Novel Applications of Paleometabolomics and Stable Isotope Ratios to Archaeobotanical Maize

Understanding how crops responded to drought stress in the past is increasingly important due to climate change. This thesis explores applications of stable isotope ratio analysis and paleometabolomics to archaeological maize cobs from Tularosa Cave, New Mexico, cultivated during two different climate regimes (wet ca. 1800 cal BP and dry ca. 700 cal BP). Attenuated total reflectance Fourier-transform infrared spectroscopy and elemental analysis have shown that the archaeological samples have good preservation across the age range.
Archaeological maize cobs from Tularosa Cave were distinguished through δ15N measurements of bulk material and individual amino acids (AAs). These findings complement previous paleogenomic analysis of Tularosa maize cobs suggesting selection for drought resistance-related genes.
Lyophilisation was shown to stabilise the maize extractable metabolome, increasing throughput and efficiency of extraction of the metabolome for analysis over more traditional tissue processing in liquid nitrogen. The lyophilisation approach was taken to explore the effect of drought upon the maize metabolome in a untargeted high resolution mass spectrometry (HRMS) metabolomics approach. It was shown that under ideal growing conditions, the biochemical make-up of the maize tissue types is different. However, under drought-stress conditions, the stress response dominates the metabolic profile. HRMS untargeted analysis showed differences in the extractable paleometabolomes of the two Tularosa maize populations. Statistical analysis reveals drought-stress biomarkers in the ancient maize metabolomes and direct evidence of increased sucrose production in the later population, possibly an indicator of adaptation to dry climate conditions.
The work presented in this thesis demonstrates the novel use of nitrogen isotope ratio analysis as a direct indicator for drought stress in archaeobotanical samples and paleometabolomics was used to test genomic inferences about the archaeological maize. Paleometabolomics can complement other `omics techniques and may be used as a novel approach to study desiccated archaeological remains.