Impact of Precooling and Controlled-Atmosphere Storage on γ‑Aminobutyric Acid (GABA) Accumulation in Longan (Dimocarpus longan Lour.) Fruit

Longan (Dimocarpus longan Lour.) fruit cultivars ‘Chuliang’ and ‘Shixia’ were analyzed for γ-aminobutyric acid (GABA) accumulation after precooling and in controlled-atmosphere storage. Fruit were exposed to 5% O₂ plus 3%, 5%, or 10% CO₂ at 4 °C, and GABA and associated enzymes, aril firmness, and pericarp color were measured. Aril softening and pericarp browning were delayed by 5% CO₂ + 5% O₂. GABA concentrations and glutamate decarboxylase (GAD; EC 4.1.1.15) activities declined during storage at the higher-CO₂ treatments. However, GABA aminotransferase (GABA-T; EC 2.6.1.19) activities in elevated CO₂-treated fruit fluctuated during storage. GABA concentrations increased after precooling treatments. GAD activity and GABA-T activity were different between cultivars after precooling. GABA concentrations in fruit increased after 3 days of 10% CO₂ + 5% O₂ treatment and then declined as storage time increased. GABA accumulation was associated with stimulation of GAD activity rather than inhibition of GABA-T activity