Disarmed by domestication? Induced responses to browsing in wild and cultivated olive
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Some theories of plant allocation to defence predict that chemical or structural defences against herbivores should be maximised when browsing is most likely to occur. However, plants are constrained by a trade-off between growth and defence such that slow-growing plants usually have higher levels of secondary compounds, such as phenolics and tannins, than faster-growing ones. Thus, it is possible that the selection for increased yield and growth rate that occurs when plants are domesticated, may cause a reduction in allocation to these compounds. We tested this hypothesis using wild (Olea europaea L. var. sylvestris Brot.) and cultivated (O. europaea L. var. europaea) olive growing in an area with high densities of ungulates. In our study, olives outside fences excluding ungulates were heavily browsed. However, browsing induced an increase in the phenolic content of olives of both varieties in winter but not in spring. In spring, new leaves of both varieties had generally higher levels of phenolics and nitrogen than old leaves, but new leaves in both varieties exposed to browsing had a lower nitrogen content compared to controls. Browsing in both olive varieties caused leaf and shoot density to increase and leaf and shoot length to decrease, but in wild olives browsed shoots lost their leaves and became similar to spines. Structural responses to browsing occurred in spring during regrowth, whilst chemical changes were more obvious in winter, in both varieties. We suggest that olive may exhibit both morphological and chemical responses to browsing, depending on the different resource allocation priorities at different times of year. In spring, independently of browsing, cultivated olive had generally longer shoots and lower levels of phenolics than wild olive. We speculate that domestication may have selected for faster growth, at the expense of allocation to secondary compounds.
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