Abstract
There is concern that modern cultivars and/or agronomic practices have resulted in reduced concentrations of mineral elements essential to human nutrition in edible crops. Increased yields are often associated with reduced concentrations of mineral elements in produce, and a number of recent studies have indicated that, when grown under identical conditions, the concentrations of several mineral elements are lower in genotypes yielding more grain or shoot biomass than in older, lower-yielding genotypes. Potato is a significant crop, grown worldwide, yet few studies have investigated whether increasing yields, through agronomy or breeding, affects the concentrations of mineral elements in tubers. This article examines the hypothesis that increasing yields, either by the application of mineral fertilizers and/or by growing higher-yielding varieties, leads to decreased concentrations of mineral elements in tubers. It reports that the application of fertilizers influences tuber elemental composition in a complex manner, presumably as a consequence of soil chemistry and interactions between mineral elements within the plant, that considerable variation exists between potato genotypes in the concentrations of mineral elements in their tubers, and that, like in other crops, higher-yielding genotypes occasionally have lower concentrations of some mineral elements in their edible tissues than lower-yielding genotypes.
MINERAL ELEMENTS REQUIRED BY HUMANS
Humans require at least 22 mineral elements for their well-being (White and Broadley, 2005a). These can all be supplied by an appropriate diet. However, it is estimated that over 60% of the world's six billion people are iron (Fe)-deficient, over 30% are zinc (Zn)-deficient, 30% are iodine (I)-deficient, and ≈15% are selenium (Se)-deficient (Welch and Graham, 2002; White and Broadley, 2005a). Deficiencies of calcium (Ca), magnesium (Mg), and copper (Cu) are also common in both developed and developing countries. This situation has been attributed to sourcing produce from land with low mineral phytoavailability, eating crops with inherently low tissue mineral concentrations, and/or consuming the refined foods of civilization. In particular, it appears that the occurrences of Fe-deficiency anemia, Zn-deficiency disorders, and Ca-deficiency disorders have increased dramatically in populations changing from traditional diets dominated by pulses, vegetables, and fruits to diets dominated by cereals (Graham et al., 2001; Welch and Graham, 2002), whose tissue concentrations of these elements appear to be constrained by an ancient evolutionary heritage (Broadley et al., 2004, 2007; White and Broadley, 2003). In practical terms, mineral malnutrition can be addressed through supplementation, food fortification, well-chosen dietary diversification, and/or increasing mineral concentrations in edible crops (biofortification). The biofortification of crops through application of mineral fertilizers, combined with breeding varieties with an increased ability to acquire mineral elements, is advocated as an immediate agronomic strategy both to increase mineral concentrations in edible produce and to improve yields on infertile soils (White and Broadley, 2005a).
MINERAL COMPOSITION OF POTATOES
Potatoes are an excellent source of carbohydrates, vitamins, and minerals. A single medium-sized potato weighing 200 g fresh weight (FW) can provide ≈26% of the U.S. Dietary Reference Intake (DRI) of Cu; 17% to 18% of the DRI of potassium (K), phosphorus (P), and Fe; and 5% to 13% of the DRI of Zn, Mg, and manganese (Mn) (Table 1). Potatoes are generally not rich in Ca but can be a valuable source of trace elements such as Se and I if fertilized appropriately (Broadley et al., 2006b). Furthermore, because potato tubers have relatively high concentrations of organic compounds that stimulate the absorption of mineral micronutrients by humans such as ascorbate (vitamin C), protein cysteine and various organic and amino acids (U.S. Department of Agriculture, Agricultural Research Service, 2006) and low concentrations of compounds that limit their absorption such as phytate [0.11% to 0.27% dry matter (DM); Frossard et al., 2000; Phillippy et al., 2004] and oxalate (0.03% DM; Bushway et al., 1984), the bioavailability of mineral elements in potatoes is potentially high.
