## Materials and Methods
### Determination of Cell Viability Using Tetrazolium Salt as a Staining Agent  
A 2,3,5-triphenyltetrazolzolium chloride solution (0.5% _w/v_) was freshly prepared and protected from light exposure to avoid precipitation and a pH change. Onion bulbs, either untreated or PEF-treated, suspended in the sodium phosphate buffer for a total contact time of 6 h were used for the staining analysis. This was because these onions had reduced natural purplish-red pigments distributed on the abaxial (outer waxy) surface of fleshy scales before staining. Therefore, the subsequent image analysis was not interfered by the presence of natural pigments in the scales. Each onion bulb was cut in half from the vertical axis of foliar stem and the fleshy scales were gently detached by hand. Four layers of fleshy scales (scale 1-4) and a terminal bud were obtained from each onion bulb. Individual onion scales were each completely submerged in 100 mL tetrazolium solution on a 90 \(\times\) 25 mm petri dish (LabServ, Thermo Fisher Scientific, Scoresby VIC, Australia) for 24 h in the dark to allow for complete diffusion of the tetrazolium salt into the onion cells. Reduction of tetrazolium salt using oxidoreductaseenzymes, present in living cell mitochondria, into the insoluble red formazan is directly proportional to the number of metabolically active viable cells [17]. Therefore, cells with formazan deposits are considered as viable cells, but this is not the case for non-viable/dead cells. Prior to photo capturing, the edges of the stained onion scales were cut to flatten the convex scales and maximize the total surface area for image analysis. The photos of stained onion fleshy scales, with the adaxial (inner/convex) surface facing upwards, were captured in the same manner as mentioned previously (Section 2.2). The stained area (in cm\({}^{2}\)) representing the viable cells was measured using an image processing software (Imagej 1.48v software package; National Institutes of Health, Bethesda, MD, USA), where the color threshold function was employed to estimate the difference in the degree of cell viability between untreated and PEF-treated samples. Since the color of natural purplish-red pigments of onion and the red formazan deposits due to staining were rather similar, values of hue (0-255), saturation (125-255), and brightness (1-255) were set using the color threshold function to a range that the software could no longer detect the natural purplish-red pigments present in the onion scale. Determination of Onion Volatile Compounds Using Headspace Solid-Phase Microextraction Coupled with Gas Chromatography-Mass Spectrometry (HS-SPME-GC-MS)  
In the present study, the sodium phosphate buffer (0.01 M NaH\({}_{2}\)PO\({}_{4}\), pH 7.0) that the onion bulb was suspended in was used as the medium to measure onion volatile compounds generated and released from the samples. This method was applied to understand the impact of PEF treatment on volatile compounds generated as a result of tissue disruption in whole, intact onion bulbs without any confounding impact of further disruption during sample preparation. Samples were taken immediately (0 h), then 1, 2, 4, and 6 h after PEF treatment from the same onion bulb. At each sampling point, 5 mL of phosphate buffer was transferred into a 20 mL headspace vial containing 1.5 g sodium chloride (BDH Chemicals, Poole, England), and the vial was immediately sealed with a PTFE-coated silicon septum screw cap. The headspace vial was heated at 40 \({}^{\circ}\)C and agitated for 5 min (PAL3 RSI 85, CTC Analytics AG, Zwingen, Switzerland) in order to aid the partitioning of volatile compounds into the headspace. Volatile compounds were then extracted using solid-phase micro-extraction (SPME) with a divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) fiber (Supelco, Bellefonte, PA, USA) for 20 min at 40 \({}^{\circ}\)C.  
Volatile compounds were analyzed using gas chromatography-mass spectrometry (GC-MS) with an Agilent 6890N GC and 5975B VL MSD (Agilent Technologies, Santa Clara, CA, USA).