Genetic variation in the concentrations of mineral elements in edible portions of crop plants
Increasing the concentrations of essential mineral elements in produce through the application of mineral fertilizers can be complemented by breeding crops with an increased ability to acquire and accumulate these minerals in their edible portions. However, it must be recognized that genotypic enhancement can only improve the acquisition, utilization or accumulation of mineral elements available to the crop. Considerable genetic variation appears to exist in the concentrations of the mineral elements most frequently lacking in human diets in the edible portions of most crop species.
Data are being amassed on within-species genetic variation in Fe, Zn, Cu, Ca, Mg and Se concentrations in the edible tissues of crop plants (Table 1). However, published data on genetic variation in I concentrations in plant tissues are scarce. Nevertheless, within-species variation in leaf I concentration has been reported for both perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) and shown to be heritable (Butler & Glenday, 1961; Alderman & Jones, 1967). These studies provide strong evidence that leaf I concentration is under genetic control, which provides the rationale to establish the extent of within-species genetic variation in the I concentration of edible portions of crop plants (Lyons et al., 2004).
Most surveys of genetic variation in mineral concentrations have been performed on the carbohydrate-rich portions of staple crops. These include rice (O. sativa), wheat (Triticum spp.) and maize (Z. mays) grains, common bean (P. vulgaris) seeds, sweet potato (Ipomoea batatas) tubers, and cassava (Manihot esculenta) and yam (Dioscorea alata) roots (Grusak & Cakmak, 2005; White & Broadley, 2005a; Pfeiffer & McClafferty, 2007). In addition, genetic variation in the mineral concentrations of the edible portions of other commonly consumed cereals (such as barley (Hordeum vulgare), millet (Panicum miliaceum) and sorghum), legumes (such as pea (Pisum sativum), soybean (Glycine max and Glycine soja), lentil (Lens culinaris), pigeonpea (Cajanus spp.), cowpea (Vigna unguiculata), groundnut (Vigna subterranea), chickpea (Cicer arietinum) and peanut (Arachis hypogaea)), vegetables (such as brassicas (Brassica spp.), spinach, carrot (Daucus carota) and potato (S. tuberosum and Solanum phureja)) and fruits (such as tomato (S. lycopersicum) and bananas/plantains (Musa spp.)) that contribute significantly to human diets has also been surveyed (Grusak & Cakmak, 2005; White & Broadley, 2005a; Johns & Eyzaguirre, 2007; Pfeiffer & McClafferty, 2007). These data suggest that there is sufficient genetic potential within cultivated species to manipulate the tissue concentrations of all the mineral elements lacking in human diets through breeding programmes.
There is genetic variation in the concentrations of mineral elements in the grains of most cereal species. Concentrations of Fe and Zn in cereal grain vary 1.5- to 4-fold among genotypes depending on the genetic diversity of the material tested (Table 1; see References S11). Some of this variation can be attributed to differences in grain yield (see section ‘Historical trends in the concentrations of mineral elements in edible tissues’). Although the cultivated germplasm of some cereals, such as wheat, may have a limited genetic variation in grain Fe and Zn concentrations, wild relatives often possess considerable variation (Monasterio & Graham, 2000; Cakmak et al., 2004; Welch et al., 2005; Chhuneja et al., 2006; Pfeiffer & McClafferty, 2007; Cakmak, 2008; Tiwari et al., 2008) and accessions with grain Fe and Zn concentrations at