Glucosinolates are a group of more than 120 secondary plant metabolites found throughout several plant families, including the Brassicaceae, Capareaceae and Caricaceae (Fahey et al. 2001). Glucosinolates are S rich, anionic natural products that upon hydrolysis by myrosinases produce several different products. The breakdown products of glucosinolates contribute to plant defense, human and livestock health, and the sensory quality of vegetables (Halkier & Gershenzon 2006). Glucosinolates are classified depending on the precursor amino acid into aliphatic glucosinolates derived from alanine, leucine, isoleucine, methionine, or valine; aromatic glucosinolates derived from phenylalanine or tyrosine and indole glucosinolates derived from tryptophan (Fahey et al. 2001; Halkier & Gershenzon 2006). Although glucosinolates represent a chemically diverse class of plant secondary compounds, the formation of these compounds consist of three major steps: (a) side chain-elongation of amino acids, (b) development of the core glucosinolate structure and (c) secondary side-chain modifications of glucosinolates (Halkier & Gershenzon 2006).
Glucosinolates occur in all parts of the plants, but in different concentrations and profiles. Up to 15 different glucosinolates can be found in the same plant species but usually contains only up to four different glucosinolates in significant amounts (Verkerk et al. 2009). Glucosinolate concentration in plants is about 1% of dry weight, although the concentrations are highly variable, and can reach 10% in seeds of some plants (Kushad et al. 1999; Fahey et al. 2001; Verkerk et al. 2009).
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Role in human health
Consumption of Brassica vegetables such as broccoli, turnip, cabbage, cauliflower, kale has been linked to a reduced risk of several types of cancers (Verkerk et al. 2009; Björkman et al. 2011). The anticarcinogenic activity of glucosinolates is thought to be due to the ability of certain hydrolysis products to induce phase II detoxification enzymes, such as quinine reductase, glutathione-S-transferase, and glucuronosyl transferased (Halkier & Gershenzon 2006). Furthermore, data suggested that, of the different dietary derived glucosinolate subgroups, aliphatic glucosinolates showed the strongest inverse association with cancer risk (Björkman et al. 2011).
Factors affecting plant levels
Genotypic differences in glucosinolate concentrations and profiles between crop species and cultivars are well documented (Kushad et al. 1999; Verkerk et al. 2009). Initially the work characterizing the genetic regulation of glucosinolates has been done to reduce glucosinolate levels in the seeds Brassica oil-crops as part of efforts to decrease the potential toxicants in animal feed supplements (Halkier & Gershenzon 2006). With this information, breeders have developed the so-called "single-low'' and "double-low'' lines that they contain reduced concentrations of glucosinolate in the seed (Scherer 2001). Moreover, breeding has been used to enhanced the health promoting glucosinolate in Brassica vegetables (Verkerk et al. 2009).
Temperature and light
A number of studies have shown that growth temperatures clearly influence the glucosinolate content in many species in the Brassicaceae. Plants exposed to high or low, rather than optimal intermediate growth temperatures, produce the highest levels (Schreiner 2005; Björkman et al. 2011). Young cabbage plants contain a higher glucosinolate concentrations in roots and higher diurnal variation at 30 oC than at 20 oC (Rosa & Rodrigues 1998) whereas, studies of broccoli heads showed that aliphatic glucosinolates increased with decreasing temperatures lower than 12 oC (Schonhof et al. 2007b). In contrast, when exposing greenhouse-grown plants to cold (0-12 oC) night temperatures, Shattuck et al. (1991) found 29% decrease of the overall glucosinolate concentration in the peel root tissues of turnip compared to normal growth conditions.
Irradiance and photoperiod affect glucosinolate concentration in plants. Long photoperiods are typical for high latitudes during summer, and found to have a positive effect on glucosinolate content (Björkman et al. 2011). In broccoli plants aliphatic glucosinolates increased at moderated mean daily radiation (10-13 mol m-2 day-1) (Schonhof et al. 1999; Schonhof et al. 2007b), whereas indole glucosinolates were higher at low irradiation (Schonhof et al. 2007b). In five B oleracea botanical groups total and indole glucosinolates had a negative linear but positive quadratic relationship with temperature and day length and a positive linear but negative quadratic relationship with photosynthetic photo flux. Glucoraphanin concentrations were influenced by average photosynthetic photo flux and day length, but not by temperature (Charron et al. 2005).
