8) and the concentration was distributed evenly among the rest of the tubers (Fig. 3).
The distribution of Cl was not detectable using ICAP. The concentration of Cl according to VP-SEM/EDS in tubers had a parabolic distribution (Fig. 3). Less Cl was measured at the stem end, the concentration rose and leveled off near the center of the tuber (Segments 3, 4, and 5), and decreased again toward the bud end of the tuber. The concentration of the bud end segment (Segment 8) was slightly lower that that of the stem end (Segment 1) (Fig. 3).
The ACD readings of the samples used for ICAP were found to be affected by fertilizer and a cultivar–segment interaction (Table 5). ACD readings were higher in tubers from fertilized plots compared with tubers from nonfertilized plots indicating that tubers from fertilized plots produce less ACD (Table 6). The distribution of ACD was found to be similar in ‘Shepody’ and ‘Russet Burbank’ with the stem end being the darkest (lowest reading), the center segment being the lightest (highest reading), and the flesh darkening again slightly toward the bud end (intermediate) (Table 6). In ‘Shepody’ tubers, there was less difference between the center and bud segments compared with ‘Russet Burbank’ tubers, which is in part because Shepody cultivar has less severe ACD in general (Table 6); therefore, the differences between stem end and bud end are less significant. When ACD was measured in Samples 1 through 8 used for VP-SEM/EDS analysis, readings were the lowest at the stem end (Segment 1), much higher in Segment 2, and then progressively higher from Segments 2 through 7. A slight decrease back to an intermediate level in the bud end segment (Segment 8) was then measured (Fig. 4). This is consistent with what was observed in the ICAP samples.
Test for effects (P values) of cultivar, segment, fertilizer regime, and their interaction (x) on after-cooking darkening readings.
Tuber after-cooking darkening values of ‘Shepody’ and ‘Russet Burbank’ tubers from fertilized and nonfertilized plots as affected by cultivar, fertilizer regime, and segment.
The severity of ACD in cooked tuber segments from samples corresponding to those used for ICAP (Treatments 1 and 2; Table 1) was evaluated. Regression equation models were generated using a stepwise regression using the element concentration from segments from raw tubers and the corresponding ACD values from the cooked tuber halves. Furthermore, regression equation models were also generated by including additional data sets (Treatments 3 through 6), which were analyzed separately using ICAP in a similar fashion as Treatments 1 and 2 (Table 1). Also evaluated, using a similar breakdown, were the complementary cooked halves of the tubers grown according to the previously described treatments (Treatments 1 through 6; Table 1). Unless otherwise stated, only Treatments 1 and 2 are discussed (Table 1).
In the raw tubers from Treatments 1 and 2, the strongest correlations between ACD and the elements studied using ICAP were with P, S, Ca, Cu, and Mg (Table 7). A stepwise regression analysis yielded P, Ca, and Cu as the best subset of elements to predict the occurrence of ACD. The regression equation generated was the following:
Pearson correlation values (r) between after-cooking darkening reading and element concentration of raw ‘Shepody’ and ‘Russet Burbank’ tuber segments from the stem, center, and bud of fertilized and nonfertilized plots analyzed using inductively coupled argon plasma.
When tubers from Treatments 3 through 6 were included along with Treatments 1 and 2, Cu was not included in the model:
Similar regression analyses were also done using the ICAP data generated from cooked tubers in Treatments 1 and 2. The strongest correlations between ACD and elements studied in cooked tubers were similar to those observed in raw tubers (data not shown). When cooked tubers from Treatments 1 and 2 were used, the regression equation was:
If Treatments 3 through 6 were added along with Treatments 1 and 2, the equation generated was:
Although both ICAP and VP-SEM/EDS techniques applied individually have merit, as a result of their complimentary characteristics, their use in combination far surpasses their individual application in this study. This study determined the distribution of a wide range of elements within tubers. To the best of our knowledge, this study contains the most comprehensive results on the tuber distribution of elements reported in a single study. Furthermore, this study described the effect of cultivar and fertilizer regime on the distribution of elements within potato tubers.
