In 2007, a report from the European Science Foundation on Investigating Life in Extreme Environments defined extreme environments as ‘having one or more environmental parameters showing values permanently close to lower or upper limits known for life in its various forms’ (CAREX, 2011).

Introduction

Plants produce thousands of chemicals that are not recognised as primary or basic metabolites (i.e. necessary for the survival of the cells). These secondary metabolites usually only occur in special, differentiated cells and are not necessary for the cells themselves, but may be useful for the plant as a whole. Plants at different taxonomic levels (family, genus, species) produce a characteristic mix of secondary metabolites that can be utilised as characters in classifying plants. Both primary and secondary metabolism overlap (Gershenzon et al., Chapter 4) and it is often not understood why a certain compound is produced.

Secondary metabolites can be classified on the basis of their chemical structure, composition, solubility in various solvents or the pathway by which they are synthesised. Three main groups are recognised: isoprenoids (composed almost entirely of carbon and hydrogen); phenolics (made from simple sugars, containing benzene rings, hydrogen and oxygen); and nitrogen-containing compounds (extremely diverse, may also contain sulfur).