Effect of prohydrojasmon on total phenolic content, anthocyanin accumulation and antioxidant activity in komatsuna and lettuce

ABSTRACT

Prohydrojasmon has been reported to improve the quality of crops. However, most previous studies have investigated its application on fruits. Here, we evaluated the effect of prohydrojasmon on the growth and total phenolic content, anthocyanin content, and antioxidant activity in komatsuna (Brassica rapa var. periviridis) and lettuce (Lactuca sativa L.). Prohydrojasmon did not show any serious inhibitory effect. Prohydrojasmon applied to komatsuna at a concentration of 0.5 µM significantly increased the total phenolic content and anthocyanin content, and a concentration of 1 µM increased the antioxidant activity. In lettuce, prohydrojasmon at a concentration of 400 µM significantly increased the total phenolic content and anthocyanin content, while a concentration of 0.5 µM significantly increased the antioxidant activity. These results suggest that prohydrojasmon positively affects the phenolic compound and anthocyanin accumulation and antioxidant activity in komatsuna and lettuce without adversely affecting growth.

Prohydrojasmon positively affects the phenolic compound and anthocyanin accumulation and antioxidant activity in komatsuna and lettuce.

Prohydrojasmon, vegetative crops, phenolic, antioxidant activity

In the last two decades, there has been increasing interest in health-promoting foods rich in secondary metabolites, which are frequently called phytochemicals [1]. One class of phytochemicals that has gained particular attention is polyphenols. Based on considerable epidemiological evidence, the presence of polyphenols along with other health-affecting substances had been proven to promote health and diminish the risk of various deadly diseases such as cancers and cardiovascular diseases [2,3]. Polyphenols can be found in various vegetables and fruits; consequently, diets rich in polyphenols have been strongly linked to reduced risk of cancers, diabetes and cardiovascular diseases [4,5].

Jasmonic acid (JA) and its derivates are plant growth regulators found in higher plants that regulate many physiological processes, including senescence, fruit ripening and coloration, and pigment accumulation. These substances are also known as elicitors that can significantly improve the quality of crops by inducing the biosynthesis of secondary compounds such as anthocyanins, glucosinolates, terpenoids, and phenolics [6–9]. Numerous studies have reported that JA improves the quality of crops by increasing these secondary metabolites through its unique role in regulating physiological processes. However, to date, most studies have focused on JA application to fruits rather than vegetables, particularly in the form of prohydrojasmon [propyl (1RS,2RS)-(3-oxo-2-pentylcyclopentyl) acetate], or PDJ.

PDJ is a synthetic analog of JA that may work identically to endogenous JA and was developed as a plant growth regulator. Unlike MeJA, most PDJ application is still focused on fruits, such as apples, and oranges [9,10]. Currently, PDJ application to vegetables is still limited. This study is the first attempt to evaluate the positive effects of PDJ on two leafy crops, komatsuna and lettuce, to increase total phenolic content, anthocyanin content, and antioxidant activity without negatively affecting growth.

Materials and methods

Plant materials, growth conditions, and prohydrojasmon treatment

Komatsuna (Brassica rapa var. periviridis) seeds were purchased from Takii & Co., Ltd. (Kyoto, Japan). Four Komatsuna seeds were sown directly into a pot (diameter = 10.5 cm and depth = 9 cm) containing soil (Metro Mix, Hyponex Japan Corp., Ltd., Japan) that was moistened with tap water. Six pots were used, one for each treatment including the control with distilled water. All pots were kept in a growth chamber (LPH-214-S, NK system, Japan). The growth conditions were set to 23°C and ca. 60% humidity with 16h light/8h dark using white fluorescent lights (FHF-16EX-N-H, NEC Lighting Ltd., Japan). The photosynthetic photon flux density (PPFD) was ca. 110 μmol m⁻² s⁻¹ at the top of each plantlet. Distilled water was added to all the pots evenly every morning to maintain the soil moisture. To support growth, additional fertilizer (Hyponex N-P-K = 6-5-10, Hyponex Japan Corp., Ltd.), diluted to 1:500, was supplied to all the pots once every week after the first true leaves completely emerged in all the pots. Jasumomeito Ekizai (Meiji Seika Pharma Co., Ltd., Tokyo, Japan), which contains 5% of the active ingredient PDJ, was prepared at various concentrations by dilution with distilled water. The treatments were divided into two groups; the low concentration group (0.1, 0.5, and 1 µM PDJ), and the high concentration group (200, and 400 µM PDJ). Foliar spraying of PDJ was performed one week prior to harvesting. To evaluate growth, the length and width of the first true leaves of komatsuna plants were measured 1 day prior to treatment and continuously measured after treatment every 2 days. On the final day (day 21), all aerial parts of the komatsuna plants were collected and weighed.

