Ecotypic responses to flood and drought in tea tree (Melaleuca alternifolia)

Plantation-grown Melaleuca alternifolia (tea tree) is the principal source of tea tree oil in Australia. Upland and coastal ecotypes of tea tree were grown in a common environment to test responses in root, shoot and developmental attributes to four hydrological conditions. Consistent with its wetland origins, tea tree exhibited morphological adaptations for flood tolerance, with both ecotypes possessing a similar maximal capacity for adventitious roots and aerenchyma. Despite adaptation to flood, growth was reduced under prolonged flood relative to a well-watered control, and to a similar degree in both ecotypes. Coastal plants responded more rapidly to flood, suggesting that upland plants may delay costly morphological modifications until flooding is more protracted. Mild water deficit (drought) had a greater impact on growth and development than flooding, and upon coastal than upland plants. Relatively lower impact of drought on biomass and branch whorl number in upland plants was probably due to a constitutively higher root : shoot biomass ratio buffering against retarded development and growth. This study was the first step in identifying genetically controlled abiotic stress tolerances that may be useful for further domestication of tea tree. The potential to improve drought tolerance appeared most promising; however, further work will require consideration of appropriate breeding strategies given the low-resource-adapted population origins of tolerance alleles, and it should be prefaced by a clear definition of the target deployment environment and include testing of yield variables of economic value in target environments.