Glucosinolate is the main phytochemical found in cole crops. There has been a growing research interest in the bioactive compounds of broccoli (Brassica oleracea L.) and their implications in human health and plant protection with regard to the influence of biotic and abiotic factors (Jahangir et al., 2009; Jeffery et al., 2003). This interest can be explained by data suggesting the importance of the dietary constituents of broccoli in the prevention and therapy of cancer, which has been recently reviewed by Herr and Büchler (2010). Sulforaphane belongs to isothiocyanates that are formed by the hydrolysis of their parent compounds, the glucosinolates (primarily glucoraphanin in broccoli). It has the most important anticarcinogenic potential (Clarke et al., 2008) for several cancer types including skin carcinogenesis (Dinkova-Kostova et al., 2006; Gills et al., 2006). Sulforaphane is also able to prevent cisplatin-induced cell death and inflammation in nephropathy during chemotherapy (Guerrero-Beltrán et al., 2012b). Guerrero-Beltrán et al. (2012a) have reviewed the effective role of sulforaphane against oxidative stress in brain and neuronal injury, renal, liver hyperglycemia and β-cell, and heart and cardiac cell damage, acting as an indirect antioxidant, which is able to induce many cytoprotective proteins, including antioxidant enzymes. Also Anupama et al. (2008) have reported that ethanol extract of broccoli flower heads possesses an antimutagenic property.
Phenolic compounds, besides glucosinolates of broccoli florets, also possess antioxidant and anticarcinogenic properties (Podsedek, 2007; Tomás-Barberán and Espín, 2001; Vallejo et al., 2002) if prepared by steaming, rather than other cooking processes (Cartea et al., 2011; Conaway et al., 2000).
The effect of broccoli genotype on glucosinolate and phenolic compound content and profile was investigated by several authors. Brown et al. (2002) observed 10-fold differences in glucoraphanin and total aliphatic glucosinolate levels of broccoli florets between the highest and lowest genotypes examined. They also found that the synthesis of aliphatic glucosinolates such as glucoraphanin was regulated primarily by genotype (60%) with environmental (5%) and environment × genotype (10%) factors exerting less effect. On the other hand, indolyl glucosinolates such as glucobrassicin were less affected by genotype (12%) and more by environment (33%) and environment × genotype (21%). Vallejo et al. (2002) also report significant differences in total glucosinolate, phenolic compounds, and vitamin C contents in 14 cultivars (genotypes) grown under uniform cultural conditions in Spain. Pérez-Balibrea et al. (2011) underline the findings of Brown et al. (2002) that aliphatic glucosinolates are more affected by genetic factors than indolic glucosinolates in the case of seeds and sprouts of three broccoli cultivars.
The effect of growing period (spring or fall) under field conditions on the glucosinolate–myrosinase system was examined by Charron et al. (2005a, 2005b) in Illinois. They showed that total glucosinolate concentrations at harvest had a negative linear but positive quadratic relationship with daylength and temperature during the 2 weeks preceding harvest and a positive linear but negative quadratic relationship with photon flux over the same 2 weeks. The regression model for indole glucosinolate concentrations similarly varied with mean daylength, temperature, and photon flux over the 4 weeks before harvest. Glucoraphanin concentrations at harvest decreased linearly with mean photon flux from transplanting to harvest and had a negative linear but positive quadratic relationship with daylength from transplanting to harvest (Charron et al., 2005a). Myrosinase activity on a fresh weight basis (U·g−1) had a negative linear relationship with temperature and a positive linear but negative quadratic relationship with photon flux. Myrosinase activity on a protein basis (U·mg−1) had a positive linear and a negative quadratic relationship with both temperature and photon flux (Charron et al., 2005b). The effect of solar radiation on the flavonol content in broccoli florets grown in spring and fall cultivation periods under field conditions was determined by Gliszczyńska-Świgło et al. (2007). Quercetin and kaempferol contents were highly positively correlated with total solar radiation from planting to harvest.
The effect of poor (15 kg·ha−1) or rich (150 kg·ha−1) sulfur soil fertilization on health-promoting compounds, during inflorescence developmental stages, under field conditions, during the spring cultivation period, in three broccoli cultivars was measured by Vallejo et al. (2003a). Phenolic compounds and vitamin C content showed a rising trend regardless of sulfur fertilization, whereas significant differences were detected in the first two developmental stages in total glucosinolate content. With low sulfur fertilization, the highest glucosinolate concentration was measured in the second stage, whereas this was true for the third stage in the case of high sulfur fertilization (Vallejo et al., 2003a). The same sulfur soil fertilization rates during early or late spring field cultivation were carried out to investigate their effect of phenolic compounds and vitamin C (Vallejo et al., 2003b) and glucosinolate contents (Vallejo et al., 2003c) in inflorescences of eight broccoli cultivars in Spain. They observed no significant difference in total glucosinolate levels between rich and poor sulfur fertilization but a higher level was detected in the late spring cultivation period (Vallejo et al., 2003c). In the case of phenolic compounds and vitamin C, the late spring cultivation period combined with rich sulfur fertilization gave the highest concentrations (Vallejo et al., 2003b).
There is no literature dealing with the effect of optimum or low soil water content on the bioactive components of broccoli under field conditions. Only under greenhouse conditions in Brazil, Cogo et al. (2011) reported that low soil water content during growth combined with post-harvest cold storage gave the best preservation of antioxidant activity and L-ascorbic acid content in broccoli florets, whereas phenolic compound content was independent of cultivation or post-harvest storage conditions.
The aim of the present study was to investigate the effect of foliar sulfur supplementation, irrigation, and different cultivation periods (spring/fall) on the bioactive constituents of broccoli florets of the same genotype grown under field conditions in Hungary.
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