The mineral content of 200 g fresh weight of potatoes and its contribution to the U.S. Dietary Reference Intake (DRI).z
HISTORICAL VARIATION IN MINERAL CONCENTRATIONS IN EDIBLE HORTICULTURAL PRODUCTS
Mayer (1997) drew attention to a possible decline in the mean DM content and mean concentrations of Ca, Cu, Mg, and sodium (Na) in raw vegetables and Cu, Fe, K, and Mg concentrations in fresh fruits available in the United Kingdom between the 1930s and 1980s. Subsequently, White and Broadley (2005b) expressed the concentrations of mineral elements in U.K. produce on a DM basis to remove any effects of tissue hydration and confirmed that the mean concentrations of Cu, Mg, and Na in the DM of vegetables and the mean concentrations of Cu, Fe, and K in the DM of fruits available in the United Kingdom had decreased significantly between the 1930s and the 1980s. White and Broadley (2005b) also analyzed comparable data from the United States and observed that the mean Ca, Cu, and Fe concentrations in the DM of vegetables and the mean concentrations of Cu, Fe, and K in the DM of fruits had decreased significantly since the 1930s. Both these publications used geometric means and t tests to support their conclusions. At the same time, Davis et al. (2004) used medians and sign (quantile) tests to conclude that the moisture-adjusted concentrations of Ca, Fe, and P in horticultural produce available in the United States had declined between 1950 and 1999. Davis (2006) noted that the conclusion of declining mineral concentrations in produce depended critically on the statistical method used, observing that nonparametric tests indicated that only Cu and Na concentrations in the DM of vegetables and the concentrations of no minerals in the DM of fruits available in the United Kingdom had decreased significantly between the 1930s and the 1980s. Nevertheless, the observation that similar changes in the geometric mean and median concentrations of mineral elements in vegetables have occurred in both the United Kingdom and United States, which share similar historical horticultural and consumer practices, suggests that this phenomenon might be a consequence of the adoption of modern varieties and/or agronomic practices (White and Broadley, 2005b). Unfortunately, none of the studies mentioned provide sufficient data to determine whether mineral concentrations of any single vegetable or fruit have altered significantly over time either in the United Kingdom or in the United States. Ultimately, different varieties and/or cultivation practices must be compared directly under the same environmental conditions to determine whether mineral concentrations in a particular crop have been affected by changing cultivars or agricultural practices (Broadley et al., 2006a; Davis, 2006; Davis et al., 2004).
It has been hypothesized that increasing yields has resulted in decreased concentrations of mineral elements in produce because of a “dilution effect” caused by plant growth rates exceeding the ability of plants to acquire these elements (Jarrell and Beverly, 1981) that is impacted by both environmental and genetic factors (Davis, 2005; Davis et al., 2004). It has long been appreciated that environmental factors accelerating plant growth rates such as higher temperatures, light intensity, and irrigation often result in reduced concentrations of macronutrient elements in plant tissues (Jarrell and Beverly, 1981), and a number of recent studies have shown that the concentrations of various mineral elements are lower in higher-yielding genotypes (Davis et al., 2004; Garvin et al., 2006; Monasterio and Graham, 2000). Monasterio and Graham (2000) observed a strong linear relationship between year of release and grain yield and negative trends between grain Fe, Zn, and P concentrations and date of release in a historical set of 26 bread wheat cultivars grown together at two locations over 3 years. Similarly, in a study of 14 U.S. hard red winter wheat varieties grown together at two locations, Garvin et al. (2006) observed negative relationships between seed Fe, Zn, and Se concentrations and both grain yields and date of release, although the extent to which this effect manifested itself was influenced by the environment. Strong negative relationships between seed P and sulfur (S) and grain yields were also observed in these field trials (Garvin et al., 2002). In leafy vegetables, Farnham et al. (2000) found strong negative relationships between head weight and Ca and Mg concentrations among 27 broccoli genotypes, and Broadley et al. (2008) observed weak negative relationships between shoot biomass and shoot Ca concentrations among genotypes of Brassica oleracea var. gongylodes (kohlrabi) and var. sabauda (Savoy cabbage) and between shoot biomass and shoot Mg concentrations among genotypes of B. oleracea var. acephala (kale/collards) and var. gongylodes (Broadley et al., 2008). This article examines the effects of fertilizer applications on the concentrations of mineral elements in potato tubers, the extent of genetic variation in the concentrations of mineral elements in potato tubers, and the hypothesis that increased yields, produced either by the application of mineral fertilizers and/or by growing higher-yielding varieties, lead to decreased concentrations of mineral elements in potato tubers.
