Dynamic labelling reveals central carbon metabolism responses to stepwise decreasing hypoxia and reoxygenation during postharvest in pear fruit

Storage of pears in low oxygen conditions after harvest and before commercialization induces several responses in their metabolism initiating survival, maintenance and low energy expenditure mechanisms. In the present study a metabolomics approach was used in combination with a 13C feeding experiment to elucidate those metabolic responses in pear fruit under a stepwise induced hypoxic profile by monitoring changes in TCA, fermentation, GABA shunt associated metabolites and amino acids. A reoxygenation treatment was added at the end of the experiment to study the post hypoxic recovery mechanisms after prolonged hypoxia. Propagation of label through metabolic pathways was monitored by GC–MS analysis. Results showed successful utilization of 13C pyruvate from pear tissue under normoxia accompanied by fractional enrichment in organic acids, pyruvate and TCA cycle derived amino acids. Initiation of hypoxia at 5 kPa O2 resulted in drop of fractional enrichment in malate, fumarate, citrate, valine, glutamate and alanine but not in α-ketoglutarate and succinate. Further induction of severe hypoxia (0.5 kPa O2) resulted in a decrease of the TCA cycle activity, initiation of fermentation and primary evidence of an upregulated alanine-α-ketoglutarate shunt, a trend that continued until the 0.2 kPa O2 incubation. Sucrose and fructose decreased below 0.5 kPa O2 with a sharp increase of phosphorylated sugars that dropped by the end of the treatment. Reoxygenation caused upregulation of the TCA cycle but not glycolysis, reversed fermentation activity and a drop in concentration of all amino acids. Results were in line with common responses of plants under hypoxia and post anoxia recovery mechanisms and suggest that different mechanisms are implemented between normoxia and reoxygenation after prolonged hypoxia.