Effects of calcium treatment on malate metabolism and γ-aminobutyric acid (GABA) pathway in postharvest apple fruit

Calcium treatment effects on malate metabolism and the GABA pathway in ‘Cripps Pink’ apple fruit during storage were investigated. Postharvest apple fruit treated with 1% and 4% calcium chloride solutions were stored at 25 ± 1 °C. The 4% calcium treatment suppressed declines in titratable acidity and malate content and increased succinate and oxalate concentrations. Calcium treatment also reduced the respiration rate and decreased ethylene production peak during storage. Moreover, 4% calcium treatment significantly enhanced cyNAD-MDH and PEPC activities and upregulated MdMDH1, MdMDH2, MdPEPC1 and MdPEPC2 expression while inhibiting cyNADP-ME and PEPCK activities and downregulating MdME1, MdME4 and MdPEPCK2 expression. Surprisingly, calcium treatment changed the content of some free amino acids (GABA, proline, alanine, aspartic acid and glutamate), two of which (glutamate and GABA) are primary metabolites of the GABA pathway. Furthermore, calcium application enhanced GABA pathway activity by increasing MdGAD1, MdGAD2, MdGABA-T1/2 and MdSSADH transcript levels.