## 4 Discussion  
Two earlier studies that investigated volatiles associated with wheat plants have been reported [17,18]. In contrast to this study, these two investigations identified many more volatiles; 105 in the case of the [PERSON] et al. [18] study and 25 by [PERSON] et al. [17]. However, [PERSON] et al. [18] used another method for VOC extraction, and [PERSON] et al. [17] investigated undamaged leaves. [PERSON] et al. [18]. used hydrodistillation of wheat plants followed by direct organic solvent extraction of crude distillate. Our approach, which is focused on in vivo _monitoring_ of wheat VOC emissions, gives information on volatiles released from plant tissue, and importantly we monitored their intensities at three fixed times (24, 48, and 72 h following a given artificial stress). In a study by [PERSON] and [PERSON] [4], which investigated the behaviour of plants during abiotic stress exposure, they focused their analysis on three known major plant volatiles, namely, (_E_)-2-hexenal, (_Z_)-3-hexenal, and hexanal. The major difference between our current study and that of [PERSON] and [PERSON] [4] is that we observed no hexanal release. In agreement with our study, other investigations have reported the stress-induced release of (_E_)-\(\beta\)-ocimene, linalool, \(\beta\)-caryophyllene, and (_E_)-\(\beta\)-farmesene as signalling and defensive volatiles of plants [2,6,19]. The correlation of increased levels of C6 aldehydes and corresponding to alcohol emission may be caused by upregulation of the biosynthetic pathway originating in linolenic acid [6], which is common for all of those compounds. It is of note that whilst a high (_Z_)-3-hexen-1-yl acetate emission was found for all treatments, it was especially so for those that did not include any physical damage to the leaves. This result may be a confirmation of [PERSON] and [PERSON]'s [20] suggestion that the conversion of C6 compounds into an acetate form is promoted while crossing the undamaged plant tissue barrier. Moreover, using GC/MS, [PERSON] et al. [21] found that _Aelia acuminata_ L. (1 or 2 adult pairs) significantly induced VOC emissions from wheat plants, where, as a result of biotic stress, larger amounts of the following VOCs were released: (_Z_)-3-hexenal, (_E_)-2-hexenal, (_Z_)-3-hexen-1-ol, (_E_)-2-hexen-1-ol, \(\beta\)-pinene, \(\beta\)-myrcene, (_Z_)-3-hexen-1-yl acetate, 1-hexyl acetate, 4-heptanone, (_Z_)-ocimene, linalool, linalool oxide, benzyl acetate, methyl salicate, indole, \(\beta\)-caryophyllene, and (_E_)-\(\beta\)-farmesene. It was also reported by them that two pairs of insects caused a significantly stronger plant reaction. Many of the studies that report VOCs emitted by plants are often carried out by harvesting the plant or cutting a representative fragment of plant tissue and then subjecting that material to some form of extraction (organic solvent, SPME) [4,22,23]. Such an approach is divorced from the reality of the various harmful situations plants face in nature, and the procedure adopted may strongly influence the behaviour of the plant material, resulting in volatiles that may not be present under real-world conditions, such as during harvesting, which enhances GLVs owing to this additional abiotic stress (actually terminal) source. Therefore, the results from solvent extraction to monitor VOC release have to be conducted with care, because the contents of phytochemicals inside the plant tissue do not necessarily translate directly to the VOC emissions [24,25]. Thus, our in vivo approach to monitor VOC release is especially valuable. Nevertheless, other approaches, including solid-phase