Discussion
Finding new oil crops is necessary to meet increasing market demands, and also to diversify our current oil crops. Results presented here indicate that bitter fennel, due to its potential to produce high amounts of oil with a high percentage of unsaturated fatty acids, is a good candidate as a new seed oil crop. The 50 Iranian fennel landraces exhibited considerable diversity for oil yield, with Sari, Haji Abad, Meshkin Shahr, Moqhan, Kohin, Alamot, Marvdasht, Fasa, Saqez, and Rafsanjan producing the highest oil yields.
The main oil compositions in the twelve studied fennels were oleic acid and linoleic acid, which is similar to previous studies on fennel (Rezaei Chiyaneh et al., 2020; Hayat et al., 2019; Sayed Ahmad et al., 2018; Agarwal et al., 2018; Rebey et al., 2016; Nguyen et al., 2015; Acimovic et al., 2015; Bogdanov et al., 2015; Barros et al., 2010; Vidrih, et al., 2009; Cosge et al., 2008; Singh et al., 2006; Gupta et al., 1995; Reiter et al., 1998). It is reported that oleic acid is also one of the major fatty acids in other Apiaceae members, such as dill, celery, cumin, coriander, and carrot (Gao et al., 2016; Uitterhaegen et al., 2016; Sowbhagya, 2014; Amin et al., 2010; Saleh et al., 2009). The oleic acid chemotypes originated from regions with a dry and warm climate and the high linoleic acid chemotypes from regions with a humid and cool climate. This pattern shows potential evolutionary adaption of biochemical pathways to the environmental condition experienced by ancestors for a long time. Changes in fatty acid profiles by factors related to climate have been observed in many plant species (Mustiga et al., 2019; Raziei et al., 2018). One reason for such a pattern could be the partially shared biosynthetic pathway for oleic acid and linoleic acid, which may be by environmental factors. These factors may shift the pathway more toward one of the components and reduce the other one’s production (negative correlation between oleic and linoleic acids). A pattern like what we found here, can help breeders in high throughput preliminary screening programs.
It has been reported that temperature is positively associated with palmitic, arachidic, and stearic acid concentrations while increasing temperature negatively impacts linoleic and oleic acid concentrations (Mustiga et al., 2019). Also, Raziei et al (2018) reported that lower temperature can increase the production of unsaturated fatty acids, such as oleic and linoleic acids. Hixson and Arts (2016) reported that phytoplankton temperature is negatively associated with omega-3 fatty acids, such as linolenic acid, while positively is associated with omega-6 fatty acids, such as linoleic. For the most part, our results are compatible with these previous studies, except about oleic acid, which was similar to what Hixson and Arts (2016) reported, but opposite of what Mustiga et al (2019) and Raziei et al (2018) reported. Definitely, analyzing a higher number of samples from different climates could clarify potential relationships between temperature and oleic acid production.
The fennel landraces comprising group 1, compared to those in group 2, had the higher amounts of monounsaturated fatty acids (62.6±0.9% vs 55±1.2%) and also saturated fatty acids (8.8%±0.7 vs 6.1%±0.6), while those from group 2 had more polyunsaturated acids than those in group 1 (37.1%±1.2 vs 28.5%±0.4). Compared to those in group 1, the landraces from group 2 originated from cool/wet climates, had a higher ratio of unsaturated to saturated fatty acids, (15.6±1.6 vs 10.8±0.8), which makes them healthier sources of oil for human use. Among the twelve fennels landraces profiled, Qazvin, Sari, Rafsanjan, Meshkin Shahr, and Chahestan landraces had the highest ratios of omega-3 to omega-6 (0.11, 0.06, 0.06, 0.05, and 0.05, respectively) fatty acids.
Taking all these points into consideration, the Meshkin Shahr landrace, with high ratios of unsaturated to saturated fatty acids (15.85), and omega-3 to omega-6 (0.05) has great potential among all the evaluated landraces as a potential source of edible oil. This landrace was also the highest oil yielding landrace. Therefore, we recommend it for further studies to be considered as a high yielding source of healthy edible oils.