Detection of gamma radiation processed onion during storage using propidium iodide based fluorescence microscopy

The determination of absorbed dose in gamma radiation processed onion (treated with 20-100 Gy for sprout inhibition) during storage is an important regulatory requirement to control unfair practices. To address this problem, a microscopy based method was developed using propidium iodide (PI) staining of onion adaxial epidermis. A proportional radiation dose dependent increase in nuclei count was observed during ambient (26 +/- 2 ?C) and low (2 +/- 1 ?C) temperature storage. The method was validated and dose of radiation could be determined accurately in stored onions using blind tests. During mechanism studies, boron-dipyrromethene (BODIPY) dye staining and malondialdehyde (MDA) estimation showed dose dependent increase in peroxidation of membrane lipids. The fluorescein diacetate (FDA) stained onion adaxial epidermis showed decrease in fluorescence indicating lowering of physiological activity. Enzyme peroxidase (POD), phenylalanine ammonia lyase (PAL) activities and phytochemicals (phenolics, flavonoids, ascorbic acid and pyruvic acid) did not change significantly with increasing dose of gamma radiation.

Automated summary

A microscopy-based method using propidium iodide staining was developed to accurately determine the radiation dose in stored onions. Mechanism studies showed an increase in membrane lipid peroxidation and a decrease in physiological activity with increasing radiation dose, while enzyme activity and phytochemical content did not significantly change.