## 2 Materials and Methods
### Electronic Nose  
An experimental electronic nose equipped with 8 metal oxide sensors (MOS), as described previously in detail [30], each with cross-sensitivity to specific chemical or VOCs, was used for electronic aroma analyses. The names of sensors in the sensor array (in order, with primary VOCs detected) are as follows: MQ3 (alcohols), TGS822 (organic solvents), MQ136 (sulfur dioxide), MQ9 (carbon monoxide and combustible gases), TGS813 (aliphatic alkanes), MQ135 (ammonia, benzene, sulfides), TGS2620 (alcohols, organic solvents), and TGS2602 (H2S, sulfides, ammonia, toluene).  
The detection parameters utilized for e-nose operation are as follows. To establish a stable sensor baseline, the e-nose system was purged with fresh air for 150 s before testing. At initial stage, air flow was initiated by a diaphragm pump in which air passed through an activated carbon filter to remove VOCs from the purge air. Purge air was injected into the sensor chamber to clean the sensors from any volatiles attached to the sensor surfaces. Sample headspace volatiles were analyzed for 150 s for one complete sample run. The flow rate of the sample gas to the sensor array chamber was set at 200 mL/min. Data responses were instantly collected for subsequent analysis. The final step in the run sequence ended with the sensor array being purged again with fresh air for 200 s. The output voltage of the sensors changed due to exposure to VOCs within gases emitted from the sample and all sensor responses were collectively recorded in data collection cards. The sensor signals were recorded and stored at 1 s intervals (Figure 1). To maintain e-nose sensor-response stability, the e-nose operating temperature for the analysis of all samples was held at 25 \(\pm\) 1 \({}^{\circ}\)C. The variation in air sample relative humidity of cucumber samples was 5 \(\pm\) 2% RH. Thus, relative humidity had minimal impact on e-nose sensor responses and recognition of fruit volatiles (VOC emissions) from cucumber samples analyzed by MOS sensors.  
Electronic aroma data, recorded for each sensor within the sensor array, were stored as electrical resistance values recorded over time within an Excel spreadsheet. E-nose response consists of three components: (1) a baseline response during the carrier gas reference phase, (2) a response curve during the sampling (data acquisition) phase, and (3) a transition of sensor responses back to baseline during the purge and recovery phase (Figure 1).  
Cucumber Fruit Volatile Analyses  
Cucumber fruits of similar same size (length 10 to 12 cm) and weight (50 to 60 g) were harvested and placed into separate sample storage containers. Five cucumber fruit samples from each of three plants, n = 15 for each treatment, were maintained at 23 \({}^{\circ}\)C for 120 min and placed singly (1 cucumber per sample) within the sampling chamber to build headspace volatiles prior to separate e-nose analysis. The e-nose sensor array was exposed to cucumber fruit sample VOCs via a tube delivery system in which individual sensors interacted with VOCs adsorbing to the sensor surface leading the changes in electrical resistance by which sensor responses were recorded. The changes in electrical resistance are proportional to the amount of chemical adsorption and response specificity of the sensor surface to specific VOC vapors. The signal responses of individual sensors, resulting in changes in electrical resistance, provided the primary data from the sensor array which were assembled to create a sensor-response distribution pattern (smellprint signature) used to identify VOC mixtures based on pattern recognition algorithms. To establish and maintain a stable sensor baseline, the e-nose system was purged with fresh air for 150 s before the testing of each cucumber air sample.