Effect of temperature and low oxygen on pericarp hardening of mangosteen fruit stored at low temperature

To investigate chilling injury of mangosteen fruit (Garcinia mangostana L.), expressed as pericarp hardening, fruit at the red-brown and red-purple stages of maturity were stored at 6 °C (87.0% RH) and 12 °C (83.5% RH) for 15 d. Fruit stored at 6 °C had greater pericarp firmness than when stored at 12 °C and red-purple fruit were firmer than the less mature red-brown fruit. When pericarp hardening occurred, pericarp firmness and lignin contents increased while total phenolics decreased. Of the phenolic acids predominant in the hardened pericarp, p-coumaric acid declined whereas sinapic acid increased throughout the storage time. Application of low O2 (0.25%) to red-purple fruit during storage at 6 °C (84% RH), or at room temperature (30 °C, 71.5% RH) following storage at 6 °C, did not reduce pericarp hardening and there were no significant differences in firmness, lignin and total free phenolics when compared with fruit in normal air conditions. Activities of enzymes involved in the lignin biosynthetic pathway, namely phenylalanine ammonia lyase (PAL), cinnamyl alcohol dehydrogenase (CAD) and peroxidase (POD), were determined. PAL and POD activities in pericarp tissue increased with storage time, whereas there was no change in CAD activity. Expression patterns of PAL and lignin peroxidase (LgPOD) genes in fruit pericarp were also determined using northern blot analysis. PAL and LgPOD mRNA accumulation increased with storage time at 6 °C coincident with their enzyme activities. Low O2 treatment had only a slight effect on PAL and LgPOD gene expression. Our results suggest that the increase in pericarp firmness of mangosteen fruit results from induction of lignin synthesis, associated with an increase in PAL and POD activity and gene expression.