least 2-fold higher than the most widely grown varieties are available for many cereal species (White & Broadley, 2005a). Differences in grain Zn and Fe concentrations between species of wild and cultivated wheat have been attributed, in part, to allelic variation at a chromosomal locus that promotes early senescence and remobilization of protein, Fe, Zn and Mn from senescing leaves to seeds, and the introgression of the high grain protein content (Gpc-B1) locus from a wild tetraploid wheat (Triticum turgidum ssp. dicoccoides) to a cultivated wheat (Triticum durum) resulted in higher concentrations of Fe and Zn in its grain (Distelfeld et al., 2007). In addition, highly significant positive correlations between grain Fe and Zn concentrations have been observed in maize (Maziya-Dixon et al., 2000; Long et al., 2004), wheat (Graham et al., 1999; Garvin et al., 2006), sorghum (Reddy et al., 2005) and pearl millet (Pennisetum glaucum; Velu et al., 2007), which increase the possibilities of breeding for increased concentrations of both Fe and Zn simultaneously. Interestingly, studies of related Triticum species showed strong positive correlations between grain protein and Zn concentrations (Ozturk et al., 2006; Distelfeld et al., 2007). Chromosomal loci affecting grain Fe and Zn have been mapped in rice (Gregorio et al., 2000) and wheat (cited by Ortiz-Monasterio et al., 2007). Grain Ca has been found to vary almost 3-fold and grain Mg c. 1.5-fold among wheat accessions (Peterson et al., 1986; Graham et al., 1999; Bálint et al., 2001; Oury et al., 2006) and genetic variation in grain Cu concentrations has been reported for wheat (McGrath, 1985; Peterson et al., 1986; Bálint et al., 2001; Garvin et al., 2006) and pearl millet (Abdalla et al., 1998). Genetic variation in grain Se has also been reported for wheat (Lyons et al., 2005; Garvin et al., 2006) and oat (Avena sativa; Eurola et al., 2004), although the expression of this trait is strongly dependent upon weather conditions, crop husbandry and selenium fertilization (Eurola et al., 2004; Lyons et al., 2005; Garvin et al., 2006). Although it is often suggested that little variation exists in grain Se concentrations amongst modern bread and durum wheats, the wild wheats (Triticum dicoccum and Triticum spelta) and their relatives (Aegilops tauschii) have significantly higher Se concentrations than cultivated wheat and can be integrated into breeding programmes (Graham et al., 2001; Lyons et al., 2003, 2005; White & Broadley, 2005a; White et al., 2007b).
Because the seeds of many cereals are often consumed after milling or polishing, it is pertinent to consider whether genetic variation in the distribution of mineral elements within the seed can be utilized in biofortification strategies. Mineral elements are nonhomogenously distributed within the seed and the concentrations of many mineral elements are highest in the husk and/or aleurone layers (see References S12). Milling or polishing cereal seeds can, therefore, remove large quantities of mineral elements from the diet and the extent of these losses is genotype dependent (Gregorio et al., 2000; Vasconcelos et al., 2003; Ma et al., 2004; Lyons et al., 2005; Prom-u-thai et al., 2007). The partitioning of mineral elements such as Fe and Zn within cereal grains is affected by aspects of grain morphology, such as grain size, embryo size, and the number and thickness of the tissue layers (Welch et al., 1993; Oikeh et al., 2003a; Cakmak et al., 2004). For example, grain Fe, Zn, Ca, Mg, and Cu concentrations in maize, rice and barley are related to the number of aleurone cell layers, which is cultivar dependent (Welch et al., 1993; Prom-u-thai et al., 2007).
