Jasmonic acid (JA) and its volatile equivalent, methyl jasmonate (MeJA), are plant hormones involved in chemical and physiological defense responses. MeJA and JA have no direct impact on plant herbivores, but instead contribute to an intracellular signal cascade that elicits secondary metabolite production important for plant defense (Wasternack and Parthier, 1997). Exogenous JA and MeJA applications have been used to induce plant responses resulting in resistance to attack from nematodes (Cooper et al., 2005; Oka et al., 1999), insect pests (Omer et al., 2000, 2001), and plant pathogens (Brader et al., 2001). Exogenous jasmonate application has also been shown to reduce chilling injury (Gonzalez-Aguilar et al., 2003) and enhance accumulation of several classes of secondary compounds (as reviewed by Memelink et al., 2001).
Glucosinolates (GSLs) are a class of secondary metabolites present in members of the Brassicaceae family. GSLs are constitutively produced thioglycosidic compounds with aliphatic, aromatic, or indolic groups depending on which modified amino acid they contain. Upon hydrolysis with myrosinase, a β-thioglucosidase, GSLs yield bioactive compounds, including volatile isothiocyanates. GSLs and their subsequent hydrolysis products are deterrents to herbivores and generalist insects and inhibit pathogen growth (reviewed by Chew, 1988); they also have potent benefits for human health (Zhang et al., 1992). For example, glucobrassicin (indole-3-ylmethyl glucosinolate), the primary glucosinolate in cabbage (Rosen et al., 2005), and its isothiocyanate derivative, indole-3-carbinol, have been shown to have potent chemopreventive properties (Jinno et al., 1999; reviewed by van Poppel et al., 1999). Increasing GSL concentration in cabbage with cultural practices such as exogenous JA application would increase the efficiency of GSL-producing plants and have implications for human health and the nutraceutical industry.
JA-induced GSL accumulation has been documented in several plant species. Exogenous JA and MeJA application has been shown to increase both the accumulation of indolic GSLs and their biosynthetic gene transcript levels in Arabidopsis (Mewis et al., 2005; Mikkelsen et al., 2003). Jasmonate application has also been documented to enhance aromatic glucosinolate levels such as glucotropaeolin (Wielanek and Urbanek, 2006). Additionally, research has shown that jasmonate-induced increases in total GSL accumulation in Brassica crops do not result in significant losses in biomass (Cipollini and Sipe, 2001; van Dam et al., 2004).
Although previous studies indicate that JA-induced GSL enhancement is possible, these studies have not been performed under field conditions using heading cabbage nor have they addressed how JA-induced GSL levels can vary between cultivars of a specific crop. Under normal field conditions, GSL levels in cabbage are, in part, cultivar-dependent with red cabbage containing more GSLs than green cabbage (Rosen et al., 2005). We wanted to examine how JA-induced GSL enhancement under field production conditions differs between commercially grown cabbage cultivars. Our objectives were: 1) to examine differences in GSL concentration in field-grown red and green cabbage exposed to various foliar application rates of JA; and 2) to determine the effect of exogenous JA application on cabbage yield. We chose to quantify both indolic and aliphatic GSLs individually because these GSL groups are under separate genetic control based on amino acid and therefore may be under different environmental controls (as reviewed by Grubb and Abel, 2006).
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