Many Brassicas grown under water deficiency have higher glucosinolate concentration than those grown under favorable conditions (Rosa et al. 1996; Schreiner 2005; Radovich et al. 2005; Zhang et al. 2008; Björkman et al. 2011). Higher glucosinolate concentrations were found in cabbage when were not irrigated during head development (Radovich et al. 2005). Ciska et al. (2000) found higher glucosinolate concentrations in cultivars of B. oleracea, B. rapa and Raphanus sativus in the year with the hot and dry summer. Zhang et al. (2008) reported that turnip, which grown in spring summer season and received 25% available soil water, had higher levels of total and individual glucosinolates compared to the 50% and 75% available soil water treatments. Rapeseed glucosinolate concentrations were found to increase linearly at midday water potential below -1.4 MPa (Jensen et al. 1996). It has been proposed that increased synthesis of amino acids and sugars, precursors in biosynthesis of glucosinolates, during drought and also the influence of S uptake are possible the reasons for this response (Ciska et al. 2000; Zhang et al. 2008).
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Generally, glucosinolate concentration and profile can be influenced by S, N and Se supply. Sulphur and nitrogen fertilization and the balance between them have a predominant effect on glucosinolate concentration in Brassicas. An increased S supply has been shown to result in higher levels of total glucosinolate concentration in broccoli, turnip, canola, mustard (Krumbein et al. 2001; Rangkadilok et al. 2004; Li et al. 2007; Malhi et al. 2007).
Chen et al. (2006), Stavridou et al. (2011a) and Krumbein et. al (2001) showed that the total glucosinolate concentration in pakchoi and broccoli was enhanced at low N supply. In cabbage, total glucosinolates were increased by high S supply and low N rates (Rosen et al. 2005). In contrast, Omirou et al. (2009) found that total glucosinolates respond to N supply, but it did not respond to N applications above 250 kg ha-1. Individual glucosinolates respond differently in N supply. For example, increase of N supply resulted in an increase of indole glucosinolate concentration in watercress and turnip (Kim et al. 2002; Kopsell et al. 2007), while alkenyl glucosinolates in rape decreased (Zhao et al. 1994)
Increasing N supply decreased seed glucosinolate concentration of oilseed rape when S was deficient, but increased it when S was applied (Zhao et al. 1993). Similarly, N by S interaction was found in a greenhouse potted experiment for red leaf mustard (Stavridou et al. 2011a). Schonhof et al. (2007a), showed that total glucosinolate concentrations were higher in broccoli plants grown with an insufficient N supply independent of the S level, while glucosinolate concentration decreased in plants given an insufficient S supply in combination with an optimal N supply.
In the case of Se the results are contradictory, Robbins et al. (2005) showed that increased Se fertilization decreased glucosinolate concentration in broccoli and this was attributed to competitive Se and S uptake by plants. However, recently Hsu et al. (Hsu et al. 2011) found that Se application did not influence glucosinolate concentrations.
Increased space between growing vegetables was found to decrease glucosinolate concentrations of different cabbage cultivars and Brussels sprouts (MacLeod & Nussbaum 1977; MacLeod & Pikk 1978). High plant density (97500 plants ha-1) led to a 37% increase of glucoraphanin concentration in broccoli (Schonhof et al. 1999). Björkman et al. (2008) found that intercropping white cabbage with red clover reduced the levels of both foliar and root glucosinolates. However, it was also concluded that glucosinolate responses to plant competition were greatly influenced by Delia floralis infestation level. In a potted experiment, total and individual glucosinolates in red leaf mustard increased when intercropped with lettuce (Stavridou et al. 2011a).