# Identification and Allelopathy of Green Garlic
## 1 Introduction  
Plants synthesize and release various volatile organic compounds (VOCs) [1]. VOCs play essential roles in attracting pollinators and seed-dispersers, defense against herbivores and pathogens, interplant signaling and allelopathy [2; 3]. Garlic (_Allium sativum_ L.), an economically important vegetable [4], contains various volatile components, including diallyl, dimethyl, and allyl methyl sulfides, disulfides, and trisulfides, as well as some other minor components, all of which are formed by the decomposition of allicin and are released upon crushing garlic. These organosulfur compounds can inhibit carcinogen activation, boost detoxifying processes, cause cell cycle arrest, stimulate the mitochondrial apoptotic pathway and increase the acetylation of histones [5]. Garlic also has potential allelopathic effects and is widely used in crop rotation and intercropping with many other crops, e.g., cucumber [6], tomato [7], eggplant [8], pepper [9]. In this kind of intercropping system, garlic has been confirmed to alleviate continuous cropping issues from the aspects of reducing plant diseases and improving the physical and chemical properties of soil [10]. Green garlic is young garlic with tender leaves that harvested at the early stage before the bulb is formed and it is consumed by people of many countries. The green garlic-cucumber intercropping system has been shown to have the benefits of improving soil fertility, promoting the activity of soil enzymes and increasing cucumber biomass [6; 11]. However, there isa lack of identification of the main allelochemicals for green garlic volatiles, and knowledge about what kind of allelochemicals regulated the increased biomass of cucumber, which are the subjects in this study. Diallyl disulfide (DADS) is a major allelochemical of the VOCs in garlic [7]. In animals, DADS has been shown having the effect of reducing cellular toxins and inhibiting the proliferation of cancer cells through several actions which include the activation of metabolizing enzymes that detoxify carcinogens, suppression of the formation of DNA adducts, antioxidant effects; regulation of cell-cycle arrest, induction of apoptosis and differentiation, histone modification and inhibition of angiogenesis and invasion [12]. [PERSON] et al. found that the active substances from garlic that can break the bud dormancy of grapevines are sulfur-containing compounds, among which the most effective one is DADS [13]. Previous studies found that DADS can also regulate tomato root growth by affecting cell division, endogenous phytohormone levels, expansin gene expression and the pathways of tomato root sulphate assimilation and glutathione (GSH) metabolism [7; 14]. Besides, a low concentration DADS is able to promote cucumber root growth and induce main root elongation by up-regulating the expression of CsCDKA and CsCDKB genes and regulating adjusting the hormone balance of roots [15]. However, the effect of DADS on scavenging of cucumber reactive oxygen species (ROS) has not been investigated. It is important to choose a suitable method to collect as much as possible the volatile compounds of plants in their natural state. For determination of garlic flavor components by gas chromatography-mass spectrometry (GC-MS), several sampling techniques including steam distillation (SD), simultaneous distillation and solvent extraction (SDE), microwave assisted hydrodistillation extraction (MWHD), ultrasound-assisted extraction (USE), solid-phase trapping solvent extraction (SPTE) and headspace solid-phase microextraction (HS-SPME) were applied and compared in previous studies [16; 17]. Compared with SD, SDE, and SPTE, the HS-SPME method had several advantages which are rapid solvent-free extraction, no apparent thermal degradation, less laborious manipulation and less sample requirement.