EFFECTS OF FERTILIZER APPLICATION ON TUBER MINERAL CONCENTRATIONS
Yields of potatoes have increased significantly over the last 50 years as a result of the widespread use of fungicides, fertilizers, and irrigation. The application of mineral fertilizers to the potato crop accelerates plant growth and increases tuber yield (e.g., Allison et al., 2001a, 2001b, 2001c; Birch et al., 1967; Eppendorfer et al., 1979; Harris, 1992; Johnston et al., 1986; Neeteson and Wadman, 1987; Simmons and Kelling, 1987; Trehan and Sharma, 2003; White et al., 2005). It is evident that applying nitrogen (N) fertilizers increases tuber N concentrations (e.g., Augustin, 1975; Eppendorfer et al., 1979; Harris, 1992; Sen Tran and Giroux, 1991), applying P fertilizers increases tuber P concentrations (Allison et al., 2001a; Alvarez-Sánchez et al., 1999; Hammond and White, 2005; Rocha et al., 1997; Simpson, 1962; Trehan and Sharma, 2003), applying K fertilizers increases tuber K concentrations (Addiscott, 1976; Allison et al., 2001b; Harris, 1992; Maier, 1986), and applying Ca or Mg fertilizers increases tuber Ca (Bamberg et al., 1993, 1998; Clough, 1994; Karlsson et al., 2006; McGuire and Kelman, 1984, 1986; Simmons and Kelling, 1987) and Mg concentrations (Allison et al., 2001c), respectively. In addition, the application of these fertilizers can also affect the concentrations of other mineral elements in tubers. For example, although the application of N fertilizers often has little effect on tuber K, Ca, and Mg concentrations, it can lead to a decrease in tuber Fe and P concentrations (Allison et al., 2001c; Augustin, 1975; Harris, 1992; Simpson, 1962), the application of P fertilizers can increase tuber N and Mg concentrations but may reduce tuber Mn concentrations (Hammond and White, 2005), the application of K fertilizers often, but not always, increases tuber Mg but reduces tuber Ca and P concentrations (Addiscott, 1974, 1976; Allison et al., 2001c; Maier, 1986), and the application of Ca fertilizers generally reduces tuber Mg concentrations but can increase tuber P, S, and K concentrations (Clough, 1994; Simmons and Kelling, 1987). These effects are the result not only of complex interactions between mineral elements in the soil and their consequences for uptake by plants, but also of the effects of tissue mineral composition on the redistribution of elements within the plant. Taken together, these studies indicate that tuber mineral concentrations depend greatly on the phytoavailability of different mineral elements to the crop. Indeed, tuber mineral concentrations vary significantly with location (True et al., 1978) allowing mineral profiling of tubers to be used to verify the regional origin of produce (Anderson et al., 1999; Casañas Rivero et al., 2003; Di Giacomo et al., 2007; Padín et al., 2001). In addition, it has been observed that tuber P, S, K, Mg, Fe, molybdenum, and Cu concentrations are often higher in organically grown potatoes, whereas tuber N and Mn concentrations are higher in conventionally grown potatoes (Warman and Havard, 1998; Wszelaki et al., 2005). Thus, the complex interactions between minerals in the soil together with interactions between mineral elements in their uptake and distribution within the plant suggest that the differences in tuber mineral concentrations observed under contrasting fertilization regimes are unlikely to be the result of a simple dilution effect resulting from increased yields per se.
Other environmental factors impacting tuber yields include irrigation and elevated CO2 concentrations. Increasing yield through irrigation appears to have little effect on tuber N or K concentrations but can increase tuber P concentration and reduce tuber Ca and Mg concentrations (Asfary et al., 1983; Simpson, 1962). Increasing yield through elevated CO2 has been found to reduce the concentrations of N, K, and Mg in tubers at crop maturity (Fangmeier et al., 2002). These studies suggest that increased tuber yield can be associated with a reduction in the concentrations of some, but not all, mineral elements in the tubers.
