Environmental stresses that hinder the growth-metabolism in plants commonly result in the accumulation of carbon-based defence compounds. However, the dynamics of nitrogen (N) containing defence compounds that could be remobilized for maintenance-metabolism is less predictable under environmental stress. Even less known is the impact of nutrient deficiency on the effective deployment of plant defences, where the deterrence is effected through the metabolic conversion of a less toxic compound to a more potent toxin. We evaluated the primary metabolism, glucosinolate (GLS) production, the conversion efficiency of GLS to more toxic isotiocyanates (ITC), and the potential activity of myrosinase enzyme (MYR) that mediate this conversion in Brassica juncea and B.nigra as influenced by nutrient availability and simulated herbivory.
In both species, N-deficiency and methyl jasmonate (MeJA) application increased the GLS production, with a concomitant decrease in carbohydrate and amino acid content. In B. juncea, application of MeJA decreased the GLS-to-ITC conversion efficiency under both N and K deficiency. Readiness of deployment of GLS, as measured by the apparent activity of MYR, decreased in both species under stress. The lower conversion efficiency of native GLS-to-ITC, and the lower potential activity of MYR in both species, despite a greater tissue content of GLS under stress indicates that the GLS-MYR-ITC defense system might be less effective under environmental stress. Thus, our study captures the vulnerabilities in the execution of GLS-ITC based defense systems under stressful environments and suggests that, contrary to the traditional notion that stems from the tissue GLS content, brassicas under stressful environments might not be effectively defended.