Defence and structure in plants

Plants have many enimies, so they have evolved defences. Photo courtesy of Photocase.com.

The world is full of large and especially small plant-eating animals, so most plants have developed some kind of defence. It can be something as simple as growing fast and the ability to re-grow eaten parts or it can involve low nitrogen concentration, low moisture content or complicated toxins or deterrents. Some plants also possess structural and morphological defences such as spines, hairs and toughened leaves. However, researchers have so far shown too little attention to structural defence mechanisms compared to chemical defences. Scientists from Australia have now begun to rectify this by synthesising the available data.

They identify four types of structural defence 1) Spinescence encompasses spines, thorns and prickles. These structures have probably mainly evolved as a defence mainly effective against vertebrates and they have only a limited effect on invertebrates. 2) Pubescence refers to a covering of hairs on the stems, leaves or fruits. These hairs have probably mainly evolved as a mechanism to avoid excessive water-loss, but studies also show an effect against many insect herbivores. 3)  Sclerophylly covers tough or hard leaves. Again these mechanisms may not primarily have evolved as herbivore defence, but more likely for structural support and to avoid water-loss. However, sclerophylly makes it harder for herbivores to get extract the nutrients. It is most effective as a deterrent for insects though. 4) Minerals. The deposit of minerals in stems and leaves of some plants make them much harder to digest. Grasses for example contains silica which makes them unpalatable for non-specialised animals.

We still only have a limited knowledge on the importance and effectiveness of structural defences compared to chemical defences and the possible trade-off. It is clear though that plants by using resources to construct structural defences can invest fewer resources in growth and reproduction. However, because many defence related structures also have other functions in the plant, it is not easy to access the actual trade-off. More research is required.

Source: Hanley M. E., Lamont B. B., Fairbanks M. M. and Rafferty C. M. (2007) Plant structural traits and their role in anti-herbivore defence. Perspectives in Plant Ecology, Evolution and Systematics 8: 157-178.

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