Lettuce (Lactuca sativa L.) plants were grown hydroponically. Lettuce seeds, which were purchased from Takii & Co., Ltd., were germinated and grown on Rockwool (Grodan, Roermond, Netherlands) in hydroponics in a growth chamber at 23°C with 16h light/8h dark using artificial white fluorescent lights. The PPFD was ca. 110 μmol m⁻² s⁻¹ at the top of each plantlet. Irrigation was performed regularly to maintain an appropriate water level. Once the first true leaf emerged, PDJ treatment at five concentrations (0.1, 0.5, 1, 200, and 400 µM) was applied once per week. Every 2 days the length of the leaves was measured and on the final day (day 21) the fresh weight was measured. Two experiments were conducted independently.

Extraction of phenolics and anthocyanins

Approximately 0.1 g of first true leaves from each plant were collected, chopped and transferred into separate plastic tubes. The exact weight from each sample was measured beforehand. One milliliter of 80% methanol + 1% hydrochloric acid was added to each tube containing chopped plant tissue. All plastic tubes were shaken gently to make sure all the plant tissue was soaked completely. The tubes were kept in the dark at temperature 4°C for 48 h to allow the extraction process.

Measurement of total phenolic content

Total phenolic content was measured using Folin-Ciocalteu (FC) reagent with gallic acid as a standard referring to a previous method with a few minor modifications [11–13]. Plant tissue was extracted with 80% methanol + 1% hydrochloric acid, was mixed with an equal volume of distilled water. The FC reagent was then added and the sample was kept in the dark at ambient temperature for 3 min. Next, 10% Na₂CO₃ was added, and the sample was kept in a dark room at ambient temperature for 30 min. The absorbance of each extract per gram fresh weight of plant tissue (A₇₅₀/g FW) was measured using a spectrophotometric microreader (Power scan-HT, BioTEK, Japan) at 750 nm. The total phenolic content is expressed as gallic acid equivalents (GAE), which was calculated from a standard curve constructed using known concentrations of gallic acid. The results are shown as milligrams of gallic acid equivalents per gram fresh weight of plant tissue (mg GAE/g FW).

Measurement of anthocyanin content

To determine anthocyanin content, the absorbance at 530 nm of extracts of plant tissue extracted with 80% methanol + 1% hydrochloric acid was measured using a Power scan-HT [14]. Each extract was transferred directly to a 96-well microplate. The anthocyanin contents are expressed as absorbance of extract per gram fresh weight of plant tissue (A₅₃₀/g FW).

Measurement of antioxidant activity

Antioxidant activity was evaluated using the DPPH (2,2-diphenyl-1-picrylhydrazyl) free radical scavenging assay referring to a previous method with a few modifications [15]. A DPPH working solution (DWS) was prepared and Trolox was prepared to provide a standard curve. Ten microliters of sample or Trolox were mixed with 190 µL of DWS. The microplate was kept in the dark for 10 min at ambient temperature to allow the reaction. The absorbance was measured in triplicate for each DPPH (+) and DPPH (−) using a Power scan-HT at 520 nm.

Statistical analysis

All statistical analysis was done using Microsoft Excel and IBM SPSS 25. Significant differences between growth parameters and total phenolic contents in different treatments were assessed by analysis of variance (ANOVA). Student’s t-test was used to assess statistical differences between the assay results of samples and negative controls. A value of p < 0.05 was considered significant (*) and p < 0.01 highly significant (**).

Results

PDJ treatment does not affect the growth of komatsuna or lettuce

The fresh weights, lengths and widths of leaves were measured on the final day of the experiment (Table 1). For komatsuna, we observed no significant difference in the length and fresh weight parameters. However, PDJ had a significant effect at a concentration of 1 µM on the width parameter. The average leaf length in komatsuna was 38.3–42.9 mm, and the highest value was observed in the control treatment (distilled water). The fresh weight of komatsuna was 2.39–3.44 g, and the highest value was observed in the 400 µM PDJ treatment. The leaf width was 16.8–22.4 mm and the 1 µM PDJ treatment showed the highest value, which was highly significantly different to the control. Although the differences among the length and fresh weight values were statistically insignificant, except for the length parameter, all PDJ treatments in komatsuna showed higher values than the control.

