Temporal dynamics of induced responses in Brassica juncea

Plants induce a suite of direct and indirect responses after herbivore attack. For utilizing these plant attributes in agriculture, it is important to compare the relative importance of these two defence types in the same plant species. Precise timing of these responses is vital for their effect on the herbivores feeding on the plant. In my thesis, I present an analysis of the temporal dynamics of various direct and indirect responses in Indian/ Brown mustard, Brassica juncea due to herbivory by a generalist Spodoptera spp, a specialist Plutella xylostella or both of these herbivores together. Morphological (leaf size and trichome densities) and chemical (volatiles, glucosinolates, amino acids, sugars) responses were analysed over temporal range of 24 hours to 20 days. The effect of direct systemic responses was assessed using a specialist (Plutella xylostella) and a generalist (Spodoptera litura) herbivore. In addition, to investigate indirect responses, orientation experiments were conducted on the generalist (Cotesia marginiventris) and specialist (C. plutellae) parasitoids. Results were validated with analysis of genes involved in signalling and biosynthetic pathways that govern the expression of these responses. For the first time, the presence of extrafloral nectar (EFN) in Brassicaceae is being reported. Their sugar and amino acid contents were measured in five different varieties of B. juncea. The indirect function of these EFN droplets was assessed by testing their inducibility and attraction and survival of parasitoids on them. These studies thus show that although morphological responses are slower than chemical responses, they also contribute to induced plant resistance in a relatively short time span. Moreover, before considering induced responses as resistance factors, their effect should be assessed at different points in time with both generalist and specialist herbivores.