Seeds of legumes, such as common bean, pea, lentil, soybean, mungbean (Vigna radiata), chickpea, peanut and groundnut, generally have higher concentrations of Fe, Zn, Ca and Mg than cereal grains, and significant within-species genetic variation in the concentrations of all these elements has been observed (Table 1; White & Broadley, 2005a; Pfeiffer & McClafferty, 2007). Seed Fe and Zn concentrations have been found to vary from 1.4- to 6.6-fold among genotypes of legume species grown together in the same environment (Table 1). Some of this variation is associated with seed morphology, as the tissue distribution of Zn and Fe in legume seeds is under genetic control (Moraghan et al., 2002; Ariza-Nieto et al., 2007). Domestication does not appear to have affected the mean concentration or range of Fe or Zn in bean seeds (Beebe et al., 2000). Seed Ca and Mg concentrations also vary considerably among genotypes of legume species grown in the same environment. For example, seed Ca concentrations varied 9-fold among 120 segregating F2,3s from a wide cross between a wild and a cultivated P. vulgaris genotype (Guzmán-Maldonado et al., 2003), and seed Ca and Mg concentrations ranged from 0.03 to 0.26% dry weight (DW) and from 0.11 to 0.25% DW, respectively, among 481 P. sativum accessions (Grusak & Cakmak, 2005). Seed Ca concentrations varied up to 2.6-fold among 10 chickpea genotypes (Abbo et al., 2000) and 2-fold among soybean cultivars (Raboy et al., 1984; Horner et al., 2005). Similarly, seed Cu concentrations show appreciable within-species genetic variation (Deosthale, 1981; Branch & Gaines, 1983; Grusak & Cakmak, 2005; Ariza-Nieto et al., 2007; Zia-Ul-Haq et al., 2007). Little information is available on within-species variation in seed Se or I concentrations in legumes, although variation in Se accumulation among soybean genotypes has been documented (Yang et al., 2003; Zhang Y et al., 2003b). Seed Fe, Zn and Ca concentrations all behave as quantitative traits in legume species and genetic loci influencing them can be mapped using QTL analysis (Beebe et al., 2000; Guzmán-Maldonado et al., 2003; Cichy et al., 2005; Gelin et al., 2007) and introgressed into commercial germplasm (Islam et al., 2004). It is thought that most genetic variation in Zn concentration in common bean is controlled by a single locus (Guzmán-Maldonado et al., 2003; Cichy et al., 2005; Gelin et al., 2007). The positive correlations among the concentrations of Zn and Fe, and Ca and Mg in beans (Beebe et al., 2000; House et al., 2002; Guzmán-Maldonado et al., 2003; Gelin et al., 2007) suggest the possibility of breeding for increased concentrations of these elements simultaneously.
The concentrations of some mineral elements, such as Ca, Mg, Fe and Zn, are often greater in leafy vegetables than in grain, seed, fruit or tuber crops (Table 1). In a core collection of B. oleracea thought to incorporate approx. 99% of the common allelic polymorphisms in this species, leaf Fe varied 45-fold and leaf Zn varied 26-fold between accessions in one glasshouse trial (M. R. Broadley, J. P. Hammond and P. J. White, unpublished observations). In the same glasshouse trial, leaf Fe varied 4.8-fold and leaf Zn varied 3.6-fold among 74 commercial varieties representing all subtaxa of B. oleracea. Among 22 accessions of B. oleracea var. acephala (kale/collards), leaf Fe varied 1.6-fold and leaf Zn varied 1.8-fold, and the rankings of cultivars were consistent between years despite substantial year-to-year variation in leaf Fe and Zn concentrations (Kopsell et al., 2004b). Among 327 accessions of spinach, leaf Fe varied 2.7-fold, and leaf Zn varied 12-fold (Grusak & Cakmak, 2005). Genetic variation in shoot Fe and Zn concentrations has also been observed in Brassica rapa (Wu et al., 2007, 2008) and onion (Allium cepa; Alvarez et al., 2003). Shoot Ca and Mg concentrations varied up to 3-fold among accessions of B. oleracea (Farnham et al., 2000; Rosa et al., 2002; Kopsell et al., 2004b; Broadley et al., 2008), B. rapa (Wu et al., 2008), spinach (Grusak & Cakmak, 2005), onion (Alvarez et al., 2003) and chickpea (Ibrikci et al., 2003), and the concentrations of these elements in shoot tissues appear to be correlated (White & Broadley, 2005a; Broadley et al., 2008; Wu et al., 2008). Several QTLs affecting shoot Ca and Mg concentrations have been found in a cross between a rapid-cycling B. oleracea var. alboglabra and an F1 B. oleracea var. italica (Broadley et al., 2008), and a QTL affecting shoot Mg concentration has been found in B. rapa (Wu et al., 2008), but no candidate genes have been identified yet. Within-species genetic variation in shoot Mg concentrations has also been reported for forage of Jerusalem artichoke (Helianthus tuberosus; Seiler & Campbell, 2006) and herbage of forage grasses, such as crested wheatgrass (Agropyron cristatum), Italian ryegrass (Lolium multiflorum), orchard grass, perennial ryegrass (Lolium perenne), reed canary grass (Phalaris arundinacea), Russian wildrye (Psathyrostachys juncea) and tall fescue (Festuca arundinacea), which have been studied because of the negative economic consequences of grass tetany in ruminant animals when their diets contain insufficient Mg (Hides & Thomas, 1981; Sleper et al., 1989; Smith et al., 1999; Jefferson et al., 2001). Leaf Cu concentrations vary many fold among genotypes of B. oleracea (M. R. Broadley, P. J. White and J. P. Hammond, unpublished observations), spinach (Grusak & Cakmak, 2005), chickpea (Ibrikci et al., 2003) and onion (Alvarez et al., 2003). Genetic variation in shoot Se concentrations has been observed in B. oleracea (Kopsell & Randle, 2001; Farnham et al., 2007) and onions (Kopsell & Randle, 1997).
