Lichen defensive compounds - an experimental approach

Secondary compounds in plants have been attributed many different roles, one being chemical defence against herbivory. Unfortunately, experimental approaches to test the defensive compound hypothesis often suffer from confounded factors: specimens in defe nsive compounds may also differ in other aspects. Lichens are well suited as test organisms for the chemical defence hypothesis because (1) they are assumed to have a strong herbivory defence, (2) their secondary chemistry is well known, and (3) secondary compounds can selectively and non-destructively be extracted from living lichens by certain solvents. By cutting one lichen specimen in two equal pieces, we can offer a herbivore two pieces of fodder that are identical in all aspects apart from the absen ce or presence of secondary compounds. This project will test the lichen defensive compound hypothesis directly by feeding land snails on natural and compound-free lichens in controlled experiments in the field and in the laboratory. Preliminary experime nts have shown that some snail species consume large amounts of some lichen species within a few days. Furthermore, we will test whether solar exposition influences the herbivory defence of lichens and whether grazing induces a defensive reaction in the l ichen. Lichens consist of up to 30 % secondary compounds. Most compounds are well known, and have been widely studied for taxonomical and pharmaceutical interests. However, one knows very little about why the lichen produces the so called lichen compound s, and what importance they have in nature. It is believed that compounds found in the cortex (above the photobiont) have light-screening functions, while medullary compounds are herbivore deterrents, antibiotics, allelopathics etc. Apart from the facts t hat most of the compounds absorb well in the UV-B and that many of them are toxic to animals, one known very little about their importance as defensive compounds in the nature. Parietin and usnic acid are induced by UV-B, but there is no evidence for any of the other compounds being produced as a response to any environmental stresses. Many lichens grow in habitats that are rich in solar irradiance, but poor in nutrients. We believe that such conditions leads to a carbon surplus in the lichen, as the lic hen also need nutrients to use all the carbon produced for growth. Lichen compounds may be produced just to get rid of the carbon surplus (waste products) or as storage. This is a proposal for an extension of my existing two-year post doc project. I want to continue my studies on lichen compounds by looking beyond the established truths about defensive functions, and concentrate on the driving forces behind the amount of compounds produced. I will also try to test the theory of lichen compounds as waste products. This will be done through a nitrogen spraying experiment, where lichens will be sprayed with nitrogen every time it rains, transplantation of lichens into habitats with contrasting light conditions and through tests of rain- and sea water as ag ents for leakage of lichen compounds.