The family Brassicaceae (Cruciferae) represents a diverse group of plant species commercially important in many parts of the world. The plants produce condiment mustard; leafy, stored, processed, and picked vegetables; seed oils for margarine, salad oils, cooking oils, and industrial uses; animal fodders; and green manure crops (Williams and Hill, 1986). The six economically important Brassica species in world production are genetically related. Three diploid species—B. nigra, B. rapa, B. oleracea—are the natural progenitors of the amphidiploids species: B. juncea, B. napus, and B. carinata. The “cole crops,” such as broccoli, brussels sprouts, cabbage, cauliflower, curly kale, and kohlrabi, make up B. oleracea. Oil seed types, as well as Chinese cabbage, turnip, and pak-choi are found within B. rapa. Brassica nigra, popularly known as black mustard, is used for its seed oil content. The cross between B. nigra and B. oleracea produces B. carinata. It is a tall, leafy plant mainly restricted to Ethiopia. The cross between B. oleracea and B. rapa produces B. napus. Rutabagas are a variety of B. napus, as well as the economically important oilseed rape. The cross between B. rapa and B. nigra produces B. juncea, the group known as mustard. The mustards are consumed in the southern United States as mustard greens, or are used as a condiment or spice (Williams and Hill, 1986).
The three common diploid species in the genus Brassica have haploid numbers of 8 (B. nigra), 9 (B. oleracea), and 10 (B. rapa) (Nwankiti, 1970). It is believed that these numbers represent a phylogenetically ascending series (Manton, 1932). The higher chromosome number species in the genus are believed to be amphidiploids and have haploid numbers of 17 (B. carinata), 18 (B. juncea), and 19 (B. napus). The genetic arrangement of the diploid and amphidiploid Brassica species is credited to the U (1935). The diagram is referred to as the triangle of U (Nwankiti, 1970) (Fig. 1). Study of interspecific hybridization and subsequent pairing within the genus led to the designation of two distinct groups: 1) monogenomic diploids of B. nigra (2n = 16; BB), B. oleracea (2n = 18; CC), and B. rapa (2n = 20; AA); and 2) amphidiploids of B. juncea (2n = 36; AABB), B. carinata (2n = 34; BBCC), and B. napus (2n = 38; AACC)(Prakash and Hinata, 1980).
Carotenoids are C40 isoprenoid polyene plant secondary compounds that form lipid-soluble yellow, orange, and red pigments (Zaripheh and Erdman, Jr., 2002). The carotenoids can be divided into two groups: 1) hydrocarbon carotenes (C40H56) and 2) their oxygenated derivative, referred to as xanthophylls. Carotenoids span the thylakoid membranes of chlorophyll (Chl) complexes and function in accessory roles for light harvesting, photoprotection, and structural stabilization (Demmig-Adams et al., 1996; Tracewell et al., 2001). Carotenoid pigments protect photosynthetic structures by quenching excited triplet Chl (3Chl) to dissipate excess energy (Frank and Cogdell, 1996) and by binding singlet oxygen (1O2) to inhibit potential oxidative damage (Demmig-Adams et al., 1996; Tracewell et al., 2001). Carotenoids are produced in the plastids and are derived from isopentenyl diphosphate (IPP). In the first step in biosynthesis, IPP is isomerized to dimethylallyl diphosphate, which becomes the substrate for the C20 geranylgeranyl diphosphate (GGPP) (Bramley, 2002). The first step unique to carotenoid biosynthesis is the condensation of two molecules of GGPP to form the first C40 carotenoid, phytoene, via phytoene synthase (Gross, 1991). The carotenoid pathway branches at the cyclization reactions of lycopene to produce carotenoids with either two β-rings (e.g., β-carotene, zeaxanthin, antheraxanthin, violaxanthin, and neoxanthin) or carotenoids with one β-ring and one ε-ring (e.g., α-carotene and lutein)(Cunningham, 2002) (Fig. 2).
There has been increased interest in the nutritional and medicinal importance of dietary carotenoids (Faulks and Southon, 2005; Frazer and Bramley, 2004; Yeum and Russell, 2002). Dietary intake of carotenoids has been associated with reduced risk of lung cancer and chronic eye diseases, including cataract and age-related macular degeneration (Johnson et al., 2000; Le Marchand et al., 1993). Studies have indicated that consumption of a variety of vegetables providing a mixture of carotenoids was more strongly associated with disease reduction than individual carotenoid supplements (Johnson et al., 2000; Le Marchand et al., 1993). Plants can be significant sources of carotenoids in the diet, and Brassica vegetables are relatively abundant sources that exhibit antioxidant and anticarcinogenic activity (Kurilich et al., 1999). Therefore, the objective of this study was to characterize the variation in accumulation of important dietary carotenoids among the six genetically related Brassica species.
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USDA 2005 U.S. Department of Agriculture, Agricultural Research Service, National Nutrient Database for Standard Reference, release 18 18 Nov. 2005<www.nal.usda.gov/fnic/foodcomp/Data/SR18/sr18.html>