Exploring secondary metabolite variation on an ancient plant clade across large environmental gradients

Diego Salazar Amoretti, Biological Sciences, Florida International University, Miami, FL and Christopher Sorich, Biological Sciences, Florida International Univesity, Miami, FL

Background/Question/Methods
Plant specialized metabolites can play a central role in modulating species interactions. In the last 60 years, ecologists have learned much about how specific specialized chemicals allow plant species to overcome or circumvent biotic and abiotic challenges. Still, plant chemical diversity remains an understudied component of the biological complexity of natural systems. Most notably, little is known about how these specialized metabolites vary across environmental gradients. Whether through the direct effect from environmental factors on plant taxa or indirectly through changes in the abundance and diversity of natural enemies, environmental gradients are likely to be associated with large variations on the composition and concentration of these specialized compounds. Here we explore the patterns of variability of leaf specialized metabolites across the order Cycadales. In this phylogenetically controlled system, we specifically ask how large environmental gradients are associated with the interspecific variation in chemical composition and the concentration of major secondary metabolite groups. We take advantage of a large outdoor living collection of cycads grown on a common garden set up to determine the leaf chemistry of 87 taxa representing all known cycad genera. We used a combination of GCMS, HPLC-DAD/ELSD, and spectrophotometric assays to assess cycad leaf chemical composition.
Results/Conclusions
Despite the large phylogenetic scale of this study, we found a very high consistency in species secondary chemical composition across the order. This is especially evident across the interspecific variation in low molecular weight/volatile compounds (e.g.: mono-, sesqui-, triterpenoids, sterols, and short-chain fatty acids). Contrastingly, we found a high degree of variability in leaf phenolics. This variability is most apparent across flavonoids. Furthermore, taxa originating from sites with higher maximum annual temperatures showed significantly higher concentrations of flavonoids, especially for species originating in sites with maximum annual temperatures of 36 C. These results suggest that cycads are likely to rely on a small set of highly effective secondary metabolites and biosynthetic pathways to overcome the multitude of natural enemies across their large environmental and geographical range. This data also shows that environmental factors are also likely to play an important role in forging and modulating plant chemical variability. We highlight to need for more comprehensive studies as well as experimental approaches to disentangle the direct and indirect effects of environmental factors on plant specialized chemicals.