Effects of insect herbivory on induced chemical defences and compensation during early plant development in Penstemon virgatus.

Abstract

The lack of studies assessing the simultaneous expression of tolerance and resistance traits during seedling development and overall seedling defences as compared with adult plants, in general, constitutes a significant research need that can greatly improve our understanding of overall investment in defences during plant ontogeny. Using two seedling and two juvenile stages of the perennial herb Penstemon virgatus (Plantaginaceae) evaluations were made of (a) patterns of investment in constitutive chemical defences [i.e. iridoid glycosides (IGs)], and (b) simultaneous variation in the short-term ability of seedling and juvenile stages to induce resistance traits, measured as induced chemical defences, or tolerance traits, measured as compensatory re-growth following moderate levels of damage by a specialist insect herbivore. Plants were highly defended during most of their transition from seedling to early juvenile stages, reaching a constant approx. 20 % dry weight total IGs. Furthermore, following 30 % above-ground tissue damage, seedlings and juvenile stages were equally able to induce resistance, by raising their IG concentration by approx. 8 %, whereas compensatory re-growth was only achieved at young juvenile but not seedling stages. Two major trends emerged from this study: (1) in contrast to expected and previously observed trends, in this perennial plant species, seedlings seem to be one of the most well-defended stages as compared with adult ones; (2) high levels of constitutive defences did not limit the ability of young developmental stages to induce resistance following damage, although this response may come with a cost (i.e. decreased compensation) in young seedling stages. Hence, as has been previously demonstrated in few other systems, these results points towards an indirect evidence for a trade-off between tolerance and resistance traits at some, but not all, developmental stages; making them often difficult to detect.