Physio-biochemical Responses of Okra (Abelmoschus esculentus) to Oxidative Stress Under Low Temperature Storage

Okra pods, commonly eaten at an immature stage, undergo quick postharvest deterioration due to high respiration, water loss, wilting, toughening and decay. As okra is a tropical crop, its pods are susceptible to chilling injury (CI) at low storage temperatures. The effects of low temperature storage on the physiobiochemical properties of okra pods were determined. Chilling injury symptoms were found only in pods stored at 4 degrees C and were more apparent after transfer to 25 degrees C. In seeds, the CI index was positively correlated with seed browning, H2O2, malondialdehyde (MDA) content, and catalase (CAT) activity. Chilling-injured seeds had lower total phenolic content (TPC), antioxidant activity (DPPH scavenging activity and FRAP assay), peroxidase (POD), and superoxide dismutase (SOD) activities than non-injured seeds. Additionally, the seed browning index was related to high polyphenol oxidase (PPO) activity. In the pericarp, the CI index was also positively correlated with the H2O2 and MDA contents. The POD and SOD activities in chilling-injured pericarp were significantly lower than in non-injured pericarp. Chilling injury resulted in an initial increase in DPPH scavenging and CAT activities which later decreased as CI became severe. These results indicate that CI in okra is due to accumulation of H2O2, and MDA, as well as its weak antioxidant defense mechanism. This resulted in development of CI symptoms, including seed browning.