Differential fitness of Eucalyptus defense phenotypes under altered nutrient and light conditions

Producing defensive chemicals is a cost for a plant. Scientists have hypothesized that there is always a trade-off between investment in chemical defence and plant physiological and developmental growth processes. It has been proposed that plants growing under high resource availability should use their carbon budget for plant growth rather than defence and that when plants are growing under resource-limited environmental conditions plants use their available carbon budget for the differentiation processes such as the production of defensive chemicals, rather than plant growth. Eucalyptus is the dominant genus of trees in Australian forests and their leaves are the main food source for many herbivores including insects and some arboreal marsupials. However, trees from the genus Eucalyptus possess a complex mixture of plant secondary metabolites (PSM), including formylated phloroglucinol compounds such as sideroxylonal, and a range of water-soluble phenolics. These compounds vary qualitatively and quantitatively between species and quantitatively within species. These plants are extensively studied for the great chemical variation they possess. However, few experiments have been conducted to test for the existence of trade-offs between growth and defence in these plants. The defensive chemistry of Eucalyptus species is mainly constitutively determined and phenotypic plasticity across the environmental variation is less well understood. I aimed to find out under which environmental circumstances this proposed trade-off will take place in Eucalyptus and also the nature of such trade-offs, under different conditions of light and nutrient availability. Eucalyptus melliodora and Eucalyptus camaldulensis seeds were grown under three different light conditions and E. melliodora seedlings were grown under two nutrient conditions and changes in growth parameters and sideroxylonal and total phenolic concentrations of seedlings were observed. Plants under different light and nutrient treatments varied greatly in their growth parameters and foliar PSM concentrations as well. Plants that were grown under higher light levels contained more sideroxylonal and more total phenolics than plants under low light conditions. Plants grown under higher nutrient levels possessed higher sideroxylonal but lower total phenolics compared to low nutrient plants. However, no trade-offs were identified between growth parameters and defensive chemical concentrations under any environmental conditions. Contrary to expectations, positive relationships between growth and concentrations of sideroxylonal were identified between and within many environmental treatments