Abstract
In their natural environment, plants are often attacked simultaneously by many insect species. The specificity of induced plant responses that is reported after single herbivore attacks may be compromised under double herbivory and this may influence later arriving herbivores. The present study focuses on the dynamics of induced plant responses induced by single and double herbivory, and their effects on successive herbivores. Morphological (leaf length, area and trichome density) and chemical changes (leaf alkenyl and indole glucosinolates) in Brassica juncea were evaluated 4, 10, 14 and 20 days after damage by the specialist Plutella xylostella alone, or together with the generalist Spodoptera litura. To assess the biological effect of the plant’s responses, the preference and performance of both herbivores on previously induced plants were measured. We found that alkenyl glucosinolates were induced 20 days after damage by P. xylostella alone, whereas their levels were elevated as early as 4 days after double herbivory. Trichome density was increased in both treatments, but was higher after double herbivory. Interestingly, there was an overall decrease in indole glucosinolates and an increase in leaf size due to damage by P. xylostella, which was not observed during double damage. S. litura preferred and performed better on undamaged plants, whereas P. xylostella preferred damaged plants and performed better on plants damaged 14 and 10 days after single and double herbivory, respectively. Our results suggest that temporal studies involving single versus multiple attacker situations are necessary to comprehend the role of induced plant responses in plant–herbivore interactions.
Highlights
Plants have to deal with various biotic and abiotic stresses in their surroundings and balance their resources to optimize growth, reproduction and defences
In order to address the specificity of induced plant responses under single and double herbivory, we studied the effect of Plutella xylostella damage alone, as well as of simultaneous damage by this specialist and the generalist Spodoptera litura on the induction of Brassica juncea resistance
Glucosinolate levels were significantly affected by damage due to herbivory by the specialist P. xylostella alone (MANOVA, Damage effect; F2,68 = 6.325, P < 0.005; Supplementary table 1) as well as by simultaneous damage by P. xylostella and S. litura (F2,67 = 34.038, P < 0.001; Supplementary table 1)
Summary
Plants have to deal with various biotic and abiotic stresses in their surroundings and balance their resources to optimize growth, reproduction and defences. Insect herbivory often serves as a significant stress factor, and plants have evolved many different forms of resistance strategies to reduce or prevent it (Schoonhoven et al.1998). Morphological structures such as trichomes, hairs, spines, and waxes and a number of secondary chemicals prevent or reduce insect herbivory. (Schoonhoven et al 2005) In addition to these constitutive resistance mechanisms, plants respond to herbivore attack by altering the levels of these traits. These changes are known as induced responses (Karban and Baldwin 1997). The effects of these responses on insect-plant interactions can differ greatly and depend on many factors including whether the feeding herbivore is a generalist or a specialist on the plant (Karban et al 1999)
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