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

•Using propidium iodide, a method developed for detection of gamma radiation treated onion.•The method can be used to calculate absorbed dose of gamma radiation during storage.•The fluorescence microscopy based method is effective in the absorbed dose range of 20–100 Gy.•The enzyme activities and phytochemicals were not affected by gamma radiation. 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.