We found that there was a higher concentration of P, Mg, K, S, Cl, and, in some cases, Zn, from the stem end toward the center of the tuber and then a lower concentration again toward the bud end. Other elements such as Fe, Al, Si, and Na showed an opposite distribution pattern with a higher concentration at the distal ends and a lower concentration near the center of the tuber. These observations are contrary to previous findings that report a linear gradient from stem to bud for Fe and P in Ulster Torch tubers (Hughes and Swain, 1962a) and for P, K, and Ca in Golden Wonder tubers (Macklon and DeKock, 1967). A notable exception to these distribution patterns is Ca, which showed a more linear decrease from stem to bud. These results, however, were unclear as a result of the high-order interaction (three factors) affecting the distribution and concentration of Ca in tubers. It was clear, however, that ‘Russet Burbank’ tubers contained more Ca that ‘Shepody’ tubers. This may explain why ‘Russet Burbank’ tubers are of better storage quality compared with ‘Shepody’ tubers because Ca is responsible for increasing tuber firmness (Bretzloff, 1971; Karlsson et al., 2006; Zaehringer et al., 1969). It may also be related to why ‘Russet Burbank’ tubers show consistently higher degrees of ACD in comparison with ‘Shepody’ tubers.
The two major studies on the distribution of minerals within potato tubers were conducted in the 1960s by Macklon and DeKock (1967) and Johnston et al. (1968). Macklon and DeKock (1967) studied gradients of the major cations and anions in Golden Wonder potato tubers, including P, Ca, Mg, K, S, Fe, Na, and Cl. The latter authors measured the concentration of these ions in 16 segments from stem to bud by either flame photometric or colorimetric techniques. The description of the methods used in their paper, however, lack details and it is difficult to assess the correctness of the method used. The latter authors found that the concentration of P and K increased from stem to bud (Macklon and DeKock, 1967). This was also measured in the present study, but, in this case, we saw a much steeper gradient in the P concentration from stem to bud. In their study, the authors found no gradient in the concentration of Ca, Mg, and Cl and a lower concentration in Fe from stem to bud (Macklon and DeKock, 1967). In our study, Mg and Cl concentration followed similar parabolic patterns as P concentrations and Ca decreases from stem to bud. Our VP-SEM/EDS results suggest that more Fe may be concentrated at the distal ends of the tubers (e.g., Segments 1 and 8). However, detection of Fe using VP-SEM/EDS was difficult as a result of the low Fe concentration in tubers. Therefore, caution must be taken in accepting these results.
Johnston et al. (1968) studied the distribution of several elements (P, Ca, Mg, K, Fe, Cu, Na, Zn, Mn, Cl, and nitrogen) in four tuber sections from stem to bud. One of the downfalls of the latter study was that the small number of sampled segments limited the precision of the distribution. In ‘Russet Burbank’ tubers grown in Eastern Canada (New Brunswick), Johnston et al. (1968) found the concentration of P and K was higher in the stem end versus bud end and that of Mg, Fe, Zn, Mn, and Cl was lower in the stem compared with the bud. In the case of Mn, they observed a slight increase again in the bud end segment. The Ca concentration was higher in the stem and bud segments and lower in the two center segments. The concentration of Cu varied little from one end of the tuber to the other (Johnston et al., 1968). We have found that P and K increased from the stem end segment to the center segment but then decreased again toward the bud end segment. Using VP-SEM/EDS, we found that the concentrations of Mg and Cl did decreased from Segments 3 to 8, but not before increasing from Segment 1 to Segment 3. For Zn, our results were conflicting with those of Johnston et al. (1968), having a lower or equivalent concentration in the stem end segment compared with the other segments. However, Zn was not detected using VP-SEM/EDS and, therefore, we could not describe its distribution in detail. Our findings showed a lower concentration of Mn from the stem end to the bud end segment in fertilized tubers, but the opposite was observed in tubers from nonfertilized plots. Similar to Johnston et al. (1968), we also found Cu to be evenly distributed from stem to bud end of tubers.