The concentrations of some elements, such as Fe, Ca and Mg, are also generally higher in root crops than in cereal grains, legume seeds, fruits or tubers (Table 1). There is considerable within-species genetic variation in the concentrations of these elements in root crops. For example, there is 51-fold variation in root Cu, 38-fold variation in root Fe, 14-fold variation in root Zn, 8-fold variation in root Ca, and 4.6-fold variation in root Mg among 600 cassava genotypes (Chávez et al., 2005). Considerable genetic variation in root Fe and Zn concentrations has also been observed in sweet potato (Pfeiffer & McClafferty, 2007; Courtney et al., 2008) and carrot (Nicolle et al., 2004b), and in root Ca and Mg concentrations in carrot (Nicolle et al., 2004b) and sugar beet (Beta vulgaris) (Baierová & Baier, 1993).
Fruits generally have low concentrations of mineral elements that are less mobile in the phloem, and the concentrations of mineral elements essential to human nutrition do not appear to vary greatly among cultivars. For example, fruit Fe, Zn, Ca and Mg concentrations differed less than 2-fold between six strawberry (Fragaria spp.) cultivars (Hakala et al., 2003), as did Fe and Zn concentrations in fruits of three apple (Malus domestica) varieties (Iwane, 1991) and Fe, Zn, Mg and Cu concentrations in representative fingers of three plantain varieties (Davey et al., 2007). However, mean fruit Ca concentrations differed 2.5-fold among these plantain varieties (Davey et al., 2007) and fruit Cu concentrations varied 2.5-fold among the apple varieties (Iwane, 1991). Similarly, there was 2.3-fold variation in Ca concentrations and 8-fold variation in Fe concentrations among 11 plum (Prunus domestica) varieties (Nergiz & Yildiz, 1997). Some genetic variation in the Se concentrations in tomato has also been reported (Shennan et al., 1990, Pezzarossa et al., 1999).
Limited within-species genetic variation in the concentrations of mineral elements has also been observed in tubers (Table 1; Pfeiffer & McClafferty, 2007; White et al., in press). In general, tuber concentrations of mineral elements differ 2- to 3-fold among genotypes, although greater variation in tuber Fe concentrations is occasionally observed (Agbor-Egbe & Trèche, 1995; White et al., in press). Genetic variation in tuber Fe, Zn, Ca, Mg and Cu concentrations has been observed in yams (Agbor-Egbe & Trèche, 1995; Pfeiffer & McClafferty, 2007), oca (Sangketkit et al., 2001) and cultivated potato species (Pfeiffer & McClafferty, 2007; White et al., in press and references therein). As tubers acquire most of their mineral elements via the phloem (Westermann, 2005), the concentrations of Ca, Mg and Zn in tubers are often lower than in other edible tissues (Table 1).
