## 3 Results and Discussion
### Profile in VOCs
#### 3.1.3 Group of Esters and Other Compounds  
Esters represented the second group of importance in the profile of wall rocket and was represented by five compounds (Table 3). Despite all populations showed similar levels for this group, significant differences were found for individual compounds (Table 4). Two esters, identified as _cis_-3-hexenyl isovalerate (E4) and _cis_-3-hexenyl butyrate (E3), contributed to approximately 11% of the total abundance (Table 4). Differences in absolute levels were found among populations for both compounds, reaching almost 20-fold. Thus, DER045 and DER040 showed the highest absolute levels for _cis_-3-hexenyl isovalerate (E4) and _cis_-3-hexenyl butyrate (E3), respectively, while the lowest values were found in DER057 and DER071, respectively. Hexyl-butyrate (E2) was the compound in this fraction showing the lowest levels. _Cis_-3-hexenyl propionate (E1) and _cis_-3-hexenyl valerate (E5) were found at intermediate levels. From them, _cis_-3-hexenyl propionate (E1) showed a high intra-population variation that reduced differences among materials. On the contrary, _cis_-3-hexenyl valerate (E5) was mainly affected by the accession and therefore significant differences were found. Thus, DER057 and DER089 showed the lowest levels, while it was found at the highest levels in DER068 (Table 4).  
On the other hand, the miscellaneous group was mainly represented by _cis_-3-hexen-1-ol (O1). This alcohol accounted on average for 6.4% of the total volatile profile. DER045 showed the lowest value in absolute terms, which was almost 4-fold times lower than in DER102 (Table 4).  
Both the alcohol _cis_-3-hexen-1-ol and the esters found in the present study are known as green leaf volatiles (GLV). This class of compounds is commonly released from leafy vegetables, flowers and fruits following damage [47; 48], for example due to herbivore attack but also when cutting during food preparation. The process is suggested to be related to the defense system of plants [25], but it is also of interest for sensorial studies and consumers acceptance as the compounds released provide a characteristic green leaf odor [47]. In agreement with our results, _cis_-3-hexen-1-ol has been previously found as the main alcohol in other _Brassicaceae_ including rocket crops, radish or horseradish [29; 31; 34; 42] and it would provide grassy, green notes [42]. This compound is commonly found together with other GLV alcohols, but especially with GLV aldehydes in leaf material [22; 33], although this co-occurrence is not always so clear [31]. On the other hand, GL esters are not commonly described as headspace volatiles in rocket crops or found at low relative abundances [29; 31; 34], but results differ in literature. For example, [PERSON] et al. [26] also found _cis_-3-hexenyl butyrate as the main ester in salad rocket leaves, sharing similar levels as the alcohol _cis_-3-hexen-1-ol as in some of the populations of wall rocket tested in the current study. Interestingly, [PERSON] et al. [25] identified esters of _cis_-3-hexen-1-ol including _cis_-3-hexenyl- propionate, butyrate and isobutyrate as prominent VOCs in tomato leaves released as defensive mechanism. Considering that our methodology provoked chemical damage by cutting, the abundance of esters in our materials could be explained by the activation of a similar defensive response. However, this issue needs more attention and should be further studied.  
Together with their involvement in the defense system, esters also provide aroma notes like green, fruity, fresh and herbal [26; 49]. In particular, the main esters determined in the current study are related to green, fresh and herbal aroma. In this sense, DER045 and DER040 could be candidates for developing varieties with enhanced green notes. Furthermore, the population DER102 may be considered as it complemented the highest levels of the alcohol _cis_-3-hexen-1-ol with also high levels of the ester _cis_-3-hexenyl isovalerate.