Table 1.

Effects of PDJ treatment on growth parameters of komatsuna and lettuce.

			Concentration of PDJ (µM)
Plant species	Parameter	Control	0.1	0.5	1	200	400
Komatsuna	Length of leaf (mm)	42.9 ± 2.01	38.7 ± 3.08	38.3 ± 2.12	38.4 ± 3.14	41.3 ± 2.32	38.9 ± 2.11
	Width of leaf (mm)	16.8 ± 1.27	18.5 ± 0.28	16.9 ± 1.71	22.4 ± 0.80*	21.3 ± 2.93	21.0 ± 1.81
Fresh weight (g)	2.48 ± 0.43	2.72 ± 0.22	2.39 ± 0.35	2.82 ± 0.21	3.19 ± 0.52	3.44 ± 0.19
Lettuce	Length of leaf (mm)	25.8 ± 0.83	30.4 ± 3.19	31.2 ± 1.78*	29.2 ± 2.28	27.7 ± 2.41	25.9 ± 1.39
Fresh weight (g)	1.58 ± 0.11	1.47 ± 0.15	1.68 ± 0.12	1.57 ± 0.15	1.50 ± 0.09	1.38 ± 0.16

Mean ± standard error. Statistically significant differences (p < 0.05) within a row are indicated by an asterisk (*).

Length and width correspond to the difference before and after PDJ treatment.

Similarly, PDJ did not show a significant effect on the fresh weight of lettuce. However, the 0.5 µM PDJ treatment significantly affected the length of lettuce leaves. The fresh weight of lettuce was 1.38–1.68 g, and the highest value was observed in the 0.5 µM PDJ treatment. The leaf length of lettuce ranged from 25.8 to 31.2 mm, and the 0.5 µM PDJ treatment showed the highest value, which was significantly different to the control. Similar to komatsuna, almost all PDJ treatments applied to lettuce produced higher values than the control.

PDJ treatment affects the total phenolic content of komatsuna and lettuce

We measured the total phenolic content in komatsuna and lettuce plants treated with PDJ at various concentrations (Figure 1). Prohydrojasmon significantly increased the total phenolic content in komatsuna. The control with distilled water showed a lower phenolic content than almost all PDJ treatments. The 0.5 µM PDJ treatment clearly showed the highest total phenolic content (0.475 mg GAE/g FW), which was very significantly higher than the total phenolic content in the control (0.351 mg GAE/g FW). PDJ treatments at 1, 200, and 400 µM had slightly lower total phenolic contents (0.446, 0.435, and 0.397 mg GAE/g FW, respectively) than the 0.5 µM PDJ treatment; however, they were also highly significantly increased compared with the control. The 0.1 µM PDJ treatment showed a value of 0.425 mg GAE/g FW, which was also statistically significantly increased compared with the control.

Figure 1.

Total phenolic contents in PDJ-treated crops. Komatsuna and lettuce plants were grown in soil and hydroponically, respectively. After leaf emergence, PDJ treatments at 0.1, 0.5, 1, 200 and 400 μM (black) or water (white) were applied to seedlings by spraying, and then the total phenolic content in the leaves of komatsuna (a) and lettuce (b) was measured. Error bar indicates standard error (SE). Asterisks (* and **) indicate statistically significant difference at p < 0.05 and p < 0.01, respectively.

In contrast to komatsuna, PDJ was more effective in lettuce at higher concentrations. PDJ had significant effects at 200 and 400 µM, with total phenolic contents of 0.414 and 0.418 mg GAE/g FW, respectively. The PDJ treatments at 1, 0.5, and 0.1 µM showed values of 0.398, 0.380, and 0.384 mg GAE/g FW, respectively. All PDJ treatments showed higher values than the control, which had a total phenolic content of 0.338 mg GAE/g FW.

Compared with their respective controls, komatsuna treated with 0.5 µM PDJ showed a 15% increase in total phenolic content, while lettuce treated with 200 µM PDJ showed only a 10.6% increase in total phenolic content.

