Accumulation of proline analogues and adaptation of Melaleuca species to diverse environments in Australia

The genus Melaleuca is native to Australia, with about 250 species spread from favourable to very stressful ecological habitats. We analysed the leaves of 125 Melaleuca species for the accumulation of proline analogues to explore relationships between the ability of the species to accumulate proline analogues and their ability to adapt to various stressful habitats in Australia. Melaleuca species that have evolved the ability to accumulate only L-proline seem to be adapted to non-saline or non-sodic soils in regions of higher rainfall. Species that accumulate N-methyl-L-proline seem to be adapted to saline and/or sodic soils with moderate amount of rainfall. Species that accumulate trans-4-hydroxy-N-methyl-L-proline seem to be adapted to saline and/or sodic soils and occur in areas with considerably lower rainfall, while species that accumulate trans-4-hydroxy-N,N-dimethyl-L-proline are widely distributed in Australia, with a particular adaptation to extremely arid (deserts) and saline and/or sodic soils. The species accumulating more than one osmoprotectant seem to have an ecophysiological advantage over plants accumulating only one osmoprotectant.