Element correlation values in plants have been widely studied, but little work has been done on potatoes. Cieslik and Sikora (1998) studied correlations among a limited number of tuber components, including nitrates, nitrites, Mg, Ca, and K. Although their study correlated the whole tuber concentration of these components among different tuber cultivars, the current study correlated the element concentration not only among tubers, but also in spatial locations within the tuber. Among Mg, Ca, and K, the only significant correlation found by Cieslik and Sikora (1998) was between Mg and K. Using different cultivars and sampling methods, we have also shown Mg and K to be the only significant correlation among those three elements. The present study has greatly expanded on the report of Cieslik and Sikora (1998) by describing the correlation for not only Mg, Ca, and K, but also P, S, Fe, Na, Al, Zn, Cu, B, and Mn. It is the first time that such correlations are reported in potato tubers. These correlations are worthy of further investigation, especially because some of these elements such as P and S may be involved in ACD.
This study has demonstrated how information from element distribution can be used in predicting the occurrence of a tuber quality trait, specifically ACD. The regression equations generated using element distribution values were found to be robust showing little change across the potato production site or whether data from raw or cooked tubers were used. P and Ca were always chosen by the stepwise regression as part of the element subset, which best predicted ACD. Although S (r = 0.53) was more highly correlated to ACD than Ca (r = –0.39), it was not selected by the stepwise regression, most likely because it is highly correlated with P and hence provided redundant information; that is not to say that S should be discounted as an element that may influence the severity of ACD. The models provided from these data have good potential to predict, in an industrial setting, the occurrence of ACD in tubers. The model was able to forecast the average ACD of a tuber lot using the element concentration data from only a 10-tuber subsample more consistently and accurately than the average ACD of the 10-tuber subsample (data not shown).
The factors currently known to predict ACD are organic components such as chlorogenic acid (CGA), a compound that is synthesized and degraded during storage (Ramamurthy et al., 1992). The concentration of inorganic constituents such as the elements studied here should not change drastically on a dry weight basis once tubers are harvested. With additional information such as how the tuber element content affects the change in ACD severity over time, it may become possible to predict the severity of ACD in tubers in late stages of storage by determining the element concentrations and their spatial distribution of the same lot of tubers at harvest. Knowledge on how a crop will behave in storage can benefit the potato processor, because they will be better equipped to evaluate whether a given crop should be processed immediately or can be processed later with limited loss in processing quality.
Although it is undeniable that CGA, controlled atmosphere, and Fe are involved in the processes that lead to the formation of ACD, this study further supports the potential role of P and Ca in influencing the severity of ACD in tubers. Other elements such as S, Cu, and Mg were also well correlated to ACD (Table 7) and hence should be further investigated. These results highlight the need to understand the mechanisms responsible for the correlation between these elements and ACD. This study substantiates the use of elemental analysis as predictors for the tuber quality trait ACD. Such predictive models have a strong potential to help the potato processing industry in predicting the occurrence of ACD and in developing agronomic treatments to minimize ACD. The combination of these results along with the concentrations of organic molecules involved in ACD such as CGA may produce better predictive models.