PDJ treatment affects the anthocyanin content of komatsuna and lettuce

We measured the anthocyanin content in komatsuna and lettuce plants treated with PDJ at various concentrations (Figure 2). In komatsuna, the 0.5 µM PDJ treatment showed the highest anthocyanin content (0.776 A₅₃₀/g FW), followed by the 0.1, 1, and 200 µM PDJ treatments, which produced similar results (0.703, 0.705, and 0.707 A₅₃₀/g FW, respectively). These four concentrations showed significantly different results to the control. The 400 µM PDJ treatment showed an anthocyanin content of 0.664 A₅₃₀/g FW, and although it did not show a significant increase like the other four concentrations, it still showed a slightly higher result compared with the control (0.631 A₅₃₀/g FW).

Figure 2.

Anthocyanin contents in PDJ-treated crops. Komatsuna and lettuce plants were grown in soil and hydroponically, respectively. After leaf emergence, PDJ treatments at 0.1, 0.5, 1, 200 and 400 μM (black) or water (white) were applied to seedlings by spraying, and then the anthocyanin content in the leaves of komatsuna (a) and lettuce (b) was measured. Error bar indicates standard error (SE). Asterisks (* and **) indicate statistically significant difference at p < 0.05 and p < 0.01, respectively.

In lettuce, the highest anthocyanin content was obtained with 400 µM PDJ treatment (0.229 A₅₃₀/g FW), and was significantly higher than in the control (0.164 A₅₃₀/g FW). The 0.1, 0.5, 1, and 200 µM PDJ treatments had anthocyanin contents of 0.181, 0.167, 0.193, and 0.201 A₅₃₀/g FW, respectively. Aside from 0.5 µM PDJ, all PDJ treatments showed higher anthocyanin contents than the control.

Overall, we observed highly significant increases of anthocyanin content in komatsuna treated with PDJ. Apart from the 400 µM PDJ treatment, all PDJ treatments showed a highly significant increase compared with the control. The 400 µM PDJ treatment showed a higher value than the control but the difference was not statistically significant. The highest anthocyanin content was found in komatsuna treated with 0.5 µM PDJ, which was 10.3% higher than in the control. Consistent with the total phenolic content, a lower concentration of PDJ was more effective at inducing anthocyanin accumulation in komatsuna.

In lettuce, we observed that the anthocyanin content was only significantly increased in plants treated with 400 µM PDJ. The 400 µM PDJ treatment showed the highest anthocyanin content, which was 16.5% higher than the control. Aside from the 0.5 µM PDJ treatment, which had a similar anthocyanin content to the control, the other PDJ treatments also showed higher anthocyanin contents than the control, although the differences were not statistically significant.

PDJ treatment affects the antioxidant activity of komatsuna and lettuce

We measured the antioxidant activity in komatsuna and lettuce plants treated with PDJ at various concentrations (Figure 3). The highest antioxidant activity was found in komatsuna treated with 1 µM PDJ (0.677 mg Trolox/g FW), which was higher than in the control (0.446 mg Trolox/g FW). The other PDJ treatments at concentrations of 0.1, 0.5, 200, and 400 µM resulted in 0.529, 0.595, 0.605, and 0.524 mg Trolox/g FW, respectively. Although all these PDJ treatments showed higher antioxidant activities, none of them were significantly different to the control.

Figure 3.

Antioxidant activities in PDJ-treated crops. Komatsuna and lettuce plants were grown in soil and hydroponically, respectively. After leaf emergence, PDJ treatments at 0.1, 0.5, 1, 200 and 400 μM (black) or water (white) were applied to seedlings by spraying, and then the antioxidant activity in the leaves of komatsuna (a) and lettuce (b) was measured. Error bar indicates standard error (SE). Asterisks (* and **) indicate statistically significant difference at p < 0.05 and p < 0.01, respectively.

In lettuce, the 0.1 and 400 µM PDJ treatments had antioxidant activities of 0.634 and 0.879 mg Trolox/g FW, respectively, which were significantly higher than the control (0.309 mg Trolox/g FW). However, the 0.5 µM PDJ treatment showed the highest activity (0.924 mg Trolox/g FW), which was considered highly significant. The other PDJ treatments at 1 and 200 µM showed values of 0.604 and 0.776 mg Trolox/g FW, and although they were higher than the control, the differences were not statistically significant.

The antioxidant activity was higher in lettuce than in komatsuna, and the increase of the antioxidant activity was also greater in lettuce than in komatsuna. Compared with their respective controls, lettuce treated with 0.5 µM PDJ showed a 49.9% increase in antioxidant activity, while komatsuna treated with 1 µM PDJ showed a 20.5% increase in antioxidant activity.

