The current study assessed the physiological and psychological effects of olfactory stimulation by peony flowers on human. Compared to the measurements for the control groups, autonomic nervous system parameters (HR, LF/HF, and HF) and POMS (T-A, A-H, F-I, D-D, C-B, and V-A) scores were determined to have significant changes after inhaling the four peony fragrances. These outcomes, showing that olfactory interaction with the peony fragrances could relax people and reduce stress, were similar to those found in previous studies [18, 20]. However, our results suggest that the differences among the four peony scents (PS, RS, WS, and LVS) were not significant. In addition, from the perspective of semantic difference scale, students' responses to the four scents of peony flowers were all positive. We conclude that the flower scents of different peony cultivars can induce optimistic emotions, and the rose scent is preferred by participants than the three others.
EEG is used to determine human comfort or stress by measuring brain functions. Basically, brainwaves are associated with a particular state of mind; for example, during relaxation, our brain starts producing alpha waves. In the present study, the theta, low alpha, and high alpha wave activities were significantly increased in the experimental (peony fragrance) groups than in the control (resting) groups. These results were consistent with the previous studies showing that plant essential oils (lavender oil, citronella oil) inhalation significantly increases the alpha wave power [13, 17]. Other studies have shown that alpha power increases when there is s feeling of happiness, and decreases when there is s feeling of sadness . Furthermore, alpha waves dominate during quiet flowing emotions , and increased alpha wave activity indicates high relaxation . Interestingly, drug addicts produce more alpha activities, which is a common example of physical relaxation . In addition, increased theta waves were observed during meditation and various situations . Thus, we can know that inhaling the four peony scents enhances the relaxation level compared with the control groups. Similarly, the participants' low and high beta brain activities were higher after inhaling the peony fragrance, suggesting that participants were more active and alert. Increased beta wave activities were observed during the alert state of mind, whereas lower beta wave activities were observed during the drowsiness state . Furthermore, beta waves occur when the individual is alert, externally focused and relaxed . An increase in alpha and beta wave activities were found during relaxation techniques . Thus, we conclude that olfactory stimulation by the four different peony scents makes participants alert and relaxed, but there was no significant difference in this study. In contrast, delta, low- and high-gamma wave activities were lower after inhaling the peony fragrance. Delta waves are the slowest waves and are produced during deepest meditation . The previous studies have shown that an increase in gamma brainwave activities might be associated with a higher meditation state . Therefore, olfactory stimulation of the four different peony scents might induce a light or general meditative state.
This study was the first one that investigated the relationship between the different types of tree peony fragrances and human physical and mental health. The study showed that robust finding for associations between inhaling the tree peony fragrance and physiological indexes and psychological scales. Compared with previous studies , this study combined heart rate variability and brain waves to comprehensively evaluate the physiological effect of peony flower fragrance on the participants.
It is worth mentioning that our research, differing from the previous [18, 36, 37, 38–41], has been completed initially by inhaling the four different peony scents. From the perspective of tree peonies application, we consider that the present study suggested three key implications. First, the four different peony scents could provide a positive effect on the participants. This indicated that the peony cultivars mentioned in this paper can be used as optimal plant materials for constructing peony fragrant landscape. Second, the essential oil, perfume, and essence can be extracted from tree peony petals [42, 43]. We suggest that the peony cultivars mentioned in this study can be used to develop the cosmetics and health care products. Finally, we suggest that the designers appropriately increase the application of tree peonies (e.g., peony cut flowers, peony pot flowers) in the indoor environment to create an aesthetic and health-promoting environment.
However, we must admit that the present study does have a few limitations. First, all of the participants were college students. We only evaluated the effects of tree peony fragrance on college students, so that the differences of the data for the psycho-physiological effects on various populations (e.g., office workers, elderly people, and patients) still remain unclear. Second, the lack of participants’ personal information of confounding variables such as cultural background (e.g., peony culture researcher), and personality (e.g., peony aroma lover) potentially affects the accuracy and reliability of the experimental results. Additionally, the present study only used heart rates, HRV to demonstrate the autonomic nervous system parameters of participants. Other experimental indices, such as skin temperatures, respiratory rates, and stress hormone concentrations, should also be assessed for a more comprehensive determination of the effect of olfactory stimulation by tree peony flowers on human physiology.
Similar to the smell of other plants in the landscape [44, 45], tree peony fragrance has health care function. As mentioned above, this may mainly benefit from some volatile components emitted by plants [2, 3]. Further, more insight is needed in the correlation between floral constituents (e.g., monoterpenol, monoterpene) of tree peony and psycho-physiological indexes. In addition, next to purposeful olfactory stimulation of flower fragrance, the plant culture may also contribute to mental health . Tree peonies may also form the smellscape in the green spaces. Further research is needed to investigate the association between the cultural connotation of tree peony smellscape (e.g., smell metaphor, smell preference, smell memory, smell symbol) and mental health.