# Identification and Allelopathy of Green Garlic
## 3 Discussion  
Garlic organosulfur compounds are biosynthesized for defensive purposes such as protection against abiotic stressors and are formed quickly once the plant tissues are damaged [24]. Fresh garlic only contains alliin, a derivative of cysteine. When the cells of fresh garlic cell are crushed, allinase can convert alliin to allicin. Similarly, the DADS content of the cut green garlic segments were higher than that of the whole garlic plants (Table 1), which suggested that the damaged green garlics can volatilize more DADS. In general, DATS, DADS, and VDTs are the major sulfides derived from the garlic extracts. When garlic oil is extracted by steam distillation or simultaneous distillation, the resulting extracts mainly contain DADS and DATS [25]. When garlic was treated by high temperature aging, crushing, and roasting, the compounds identified by SPME-GC/MS were also DADS and DATS [26]. DADS (97.85%), DAS (0.01%), and DATS (0.01%) were found to be the predominant flavor components of garlic samples extracted by HS-SPME using a 50/30-micron DVB/CAR/PDMS fiber [16]. In cut green garlic segments, we can detect two sulfur compounds, DADS and DAS, while in whole green garlic, only DADS can be detected (Figure 1). The green garlic allelochemicals were determined by fiber-HS-SPME-GC-MS in this study. The analytical performances of needle trap micro-extraction (NTME) coupled with GC-MS were evaluated by analyzing a mixture of twenty-two representative breath volatile organic compounds (VOCs) belonging to different chemical classes (i.e., hydrocarbons, ketones, aldehydes, aromatics and sulfurs), which confirmed the reliability of this method [27; 28]. Previous studies were mainly focused on the identification of garlic compounds produced from garlic cloves, which might not be identical to those seen this study, because we collected the volatile compounds of green garlic plants  
By calculation and comparison, in general, the amount of DADS released by green garlic was approximately 0.08 mg/g. The four treatments of 0, 6, 12 and 18 bulbs of green garlic in the mini-greenhouse, corresponded to the DADS concentrations of 0, 0.025, 0.046 and 0.057 mmol/L per cucumber plant (Table S1). This means that the DADS concentration of a 6-bulb treatment in a mini-greenhouse is approximately equivalent to 50 mL of 1 mmol/L DADS treatment (0.024 mmol/L per cucumber plant). It has been reported that lower concentrations (0.01-0.62 mmol/L) of DADS can significantly promoted root growth, whereas higher levels (6.20-20.67 mmol/L) will have inhibitory effects [6]. Therefore, considering the effect and cost, 1 mmol/L DADS concentration is suitable for use. Many studies have shown that allelochemicals significantly inhibit the activities of antioxidant enzymes, increase the level of free radicals, lead to membrane lipid peroxidation and membrane potential changes, thereby reducing the scavenging effect on ROS and destroying the entire membrane system of plants [9]. ROS play an important role in plants' signal transduction pathways, as key regulators of processes such as growth, development, response to biotic and environmental stimuli, and plant metabolism, especially H\({}_{2}\)O\({}_{2}\) which is important for programmed cell death to resist disease [20]. Green garlic volatiles and DADS significantly decreased the O\({}_{2}^{\bullet-}\) contents (Figure 3B). However, they significantly increased the H\({}_{2}\)O\({}_{2}\) content in cucumber leaves (Figure 3C). Plants have complex antioxidant systems to deal with ROS damage, including enzymatic systems (such as SOD, CAT and POD) and non-enzymatic systems (such as MDA, GSH and ASA) [23]. Previous studies have shown that the activities of POD, CAT, SOD and MDA contents in the leaves and roots of tomato were increased after the treatment of DADS [14]. In this study, the activity of POD was increased, while that of CAT was decreased, and the MDA content was also decreased after when treated with DADS.