GENETIC VARIATION IN TUBER MINERAL CONCENTRATIONS
There is considerable genetic variation in tuber mineral concentrations both between and within Solanum species (Table 2). Tuber Ca, Fe, and Zn concentrations have been shown to vary significantly between Solanum species grown under identical conditions (Andre et al., 2007; Bamberg et al., 1993, 1998). Among the Solanum species, S. gourlayi and S. microdontum had the highest tuber Ca concentrations, whereas S. kurtzianum and S. tuberosum had the lowest tuber Ca concentrations when supplied with ample Ca (Bamberg et al., 1993). Although the skin generally has a greater Ca concentration than the flesh (Ereifej et al., 1998; McGuire and Kelman, 1984, 1986; Wszelaki et al., 2005), differences in tuber Ca concentration between Solanum species do not appear to be associated simply with differences in skin-to-flesh ratios (Bamberg et al., 1993). Andre et al. (2007) observed a strong relationship between tuber Ca and Fe concentrations and a weak but significant correlation between Zn and Fe concentrations among 74 Andean landraces. They observed that some genotypes from the Ajanhuiri group had exceptionally high tuber Ca and Fe concentrations and that tuber size explained ≈13% of the variability in tuber Fe concentrations. When grown under identical conditions, S. tuberosum genotypes have been shown to differ in tuber N (Augustin, 1975; Fitzpatrick et al., 1969; Rexen, 1976), K (Brown et al., 2005; Ereifej et al., 1998; Tekalign and Hammes, 2005; Van Marle et al., 1994; Workman and Holm, 1984), P (Dampney et al., 2002; Ereifej et al., 1998; Randhawa et al., 1984; Tekalign and Hammes, 2005; Trehan and Sharma, 2003), S (Tekalign and Hammes, 2005), Ca (Ereifej et al., 1998; Karlsson et al., 2006; McGuire and Kelman, 1986; Randhawa et al., 1984; Tekalign and Hammes, 2005; Tzeng et al., 1990; Van Marle et al., 1994), Mg (Allison et al., 2001c; Ereifej et al., 1998; Randhawa et al., 1984; Tekalign and Hammes, 2005), Fe (Brown et al., 2005; Ereifej et al., 1998; Randhawa et al., 1984), Zn (Brown et al., 2005; Ereifej et al., 1998; Randhawa et al., 1984; Tekalign and Hammes, 2005), Cu (Ereifej et al., 1998; Randhawa et al., 1984; Tekalign and Hammes, 2005), and Mn concentrations (Brown et al., 2005). Systematic differences in tuber K, Mg, Fe, Zn, Cu, and Mn concentrations have also been observed between potato varieties obtained commercially (Casañas Rivero et al., 2003; Di Giacomo et al., 2007). It is likely, therefore, that tuber mineral concentrations can be manipulated genetically through commercial breeding programs.
Variation in tuber mineral concentrations among Solanum genotypes in diverse trials.z
LOWER TUBER MINERAL CONCENTRATIONS ARE SOMETIMES OBSERVED IN HIGHER-YIELDING GENOTYPES
There is some evidence in the literature to suggest that higher-yielding potato genotypes have lower concentrations of mineral elements in their tubers than lower-yielding genotypes when grown in the same environment (Table 3). For example, in studies comparing a small number of genotypes, it has been observed that tuber K (Tekalign and Hammes, 2005), P (Randhawa et al., 1984; Tekalign and Hammes, 2005; Trehan and Sharma, 2003), S (Tekalign and Hammes, 2005), Ca (Randhawa et al., 1984; Tekalign and Hammes, 2005), Mg (Allison et al., 2001c; Tekalign and Hammes, 2005), Fe (Randhawa et al., 1984; Tekalign and Hammes, 2005), Zn (Tekalign and Hammes, 2005), Cu (Tekalign and Hammes, 2005), and Mn (Tekalign and Hammes, 2005) concentrations are lower in higher-yielding varieties than in lower-yielding varieties grown in the same trial, but none of these relationships are statistically robust. Moreover, Randhawa et al. (1984) found nonsignificant positive relationships between tuber FW yield and tuber K, Mg, Zn, and Cu concentrations in their comparison of eight potato varieties.