Anderson, K.A., Magnuson, B.A., Tschirgi, M.L. & Smith, B. 1999 Determining the geographic origin of potatoes with trace metal analysis using statistical and neural network classifiers J. Agr. Food. Sci. 47 1568 1575
Arteca, R.N., Poovaiah, B.W. & Hiller, L.K. 1980 Electron microprobe and neutron activation analysis for the determination of elemental distribution in hollow heart potato tubers Amer. Potato J. 57 241 247
Cieslik, E. & Sikora, E. 1998 Correlation between the levels of nitrates and nitrites and the contents of potassium, calcium and magnesium in potato tubers Food Chem. 63 525 528
Cubbada, F. & Raggi, A. 2005 Determination of cadmium, lead, iron, nickel, and chromium in selected foods matrices using plasma spectrometric techniques Microchem. J. 79 91 96
Dale, M.F.B. & Mackay, G.R. 1994 Inheritance of table and processing quality 296 297 Bradshaw J.E. & Mackay G.R. Potato genetics CAB International Wallingford, UK
Di Giacomo, F., Del Signore, A. & Giaccio, M. 2007 Determining the geographic origin of potatoes using mineral and trace element content J. Agr. Food Chem. 55 860 866
Haase, T., Schüler, C. & Heb, J. 2007 The effect of different N and K sources on tuber nutrient uptake, total and graded yield of potatoes (Solanum tuberosum L.) for processing Eur. J. Agron. 26 187 197
Hughes, J.C. & Swain, T. 1962b After-cooking blackening in potatoes. III.*—Examination of the interaction of factors by in vitro experiments J. Sci. Food Agr. 13 358 363
Johnston, F.B., Hoffman, I. & Petrasovits, A. 1968 Distribution of mineral constituents and dry matter in the potato tuber Amer. Potato J. 45 287 292
Karlsson, B.H., Palta, J.P. & Crump, P.M. 2006 Enhancing tuber calcium concentration may reduce incidence of blackspot bruise injury in potatoes HortScience 41 1213 1221
LeRiche, E.L., Wang-Pruski, G. & Zheljazkov, V.D. 2006 Mineral concentration and distribution in tubers of fertilized and unfertilized potato cultivars Russet Burbank and Shepody as determined by VP-SEM/EDS Can. J. Plant Sci. 86 1349 1353
Mondy, N.I. & Ponnampalam, R. 1986 Potato quality as affected by sources of magnesium fertilizer: Nitrogen minerals, and ascorbic acid J. Food Sci. 51 352 354
Ramamurthy, M.S., Maiti, B., Thomas, P. & Nair, P.M. 1992 High-performance liquid chromatography determination of phenolic acids in potato tubers (Solanum tuberosum) during wound healing J. Agr. Food Chem. 40 569 572
Randhawa, K.S., Sandhu, K.S., Kaur, G. & Singh, D. 1984 Studies of the evaluation of different genotypes of potato (Solanum tuberosum L.) for yield and mineral content Qual. Plant. Plant Foods Hum. Nutr. 34 239 242
Smith, O. 1987 Effect of cultural and environmental conditions on potatoes for processing 107 119 Talburt W.F. & Smith O. Potato processing 4th Ed Van Nostrand Reihold Company Inc New York, NY
Takahashi, N., Kato, Y., Isogai, A. & Kurata, K. 2006 Silica Distribution on the husk epidermis at different parts of the panicle in rice (Oryza stativa L.) determined by x-ray microanalysis Plant Prod. Sci. 9 168 171
Wang-Pruski, G. 2006 Digital photo-imaging for evaluation of potato after-cooking darkening and its comparison with other methods Intl. J. Food Sci. Technol. 41 885 891
Warman, P.R. & Havard, K.A. 1998 Yield, vitamin and mineral contents of organically and conventionally grown potatoes and sweet corn Agr. Ecosyst. Environ. 68 207 216
White, P.J., Bradshaw, J.E., Dale, F.B., Ramsey, G., Hammond, J.P. & Broadley, M.R. 2009 Relationship between yield and mineral concentration in potato tubers HortScience 41 6 11
Wszelaki, A.L., Delwiche, J.F., Walker, S.D., Liggett, R.E., Scheerens, J.C. & Kleinheinz, M.D. 2005 Sensory quality and mineral and glycoalkaloid concentrations in organically and conventionally grown redskin potatoes (Solanum tuberosum L.) J. Sci. Food Agr. 85 720 726