Abstract/Summary

Eruca sativa, also known as ‘salad’ rocket, is a ready-to-eat leafy salad of the Brassicaceae family that is gaining popularity. It contains various important phytochemicals such as glucosinolates, flavanols, vitamins, and minerals that are thought to benefit human health. ‘Salad’ rocket is known for its distinct sensory characteristics, such as hot, pungent, peppery, and bitter; sometimes not widely accepted by many consumers. Numerous factors such as genetics (crop and human), stresses (abiotic and biotic), seasons, and cultivation practices influence the sensory attributes such as taste and flavour of ‘salad’ rocket, resulting in inconsistent nutritional ‘quality’. Due to growing demand for rocket crops, growers and producers are increasingly under pressure to provide supermarkets and consumers with consistent high ‘quality’. Moreover, the ability of human taste receptors to assess sensory attributes is highly subjective. However, it is known that sugars play a key role in determining the overall taste and flavour of fruit and vegetables as they can mask other tastes, such as bitterness. The overall aim of the present study was to identify molecular markers for sugars in a mapping population of E. sativa for increased consumer acceptance while maintaining the health benefits associated with the crop. Instruments such as high-performance liquid chromatography, liquid chromatography-mass spectrometry, and inductively coupled plasma optical emission spectroscopy were used to measure sugars, organic acids, glucosinolates, and sulphur content present in the ‘salad’ rocket. Sensory analysis was carried out using two trained panels, differing in genotype for the TAS2R38 bitter taste receptor that was associated with the perception of a bitter taste for glucosinolates. Three objectives were set to achieve the overall aim of the present study. ii Firstly, to measure the abundance of phytochemicals (sugars, organic acids, and GSLs) present in an F3 mapping population of 141 recombinant inbred lines of ‘salad’ rocket (Eruca sativa) grown at two separate locations: Italy and the UK and to understand the influence of environmental conditions on the accumulation of phytochemicals present. Secondly, to identify the quantitative trait loci responsible for the accumulation of primary metabolites that will be utilised in the future breeding programme of E. sativa for targeted nutritional ‘quality’. Finally, to understand the sensory perceptions of ‘salad’ rocket on human genotype by investigating the relationship between environmental factors and phytochemical (sugars, glucosinolates, and sulphur) constituents on selected six lines of E. sativa, on the first and second cut with two-time points (day 0 and day 5). The six lines (21, 25, 68, 72, 112, and 130) were chosen based on their high or low abundance of glucosinolate content from a previously developed mapping population. The results from the first objective suggested a clear influence of the growth environment on the accumulation of phytochemicals with the UK-grown plants showing a two-fold higher total sugar concentration compared to Italian-grown plants. Other phytochemicals such as total organic acids and total glucosinolates did not show any statistically significant differences between the trial locations, however, individual glucosinolate and organic acid varied significantly (p ≤ 0.05). In the second objective, a total of 20 quantitative trait loci were identified across the two trials, with 13 quantitative trait loci identified from the UK trial and 7 quantitative trait loci from the Italian trial. Here, we presented a first linkage and quantitative trait loci map for metabolites such as sugars, organic acid, and glucosinolates using a mapping population of 141 F3 recombinant inbred lines of E. sativa. The linkage map was constructed using 285 high-quality single nucleotide polymorphism markers having a map length of 889.2 cM, distributed onto 18 linkage groups covering all 11 chromosomes. The results from the third objective showed a significant difference iii (p≤0.05) in phytochemical content and sensory attributes, which were influenced by both locations and selected six lines (21, 25, 68, 72, 112, and 130). The second cut of UK-grown leaves showed a 3.5-fold higher total sugar concentration compared to the first cut. Total glucosinolates and sulphur contents were higher in the Italian trial and were positively correlated with sensory attributes such as bitterness and pepperiness. Sugars were higher in the UK-grown trial and were positively correlated with a sweet taste. Furthermore, individuals with PAV/PAV TAS2R38 diplotypes showed a reduced perception of the subtle flavour component of rocket leaves compared with AVI/AVI diplotypes. Lines 68, 112, and 130 were positively correlated with sensory attributes such as pepperiness, pungency, and sweetness, while lines 21, 25, and 72 were positively associated with green flavour, green aroma, and moistness. The results from the present study highlighted the components important for determining the taste/ flavour of E. sativa. Lines 68, 112, and 130 could be used as the potential candidates in a breeding programme for those who prefer their rocket ‘hot’, ‘peppery’, and ‘sweet’, while lines 21, 25, and 72 for those who prefer ‘mild’ rocket. This information will enable breeders to select specific cultivars to cater for the specific consumer groups that have known sensory profiles. Combining the knowledge of genetic and chemical information will help to breed a ‘salad’ rocket for increased consumer acceptance while maintaining the maximum health benefits associated with the crop.