Genetic variation in the concentrations of promoters and antinutrients in the edible portions of crop plants
The bioavailability of essential mineral elements depends greatly on the presence in a meal of substances that promote or inhibit their absorption by the gut (Frossard et al., 2000; Reddy, 2002; Hotz & Brown, 2004; Welch & Graham, 2004; White & Broadley, 2005a; Slingerland et al., 2006; Bohn et al., 2008). The main substances known to inhibit the absorption of Fe, Zn, Ca and Mg are phytate from cereal grains and legume seeds and polyphenolics from beverages such as tea and coffee, beans and sorghum. The absorption of Ca is inhibited additionally by oxalate present in certain fruit and vegetables (Franceschi & Nakata, 2005; Titchenal & Dobbs, 2007). It is estimated that only c. 5% of the Fe and 25% of the Zn present in legume and cereal seeds is bioavailable (Pfeiffer & McClafferty, 2007). The bioavailability of Fe is reduced at dietary phytate/Fe molar quotients greater than 1 and the bioavailability of Zn is reduced when the phytate/Zn molar quotient exceeds about 6 (Lönnerdal, 2002; Hurrell, 2003). Substances enhancing the absorption of Fe and Zn include ascorbic acid and β-carotene from fruits and vegetables, whilst cysteine-rich polypeptides from plant and animal sources promote the absorption of Fe, Zn and Cu. Inulin, a polysaccharide of fructose often with a terminal glucose unit that is present in significant amounts in a wide range of edible crops, promotes the absorption of both Ca and Mg (Roberfroid, 2005, 2007).
Phytate concentrations vary considerably among cereal, legume and vegetable crops (Holland et al., 1991, 1992; Frossard et al., 2000; Reddy, 2002; Hotz & Brown, 2004; Vreugdenhil et al., 2005). Significant within-species genetic variation has been found for grain phytate concentration in rice (Welch et al., 2000, Glahn et al., 2002), wheat (Lolas et al., 1976; Raboy et al., 1991; Erdal et al., 2002; Welch et al., 2005), barley (Dai et al., 2007), pearl millet (Abdalla et al., 1998), oat (Lolas et al., 1976), triticale (Feil & Fossati, 1997) and sorghum (Reddy et al., 2005; Slingerland et al., 2006). Similarly, seed phytate concentrations vary among genotypes of common bean (Lolas & Markakis, 1975; Coelho et al., 2002; House et al., 2002; Cichy et al., 2005; Ariza-Nieto et al., 2007) and soybean (Lolas et al., 1976; Raboy et al., 1984; Horner et al., 2005) and among accessions of B. rapa (Zhao et al., 2007, 2008). In addition, natural and induced mutants with similar total P concentrations to conventional varieties, but reduced seed phytate concentrations, named low phytic acid (lpa) mutants, have been described in wheat, barley, maize, rice and soybean (reviewed by Raboy, 2003, 2007; Bohn et al., 2008). Although many of these mutants have reduced rates of germination and agronomic yield, this is not always the case (Ertl et al., 1998, Pilu et al., 2003; Hulke et al., 2004; Oltmans et al., 2005; Bregitzer & Raboy, 2006; Guttieri et al., 2006a; Raboy, 2007). Two phenotypes of lpa mutants are commonly observed. Seeds of one phenotype do not accumulate greater concentrations of inositol phosphates (IPs), whereas seeds of the other phenotype accumulate IP3, IP4 and IP5 (Raboy, 2003, 2007). The lpa mutations do not appear to have much effect on the concentrations or distributions of mineral elements within the seed (Hatzack et al., 2000; Guttieri et al., 2004, 2006b; Liu et al., 2004a, 2007; Ockenden et al., 2004; Bryant et al., 2005; Joyce et al., 2005; Lin et al., 2005), but replacing conventional varieties with lpa mutants in diets improves the mineral nutrition of monogastric animals, including humans (Adams et al., 2002; Mendoza et al., 1998, 2001; Hambidge et al., 2004, 2005; Mazariegos et al., 2006; see reviews by Mendoza, 2002; Raboy, 2007). In humans, this beneficial effect is most apparent when the dietary consumption of minerals is low. Several QTLs affecting the phytate concentration in seeds of rice (Stangoulis et al., 2007) and common bean (Cichy et al., 2005) and in seeds and leaves of B. rapa (Zhao et al., 2007, 2008) have been identified for use as molecular markers in breeding programmes. Weak correlations between phytate and Fe, Zn, Ca or Mg concentrations in sorghum grain (Reddy et al., 2005), soybean seeds (Raboy et al., 1984) and common bean seeds (Cichy et al., 2005), and a lack of coincidence of QTLs affecting seed phytate and those affecting Fe or Zn concentrations (Vreugdenhil et al., 2004; Cichy et al., 2005; Stangoulis et al., 2007; Waters & Grusak, 2008b) suggest that these traits are controlled by different genes and that it would be possible to breed for Fe-, Zn-, Ca- and/or Mg-dense edible portions with low phytate concentrations.