Relationships between the concentrations of mineral elements in tubers and tuber yield among N genotypes of Solanum tuberosum in diverse trials.z
Recently, a more extensive investigation of genetic variation in concentrations of mineral elements in potato tubers was initiated at the Scottish Crop Research Institute. During this investigation, field trials incorporating 26 commercial potato varieties provided no support for the hypothesis that higher-yielding varieties have lower concentrations of mineral elements in their tubers than lower-yielding varieties (Fig. 1; Table 3). In these trials, there were no significant relationships between tuber FW yield and tuber N, K, S, Ca, Mg, Fe, Zn, or Mn concentrations, but tuber P (P = 0.014) and Cu concentrations (P < 0.001) increased significantly with increasing tuber FW yield. This is possibly the first report in which concentrations of mineral elements in edible produce are significantly positively correlated with yields and suggests that breeding for both increased yields and increased concentrations of mineral elements is feasible.
Relationships between tuber mineral concentrations and tuber yield in a core collection of 26 commercial Solanum tuberosum varieties trialled in the field at SCRI in 2006. Data are means of two replicate plots each containing eight plants at 40 cm spacing. Yields approximate 32,000 to 92,000 kg fresh weight/ha. Plant husbandry followed local practice. Statistical parameters for linear regressions are presented in Table 3.
Citation: HortScience horts 44, 1; 10.21273/HORTSCI.44.1.6
Relationships between tuber mineral concentrations and tuber yield in a core collection of 26 commercial Solanum tuberosum varieties trialled in the field at SCRI in 2006. Data are means of two replicate plots each containing eight plants at 40 cm spacing. Yields approximate 32,000 to 92,000 kg fresh weight/ha. Plant husbandry followed local practice. Statistical parameters for linear regressions are presented in Table 3.
Citation: HortScience horts 44, 1; 10.21273/HORTSCI.44.1.6
Relationships between tuber mineral concentrations and tuber yield in a core collection of 26 commercial Solanum tuberosum varieties trialled in the field at SCRI in 2006. Data are means of two replicate plots each containing eight plants at 40 cm spacing. Yields approximate 32,000 to 92,000 kg fresh weight/ha. Plant husbandry followed local practice. Statistical parameters for linear regressions are presented in Table 3.
Citation: HortScience horts 44, 1; 10.21273/HORTSCI.44.1.6
CONCLUSIONS
The concentrations of mineral elements in potato tubers are influenced by both environmental and genetic factors. One of the most significant environmental factors is the phytoavailability of mineral elements in the soil. The literature contains some evidence to support the hypothesis that increasing tuber yields, either by elevating CO2 concentrations or by growing higher-yielding varieties, can lead to decreased concentrations of some mineral elements in tubers, but this is not universally observed. A reduction in tuber mineral concentrations might be addressed through appropriate applications of mineral fertilizers. Increasing yields through the application of mineral fertilizers has diverse effects on tuber mineral concentrations depending on the composition of the fertilizer, the soil type, and the crop genotype. Interactions between mineral elements in the soil, and also within the plant, are likely to affect the concentrations of mineral elements in tubers independent of any “yield dilution” phenomenon. Consequently, it should be possible to increase tuber mineral concentrations by combining genotypes that have naturally higher tuber mineral concentrations with appropriate fertilization strategies to deliver more essential minerals to the diet without compromising yield.
