Metabolic adaptation of ‘Conference’ pear to postharvest hypoxia: The impact of harvest time and hypoxic pre-treatments

Abstract

The choice of harvest time and low oxygen storage conditions are crucial parameters in the commercialization of ‘Conference’ pear and the prevention of market losses. To elucidate the effect of harvesting time and hypoxia on the metabolic adaptational responses in pear fruit, a 13C-feeding experiment was performed. Pear tissue disks obtained from fruit harvested at optimum harvest time, or 1 week earlier or later (commercial, early, and late maturity stages, respectively) were exposed for 24 h to 21 kPa O2 followed by 24 h incubation at 0.2 kPa O2 at a constant temperature of 18 °C in a liquid medium containing [U13C] sodium pyruvate. Results showed high fractional enrichment of organic acids, succinate, α-ketoglutarate and lactate, and amino acids, glutamate, and alanine, during hypoxia for all harvesting times suggesting a downregulated TCA cycle stopping at succinate, as suggested by its accumulation, and a possible upregulation of the γ-aminobutyric acid (GABA) shunt. Increased concentration of alanine under hypoxia only in late pears was connected to increased endogenous levels of ethanol and acetaldehyde prior the experiment and lower lactate enrichment. Concentration and fractional enrichment of the optimum harvest time under 0.2 kPa O2 were compared to previously published data from a similar experimental setup where a 4-day gradually declining hypoxic prolife from 21 kPa to 0.2 kPa was implemented. Results showed similar enrichment for all TCA cycle organic acids. Faster label propagation of lactate in the pre-treated tissue suggested faster adaptational mechanisms and an already initiated hypoxic response while higher concentration and enrichment of alanine in the non-acclimated fruit could be evidence of a tissue specific alanine response to sudden oxygen shortage or aneffect of the age of the acclimated sample.