Testing the optimal defence hypothesis for two indirect defences: extrafloral nectar and volatile organic compounds

Many plants respond to herbivory with an increased production of extrafloral nectar (EFN) and/or volatile organic compounds (VOCs) to attract predatory arthropods as an indirect defensive strategy. In this study, we tested whether these two indirect defences fit the optimal defence hypothesis (ODH), which predicts the within-plant allocation of anti-herbivore defences according to trade-offs between growth and defence. Using jasmonic acid-induced plants of Phaseolus lunatus and Ricinus communis, we tested whether the within-plant distribution pattern of these two indirect defences reflects the fitness value of the respective plant parts. Furthermore, we quantified photosynthetic rates and followed the within-plant transport of assimilates with 13C labelling experiments. EFN secretion and VOC emission were highest in younger leaves. Moreover, the photosynthetic rate increased with leaf age, and pulse-labelling experiments suggested transport of carbon to younger leaves. Our results demonstrate that the ODH can explain the within-plant allocation pattern of both indirect defences studied

Effects of stress and defence allocation on tree growth: Simulation results at the individual and stand level.

The long life span of trees implies that they are more or less frequently confronted with different biotic and abiotic stress situations during their lives. However, biotic stress such as attacks by herbivores or pathogens and abiotic stress such as frost or drought could strongly vary in frequency, intensity, duration, time of occurrence as well as in the involved tissues. This urged trees to develop flexible defence mechanisms during their evolution ensuring a high probability of survival to regenerate successfully. Based on an analysis of existing literature on plant response to herbivory, McNaughton (1983) concludes that “. . .the yield of the tissue affected and other tissues is not affected in proportion to the amount of tissues damaged by the herbivore”, referring also to Lee and Bazzaz (1980) and Neilsen (1981). McNaughton presents a set of alternative patterns about the eff