Literature Cited
Addiscott, T.M. 1974 Potassium and the distribution of calcium and magnesium in potato plants J. Sci. Food Agr. 25 1173 1183
Addiscott, T.M. 1976 Nutrient concentrations and interactions in young leaves of potato plants growing with and without tubers Ann. Bot. (Lond.) 40 65 72
Allison, M.F., Fowler, J.H. & Allen, E.J. 2001a Effects of soil- and foliar-applied phosphorus fertilizers on the potato (Solanum tuberosum) crop J. Agr. Sci. 137 379 395
Allison, M.F., Fowler, J.H. & Allen, E.J. 2001b Responses of potato (Solanum tuberosum) to potassium fertilizers J. Agr. Sci. 136 407 426
Allison, M.F., Fowler, J.H. & Allen, E.J. 2001c Factors affecting the magnesium nutrition of potatoes (Solanum tuberosum) J. Agr. Sci. 137 397 409
Alvarez-Sánchez, E., Etchevers, J.D., Ortiz, J., Núñez, R., Volke, V., Tijerina, L. & Martínez, A. 1999 Biomass production and phosphorus accumulation of potato as affected by phosphorus nutrition J. Plant Nutr. 22 205 217
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 Chem. 47 1568 1575
Andre, C.M., Ghislain, M., Bertin, P., Oufir, M., del Rosario Herrera, M., Hoffmann, L., Hausman, J.-F., Larondelle, Y. & Evers, D. 2007 Andean potato cultivars (Solanum tuberosum L.) as a source of antioxidant and mineral micronutrients J. Agr. Food Chem. 55 366 378
Asfary, A.F., Wild, A. & Harris, P.M. 1983 Growth, mineral nutrition and water use by potato crops J. Agr. Sci. 100 87 101
Augustin, J. 1975 Variations in the nutritional composition of fresh potatoes J. Food Sci. 40 1295 1299
Bamberg, J.B., Palta, J.P., Peterson, L.A., Martin, M. & Krueger, A.R. 1993 Screening tuber-bearing Solanum (potato) germplasm for efficient accumulation of tuber calcium Amer. Potato J. 70 219 226
Bamberg, J.B., Palta, J.P., Peterson, L.A., Martin, M. & Krueger, A.R. 1998 Fine screening potato (Solanum) species germplasm for tuber calcium Amer. J. Potato Res. 75 181 186
Birch, J.A., Devine, J.R., Holmes, M.R.J. & Whitear, J.D. 1967 Field experiments on the fertilizer requirements of maincrop potatoes J. Agr. Sci. 69 13 24
Broadley, M.R., Bowen, H.C., Cotterill, H.L., Hammond, J.P., Meacham, M.C., Mead, A. & White, P.J. 2004 Phylogenetic variation in the shoot mineral concentration of angiosperms J. Expt. Bot. 55 321 336
Broadley, M.R., Hammond, J.P., King, G.J., Astley, D., Bowen, H.C., Meacham, M.C., Mead, A., Pink, D.A.C., Teakle, G.R., Hayden, R.M., Spracklen, W.P. & White, P.J. 2008 Shoot calcium and magnesium concentrations differ between subtaxa, are highly heritable, and associate with potentially pleiotropic loci in Brassica oleracea Plant Physiol. 146 1707 1720
Broadley, M.R., Mead, A. & White, P.J. 2006a Reply to Davis (2006) commentary on ‘Historical variation in the mineral composition of edible horticultural products’ J. Hort. Sci. Biotechnol. 81 554 555
Broadley, M.R., White, P.J., Bryson, R.J., Meacham, M.C., Bowen, H.C., Johnson, S.E., Hawkesford, M.J., McGrath, S.P., Zhao, F.-J., Breward, N., Harriman, M. & Tucker, M. 2006b Biofortification of UK food crops with selenium Proc. Nutr. Soc. 65 169 181
Broadley, M.R., White, P.J., Hammond, J.P., Zelko, I. & Lux, A. 2007 Zinc in plants New Phytol. 173 677 702
Brown, C.R., Moore, M., Alva, A.K., Boge, W.L. & Yang, C. 2005 Genetic variation of mineral content in potato and nutritional considerations 44th Annual Washington State Potato Conference 1–3 Feb. 2005 Moses Lake, WA 5 8
Bushway, R.J., Bureau, J.L. & McGann, D.F. 1984 Determinations of organic acids in potatoes by high performance liquid chromatography J. Food Sci. 49 75 81
Casañas Rivero, R., Suárez Hernández, P., Rodríguez Rodríguez, E.M., Darias Martín, J. & Díaz Romero, C.D. 2003 Mineral concentrations in cultivars of potatoes Food Chem. 83 247 253
Clough, H.G. 1994 Potato tuber yield, mineral concentration and quality after calcium fertilization J. Amer. Soc. Hort. Sci. 119 175 179
Dampney, P., P. Johnson, G. Goodlass, C. Dyer, A. Sinclair, and T. Edwards. 2002. Review of the response of potatoes to phosphate. Final Report on Defra Project PE0108.