It has long been known that P and phytate concentrations in cereal grains and legume seeds are increased greatly by the application of P fertilizers and reduced by the application of Zn fertilizers (Reddy et al., 1989; Ryan et al., 2008). Thus, judicious applications of inorganic fertilizers could complement any genetic approaches to reduce dietary phytate intake and increase dietary Zn/phytate quotients.
Genetic variation in the concentrations of polyphenolics in seeds of sorghum (Dicko et al., 2002) and common bean (Guzmán-Maldonado et al., 2000, 2003; House et al., 2002) has also been reported. However, because polyphenolic compounds differ in their abilities to bind Fe (Brune et al., 1989; Hurrell et al., 1999), and many polyphenols have been shown to be beneficial to human health (Scalbert et al., 2005), it will be necessary to reduce the concentrations of only those polyphenols that bind Fe most avidly to increase Fe bioavailability.
Cereal grains, vegetables and fruits vary widely in their oxalate concentrations (USDA, 1984; Libert & Franceschi, 1987; Holland et al., 1992; Kim et al., 2007; Massey, 2007). Most angiosperms deposit Ca-oxalate crystals in cell vacuoles or, occasionally, the cell wall, although members of some commelinoid and noncommelinoid monocot families, including the Poaceae, Liliaceae and Zingiberaceae, appear to lack Ca-oxalate crystals (Prychid & Rudall, 1999; Franceschi & Nakata, 2005). The shapes and tissue distributions of oxalate crystals differ among plant species. Plants that can accumulate large quantities of oxalate (> 5% DM) in their edible tissues include members of the Caryophyllales (e.g. amaranth, beet/chard, purslane (Portulaca oleracea), spinach, tetragona (Tetragonia tetragonioides) and rhubarb), Araceae (e.g. taro (Colocasia esculenta)), and Oxalidaceae (e.g. carambola and oca). Seeds of some legumes, and roots of carrot and cassava can also accumulate high concentrations of oxalate on occasion (USDA, 1984; Massey, 2007). In general, oxalate concentrations are far greater in leaves than in roots or fruits (Libert & Franceschi, 1987). In some species, such as amaranth, beet/chard and spinach, much of the oxalate is insoluble, whereas in other species, such as oca, sweet pepper (Capsicum annuum), eggplant (Solanum melongena) and carrot, the oxalate is soluble (Libert & Franceschi, 1987; Albihn & Savage, 2001; White & Broadley, 2003; Franceschi & Nakata, 2005; White, 2005; Kim et al., 2007). In plants that precipitate Ca-oxalate, the accumulation of oxalate is directly proportional to tissue Ca concentration and is, therefore, strongly dependent upon Ca phytoavailability and plant growth rate (Libert & Franceschi, 1987; Kinzel & Lechner, 1992; Franceschi & Nakata, 2005). Some within-species genetic variation in oxalate concentrations has been observed in beet (Libert & Franceschi, 1987), spinach (Kitchen et al., 1964; Libert & Franceschi, 1987; Kawazu et al., 2003; Mou, 2008), rhubarb (Libert & Creed, 1985; Libert, 1987; Libert & Franceschi, 1987), carambola (Wilson et al., 1982), oca (Ross et al., 1999; Albihn & Savage, 2001; Sangketkit et al., 2001), taro (Tanaka et al., 2003) and soybean (Massey et al., 2001; Horner et al., 2005), and several mutants have been identified in the forage legume Medicago truncatula with reduced or altered accumulation of Ca-oxalate in their leaves (Nakata & McConn, 2000; McConn & Nakata, 2002). For two of the M. truncatula calcium oxalate deficient mutants (cod5 and cod6), reduced accumulation of Ca-oxalate has been correlated with increased Ca bioavailability in herbage (Nakata & McConn, 2006, 2007; Morris et al., 2007).
