Chemical‐based resistance of Brassica oleracea and Rorippa dubia in response to Spodoptera litura attack.

A plant's chemical‐based resistance against insects can be characterized by an accumulation of phytochemicals that can severely harm or deter the generalist and non‐adapted specialist insects. In the absence of an herbivore threat, plants maintain a basic concentration of defensive compounds as a constitutive resistance mechanism. On the other hand, plants are capable of sharply increasing their production of defensive compounds as induced defence in response to herbivore attacks. In this study, we highlight the role of chemical‐based defence by comparing the constitutive and herbivore‐induced resistances of cauliflower (Brassica oleracea var. botrytis L.) and yellow cress (Rorippa dubia (Persoon)) with respect to time‐dependent changes of phytochemicals that directly influence the performance of the generalist herbivore insect, Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae). The results of the study showed that both plant species were able to switch, albeit different degrees, between constitutive and induced resistances to combat herbivore attack. Brassica oleracea amassed lesser defensive compounds of constitutive resistance (trypsin inhibitor, polyphenol oxidase and sinigrin) than R. dubia. This led to a higher relative growth rate (RGR) of the S. litura larvae fed on cauliflower than in those fed on yellow cress. Brassica oleracea also demonstrated a lower induced response capability than R. dubia in its production of allelochemicals in herbivore‐induced resistances. This was shown by the decrease of the RGR of larvae fed on induced foliage of the cauliflower compared to those fed on yellow cress. These results of the study proved and added to the belief that modern crop variety is not only lower in constitutive resistance but also weaker in induced resistance capability against herbivores, which is likely to be an effect of domestication even Rorippa and Brassica are relatively far from each other in Brassicaceae phylogeny. [ABSTRACT FROM AUTHOR]