Davis, D.R. 2005 Trade-offs in agriculture and nutrition Food Technol. 59 120
Davis, D.R. 2006 Commentary on ‘Historical variation in the mineral composition of edible horticultural products’ J. Hort. Sci. Biotechnol. 81 553 554
Davis, D.R., Epp, M.D. & Riordan, H.D. 2004 Changes in USDA food composition data for 43 garden crops, 1950 to 1999 J. Amer. Coll. Nutr. 23 669 682
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
Eppendorfer, W.H., Eggum, B.O. & Bille, S.W. 1979 Nutritive value of potato crude protein as influenced by manuring and amino acid composition J. Sci. Food Agr. 30 361 368
Ereifej, K.I., Shibli, R.A., Ajlouni, M.M. & Hussein, A. 1998 Mineral contents of whole tubers and selected tissues of ten potato cultivars grown in Jordan J. Food Sci. Technol. 35 55 58
Fangmeier, A., De Temmerman, L., Black, C., Persson, K. & Vorne, V. 2002 Effects of elevated CO2 and/or ozone on nutrient concentrations and nutrient uptake of potatoes Eur. J. Agron. 17 353 368
Farnham, M.W., Grusak, M.A. & Wang, M. 2000 Calcium and magnesium concentration of inbred and hybrid broccoli heads J. Amer. Soc. Hort. Sci. 125 344 349
Fitzpatrick, T.J., Akeley, R.V., White, J.W. & Porter, W.L. 1969 Protein, non-protein, and total nitrogen in seedlings of potatoes Amer. Potato J. 46 273 286
Food Standards Agency 2002 McCance and Widdowson's The composition of foods. Sixth summary edition Royal Society of Chemistry Cambridge, UK
Frossard, E., Bucher, M., Mächler, F., Mozafar, A. & Hurrell, R. 2000 Potential for increasing the content and bioavailability of Fe, Zn and Ca in plants for human nutrition J. Sci. Food Agr. 80 861 879
Garvin, D.F., Welch, R.M. & Finley, J.W. 2006 Historical shifts in the seed mineral micronutrient concentration of US hard red winter wheat germplasm J. Sci. Food Agr. 86 2213 2220
Garvin, D.F., Welch, R.M., Finley, J.W., Fritz, A.K., Donmez, E., Shroyer, J.P. & Paulsen, G.M. 2002 Seed micronutrient contents of a historical collection of hard red winter wheats Abstracts of the Annual Meeting, Am. Soc. Agron./Crop Sci. Soc. Amer./Soil Sci. Soc. Amer Madison, WI
Graham, R.D., Welch, R.M. & Bouis, H.E. 2001 Addressing micronutrient malnutrition through enhancing the nutritional quality of staple foods: Principles, perspectives and knowledge gaps Adv. Agron. 70 77 142
Hammond, J.P. and P.J. White. 2005. Is struvite a valuable phosphate source for agriculture? Final Report on Entrust Project 675382.006.
Harris, P.M. 1992 Mineral nutrition 162 213 Harris P.M. The potato crop: The scientific basis for improvement Chapman and Hall London, UK
Jarrell, W.M. & Beverly, R.B. 1981 The dilution effect in plant nutrition studies Adv. Agron. 34 197 224
Johnston, A.E., Lane, P.W., Mattingley, G.E.G., Poulton, P.R. & Hewitt, M.V. 1986 Effects of soil and fertilizer P on yields of potatoes, sugar beet, barley and winter wheat on a sandy clay loam soil at Saxmundham, Suffolk J. Agr. Sci. 106 155 167
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
Maier, N.A. 1986 Potassium nutrition of irrigated potatoes in South Australia. 2. Effect on chemical composition and the prediction of tuber yield response by plant analysis Aust. J. Exp. Agr. 26 727 736
Mayer, A.-M. 1997 Historical changes in the mineral content of fruits and vegetables Br. Food J. 99 207 211
McGuire, R.G. & Kelman, A. 1984 Reduced severity of Erwinia soft rot in potato tubers with increased calcium content Phytopathol. 74 1250 1256
McGuire, R.G. & Kelman, A. 1986 Calcium in potato tuber cell walls in relation to tissue maceration by Erwinia carotovora pv. atroseptica Phytopathol. 76 401 406
Monasterio, I. & Graham, R.D. 2000 Breeding for trace minerals in wheat Food Nutr. Bull. 21 392 396
National Academy of Sciences 2004 Dietary reference intakes (DRIs): Recommended intakes for individuals, elements 6 Nov. 2008 <http://fnic.nal.usda.gov/nal_display/index.php?info_center=4&tax_level=3&tax_subject=256&topic_id=1342&level3_id=5140>.