Discussion

Jasmonic acid and its derivates is produced naturally when plants are exposed to various environmental stresses. This compound is known to be part of the defense mechanism toward insects and pathogens; in other words, it functions to improve plant resistance toward biotic and abiotic stresses [16]. As a synthetic analog of JA, PDJ shows promising potential to overcome the severe inhibitory effect of MeJA application.

In this study, we investigated and evaluated the effects of PDJ in inhibiting or inducing the growth of two vegetable crops, komatsuna and lettuce. For ease of evaluation, the PDJ treatments were divided into two groups; a low concentration group and a high concentration group. Our findings suggested that PDJ in general did not show any remarkable effect on the growth of either crop; in particular, there were no significant differences in plant weight. This trend was similar in both komatsuna and lettuce. However, we also found that there was a similar trend of a significant increase in dimensional growth with a lower concentration of PDJ. In komatsuna, PDJ treatment at 1 µM significantly increased the width of leaves, and in lettuce, PDJ treatment at 0.5 µM significantly increased the length of leaves.

The application of exogenous JAs inhibits the growth of aerial parts of plants [17]. In some studies, JA, particularly MeJA, tended to inhibit plant growth, particularly at a higher concentration. Tomato plants sprayed with MeJA at 1 mM were reduced their plant height and dry weight [18]. Scot pine seeding sprayed with MeJA at 10 mM also found growth reduction of plant height [19]. Arabidopsis leaves treated with 100 µM MeJA inhibited the growth of the leaf area [20]. However, in other studies, JA did not affect or only slightly increased plant growth of St. John’s wort (Hypericum perforatum L.) and thyme [21,22]. These contradictory results for the effects of JA may be related to the dose, application time, plant species, and growth conditions.

In our preliminary experiments, treatment with MeJA at 5 µM led to significantly reduced fresh weight of komatsuna (data not shown). In contrast, PDJ did not inhibit the growth of vegetable crops, but instead had a mild positive effect on dimensional growth. Our findings suggest that vegetable crops can grow normally after foliar spray treatment with PDJ in both soil and hydroponic culture conditions.

Some studies found that JA application did not affect some plant growth indices, but positively affected the production and accumulation of various secondary metabolites. For example, a foliar spray of JA did not affect the growth and development of St. John’s wort (Hypericum perforatum L.), but it significantly increased the hypericin content [21]. A foliar spray of JA did not affect the growth of thyme, but it promoted the accumulation of some secondary metabolites in its essential oil [22]. Another study reported the same trend as that detected in our study, that is, a foliar spray of MeJA did not affect the growth of romaine lettuce, but resulted in a slight dimensional growth increase, and significantly affected the composition of secondary metabolites in its essential oil [23]. Similar to the results of those studies, our findings suggest that vegetable crops can grow normally after treatments with appropriate concentrations of PDJ in both soil and hydroponic culture conditions.

A few studies have reported that PDJ improves the coloration of fruits such as apple [9], mandarin orange [10], and other fruits. The improvement of color might be explained by the fact that exogenous application of JA has a stimulatory effect on anthocyanin content that consequently improves fruit coloration [24,25]. It has been suggested that the synthetic PDJ is more stable in intensifying anthocyanin formation and thus promoting fruit coloration. The exogenous application of JA might affect several physiological processes including inhibiting callus growth, seed germination, leaf abscission, and leaf senescence [26–29]. Furthermore, it has been suggested that PDJ probably works more effectively at the reproductive growth stage than the vegetative growth stage [18]. This explains why PDJ application has been emphasized more on fruits than vegetables. Nevertheless, we found interesting results with PDJ spray application on vegetable crops. Our results showed that PDJ spraying has a slight positive impact on vegetable crops. In general, MeJA tends to upregulate genes involved in secondary metabolism that might be correlated to fruit coloration in reproductive growth [19]. However, it also tends to downregulate genes involved in photosynthesis that might be correlated to plant development during vegetative growth [30].

