In recent years, the consumption of cruciferous vegetables with increased recognition of their nutraceutical properties has become very important because of the associated benefit of reducing the incidence of cancer and other chronic diseases (Talalay and Fahey, 2001; Wargovich, 2000). A group of secondary compounds, called glucosinolates, is widely present in crucifers. On cellular damage or disruption, hydrolysis of glucosinolates, through the enzyme myrosinase, produces isothiocyanates, which may, in part, account for the chemopreventive properties associated with these vegetables.
The hydrolysis of the aromatic glucosinolate gluconasturtiin releases phenethyl isothiocyanate (PEITC). Isothiocyanates are known to induce phase II detoxification enzymes (Wattenberg, 1990). Previous studies have shown that PEITC provides significant chemoprevention, especially against human prostate cancer (Powolny et al., 2003; Wargovich, 2000; Xiao et al., 2003). Gluconasturtiin is present in several crucifers such as watercress (Nasturtium officinale L.) and Chinese cabbage (Brassica campestris ssp. pekinensis L.). Because of the chemopreventive properties of PEITC, it is desirable to characterize gluconasturtiin (precursor of PEITC) concentration and explore the potential of plants to produce increased amounts of this compound.
The variation in total and individual glucosinolate concentration in a plant is the result of several factors, both abiotic and biotic (Rosa et al., 1997; Smith et al., 2003). Research has also shown that glucosinolate levels are not constant throughout the phenology of a plant, particularly in early growth stages. Young photosynthetically active tissues are believed to be the major sites of synthesis or storage of indole glucosinolates (Chong and Bible, 1974; Ju et al., 1980).
The cabbage looper (Trichoplusia ni Hübner) is a generalist insect native to the United States. It has been extensively used in controlled feeding trials and it was chosen for the present study because of its voracity, lower susceptibility to pesticides, and especially because cabbage plants have been shown to tolerate moderate defoliation before a significant reduction in plant weight occurs (Andaloro and Shelton, 1981). Changes in leaf nutrients incited by herbivory depend on the timing and intensity of feeding (Rostás et al., 2002). However, little has been done to correlate the concentration of glucosinolates with the level of insect pressure.
The possibility of manipulating the metabolism of cruciferous plants to modify its glucosinolate content has generated considerable interest. The potential impact this will have on both consumers and producers will depend on the plant's ability to offer improved nutritional quality, less susceptibility to herbivores/pathogens, increased therapeutic properties, and desirable agronomic, storage, and sensory characteristics (Rosa et al., 1997).
Our objectives were to: 1) determine if there was an effect of cabbage looper larvae feeding on gluconasturtiin concentration in Chinese cabbage by correlating it with the insect pressure imposed on the plant; 2) evaluate the effect after the larvae were removed; and 3) determine if reduced light conditions had an effect on gluconasturtiin concentration.
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