Neeteson, J.J. & Wadman, W.P. 1987 Assessment of economically optimum application rates of fertilizer N on the basis of response curves Fert. Res. 12 37 52
Padín, P.M., Peña, R.M., García, S., Iglesias, R., Barro, S. & Herrero, C. 2001 Characterization of Galician (N.W. Spain) quality brand potatoes: A comparison study of several pattern recognition techniques Analyst (Lond.) 126 97 103
Phillippy, B.Q., Lin, M. & Rasco, B. 2004 Analysis of phytate in raw and cooked potatoes J. Food Compost. Anal. 17 217 226
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 contents Qual. Plant. 34 239 242
Rexen, B. 1976 Studies of protein of potatoes Potato Res. 19 189 202
Rocha, F.A.T., Fontes, P.C.R., Fontes, R.L.F. & Reis, F.P. 1997 Critical phosphorus concentrations in potato plant parts at two growth stages J. Plant Nutr. 20 573 579
Sen Tran, T. & Giroux, M. 1991 Effects of N rates and harvest dates on the efficiency of 15N-labelled fertilizer on early harvested potatoes (Solanum tuberosum L.) Can. J. Soil Sci. 71 519 532
Simmons, K.E. & Kelling, K.A. 1987 Potato responses to calcium application on several soil types Amer. Potato J. 64 119 136
Simpson, K. 1962 Effects of soil-moisture tension and fertilizers on the yield, growth and phosphorus uptake of potatoes J. Sci. Food Agr. 13 236 248
Tekalign, T. & Hammes, P.S. 2005 Growth and productivity of potato as influenced by cultivar and reproductive growth. II. Growth analysis, tuber yield and quality Sci. Hort. 105 29 44
Trehan, S.P. & Sharma, R.C. 2003 External phosphorus requirement of different potato (Solanum tuberosum) cultivars resulting from different internal requirements and uptake efficiencies Indian J. Agr. Sci. 73 54 56
True, R.H., Hogan, J.M., Augustin, J., Johnson, S.J., Teitzel, C., Toma, R.B. & Shaw, R.L. 1978 Mineral composition of freshly harvested potatoes Amer. Potato J. 55 511 519
Tzeng, K.-C., McGuire, R.G. & Kelman, A. 1990 Resistance of tubers from different potato cultivars to soft rot caused by Erwinia carotovora subsp. atroseptica Amer. Potato J. 67 287 305
U.S. Department of Agriculture, Agricultural Research Service 2006 USDA National Nutrient Database for Standard Reference, Release 19. Nutrient Data Laboratory home page May 2007 <http://www.ars.usda.gov/ba/bhnrc/ndl>.
Van Marle, J.T., Van Dijk, C., Voragen, A.G.J. & Biekman, E.S.A. 1994 Comparison of the cooking behaviour of the cultivars Nicola and Irene with respect to pectin breakdown and the transfer of ions Potato Res. 37 183 195
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
Welch, R.M. & Graham, R.D. 2002 Breeding crops for enhanced micronutrient content Plant Soil 245 205 214
White, P.J. & Broadley, M.R. 2003 Calcium in plants Ann. Bot. (Lond.) 92 487 511
White, P.J. & Broadley, M.R. 2005a Biofortifying crops with essential mineral elements Trends Plant Sci. 10 586 593
White, P.J. & Broadley, M.R. 2005b Historical variation in the mineral composition of edible horticultural products J. Hort. Sci. Biotechnol. 80 660 667
White, P.J., Broadley, M.R., Hammond, J.P. & Thompson, A.J. 2005 Optimising the potato root system for phosphorus and water acquisition in low-input growing systems Aspects App. Biol. 73 111 118
Workman, M. & Holm, D.G. 1984 Potato clone variation in blackspot and soft rot susceptibility, redox potential, ascorbic acid, dry matter and potassium Amer. Potato J. 61 723 733
Wszelaki, A.L., Delwiche, J.F., Walker, S.D., Liggett, R.E., Scheerens, J.C. & Kleinhenz, M.D. 2005 Sensory quality and mineral and glycoalkaloid concentrations in organically and conventionally grown redskin potatoes (Solanum tuberosum) J. Sci. Food Agr. 85 720 726