The effect of JA application in plants can be explained as mimicking the environmental stress that occurs in nature. Although it can adversely affect plant growth as well as yield, on the positive side, it can also stimulate the production and accumulation of some plant secondary metabolites, including terpenoids, flavonoids, glucosinolates, anthocyanins, and phenolics. The practice of PDJ foliar spraying can be referred to as elicitation, which has been practiced and studied in the past two decades to purposefully improve plant resistance, which may affect the quality of crops. Jasmonic acid and its derivates including PDJ can be considered elicitors. As an elicitor, PDJ is thought to act as a plant signaling molecule that plays an important role in inducing the expression levels of genes in secondary metabolic pathways such as phenylalanine ammonia lyase (PAL). PAL is known as an upstream enzyme of many phenolic compounds synthesized via the phenylpropanoid pathway. Consequently, it increases the amount of phenolic compounds [31–33]. The presence of phenolic compounds in vegetables, fruit, and other plant-based foods has been linked to high antioxidant activity. The high antioxidant activity of phenolics might be explained by their physiological redox properties [34].

Our findings revealed that, statistically, PDJ indeed significantly improved the total phenolic content and anthocyanin content and increased antioxidant activity without adversely affecting the growth of the crops.

In komatsuna, all PDJ treatments showed a highly significant increase in the total phenolic content in comparison with the control, except 0.1 µM PDJ, which still showed a significant increase. The results indicated that PDJ treatments of komatsuna were more effective at low concentrations than at high concentrations. In particular, the 0.5 µM PDJ treatment resulted in the highest content of phenolic compounds. Treatment with higher PDJ concentrations (1, 200, and 400 µM PDJ) led to slight decreases in phenolic contents. Some previous reports have shown that higher concentrations of MeJA can lead to reduced accumulation of phenolics and anthocyanins in some plant species and plant cell cultures [35–38]. To avoid energy-consuming responses during normal condition, some negative feedback loops for JA signaling were known and they are regulated by several transcriptional repressors, which have the ERF-associated amphiphilic repression (EAR) domain containing proteins [39–41]. In our experimental conditions, these mechanisms may act to repress the production of phenolics at higher PDJ concentrations.

In lettuce, we found significant increases of total phenolic content only in the plants treated with PDJ at concentrations of 200 and 400 µM. We observed the highest total phenolic content in 400 µM PDJ-treated lettuce. Nevertheless, even though the differences were not significant, the lower concentrations of PDJ also increased the total phenolic content in comparison with the control.

Overall, we observed that the total phenolic content and anthocyanin content shared a similar tendency in komatsuna, where PDJ application was more effective at lower concentrations (0.5–1 µM) and the effect was somehow lessened as the PDJ concentration increased. Conversely, PDJ application in lettuce was more effective at higher concentrations (200 and 400 µM), and concentrations less than 1 µM did not induce a significant increase in total phenolic or anthocyanin content. We speculate that this might be due to the differences in growth conditions between soil culture and hydroponic culture.

PDJ also had a positive effect on the antioxidant activity. In komatsuna, all treatments resulted in higher activities than the control, particularly 1 µM, which showed the highest antioxidant activity. Although the PDJ treatments showed higher values, none of them were significantly different to the control. However, we found that the antioxidant activity was similar to the total phenolic content in that a smaller dose of PDJ (0.5–1 µM) led to a higher value.

Unlike in komatsuna, the effect of PDJ on the antioxidant activity in lettuce was more obvious. We observed that PDJ at concentrations 0.5 and 400 µM resulted in significant increases in comparison to the control. Furthermore, the 0.5 µM PDJ treatment resulted in the highest activity, which was considered highly significant compared with the control. Although the 1 and 200 µM PDJ treatments did not result in statistically significant differences, they still showed remarkably higher values than the control. When we compared the antioxidant activities in komatsuna and lettuce, we found that the activity in lettuce was clearly higher than in komatsuna.

Conclusion

In this study, we showed that foliar spray application of PDJ had a positive impact on the total phenolic content, anthocyanin content and antioxidant activity in the vegetable crops komatsuna and lettuce. PDJ showed promising results that almost met our expectations without causing any serious adverse growth effects. We propose that PDJ can be used on vegetable crops to increase the contents of some beneficial secondary metabolites including phenolic compounds and anthocyanins, as well as to increase the antioxidant activity.

Acknowledgments

We thank Robbie Lewis, MSc, and Jennifer Smith, PhD, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

Author contributions

H.A., S.E, and S.T. carried out the experiments. M.K., H.F., and H.I. contributed to the analysis and discussion. H.A., S.E., S.T., and H.I. prepared the manuscript. S.T., M.K., H.F., and H.I designed the project.

Disclosure statement

No potential conflict of interest was reported by the authors.

Author notes

*

These authors contributed equally to this work.