Metabolic acclimation to hypoxia revealed by metabolite gradients in melon fruit

A metabolomics approach using 1H NMR and GC–MS profiling of primary metabolites and quantification of adenine nucleotides with luciferin bioluminescence was employed to investigate the spatial changes of metabolism in melon fruit. Direct 1H NMR profiling of juice collected from different locations in the fruit flesh revealed several gradients of metabolites, e.g. sucrose, alanine, valine, GABA or ethanol, with increase in concentrations from the periphery to the center of the fruit. GC–MS profiling of ground samples revealed gradients for metabolites not detected using 1H NMR, including pyruvic and fumaric acids. The quantification of adenine nucleotides highlighted a strong decrease in both ATP and ADP ratios and the adenylate energy charge from the periphery to the center of the fruit. These concentration patterns are consistent with an increase in ethanol fermentation due to oxygen limitation and were confirmed by observed changes in alanine and GABA concentrations, as well as other markers of hypoxia in plants. Ethanol content in melon fruit can affect organoleptic properties and consumer acceptance. Understanding how and when fermentation occurred can help to manage the culture and limit ethanol production.