The concentrations of β-carotene and ascorbic acid vary among plant tissues and among plant species (Bhaskarachary et al., 1995, 2007; Frossard et al., 2000; FSA, 2002; USDA-ARS, 2007). Fruits and vegetables are important dietary sources of ascorbic acid and β-carotene. Significant within-species genetic variation in β-carotene concentration has been found in edible portions of a number of crop species, including cassava (Chávez et al., 2000, 2005; Maziya-Dixon et al., 2000; Thakkar et al., 2007), carrot (Nicolle et al., 2004b; Baranska et al., 2006), lettuce (Lactuca sativa; Nicolle et al., 2004a; Mou, 2005), B. oleracea (Schonhof & Krumbeln, 1996; Kurilich et al., 1999; Kopsell et al., 2004a), sweet potato (Nestel et al., 2006), potato (Nesterenko & Sink, 2003; Morris et al., 2004), sweet pepper (Simonne et al., 1997; Howard et al., 2000), tomato (Fraser & Bramley, 2004), chickpea (Abbo et al., 2005), immature soybean (Simonne et al., 2000), common bean, rice, wheat (Welch & Graham, 2005; Howitt & Pogson, 2006), sorghum (Kapoor & Naik, 1970; Reddy et al., 2005), pearl millet (Kapoor & Naik, 1970), maize (Kurilich & Juvik, 1999; Maziya-Dixon et al., 2000; Hulshof et al., 2007; Ortiz-Monasterio et al., 2007; Harjes et al., 2008) and plantains (Davey et al., 2007). In addition, natural mutations in either LeDET1 or LeDDB1 have been found to lead to increased concentrations of β-carotene and ascorbic acid in tomato fruit (Liu et al., 2004b; Bino et al., 2005), a spontaneous, semidominant mutation of the orange (Or) gene encoding a plastid-associated DnaJ-related protein (Lu et al., 2006) has been found to lead to high concentrations of β-carotene in cauliflower (Brassica oleracea var. botrytis) curd (Li et al., 2001a), and natural mutations in the promoter of the yellow1 (Y1) gene encoding a phytoene synthase have been found to be associated with greater concentrations of β-carotene in the endosperm of maize (Palaisa et al., 2003). Both cauliflower Or hybrids and maize with yellow endosperm are available commercially.
There is also significant genetic variation in the concentration of ascorbic acid in edible tissues of many crop species including carrot (Nicolle et al., 2004b), cassava (Chávez et al., 2000), lettuce (Nicolle et al., 2004a), B. oleracea (Kurilich et al., 1999), sweet pepper (Simonne et al., 1997; Howard et al., 2000), strawberry (Hakala et al., 2003) and banana (Wall, 2006). In addition to genotypic effects, ascorbate concentrations in plant tissues are affected by other factors such as developmental stage, time of day, and growth environment (Smirnoff et al., 2001; Ishikawa et al., 2006; Smith et al., 2007). Ascorbate concentrations increase in response to diverse environmental stresses and, in particular, high light intensities. This is thought to be related to the role of ascorbate as a key antioxidant in plants (Smirnoff et al., 2001; Noctor, 2006).