Ginger and Turmeric Essential Oils for Weed Control and Food Crop Protection

Ginger and turmeric are two food ingredients that are in high demand due to their flavor and positive effects on health. The biological properties of these spices are closely related to the aromatic compounds they contain. The chemical compositions of their essential oils and their in vitro phytotoxic activity against weeds (Portulaca oleracea, Lolium multiflorum, Echinochloa crus-galli, Cortaderia selloana, and Nicotiana glauca) and food crops (tomato, cucumber, and rice) were studied. Forty-one compounds, accounting for a relative peak area of 87.7% and 94.6% of turmeric and ginger essential oils, respectively, were identified by Gas Chromatography–Mass Spectrometry analysis. Ginger essential oil with α-zingiberene (24.9 ± 0.8%), β-sesquiphelladrene (11.7 ± 0.3%), ar-curcumene (10.7 ± 0.2%), and β-bisabolene (10.5 ± 0.3%) as the main compounds significantly inhibited the seed germination of P. oleracea, L. multiflorum, and C. selloana at the highest dose (1 µL/mL) assayed, as well as the hypocotyl and radicle growth of the weeds. Turmeric essential oil with ar-turmerone (38.7 ± 0.8%), β-turmerone (18.6 ± 0.6%), and α-turmerone (14.2 ± 0.9%) as principal components significantly inhibited the seed germination of C. selloana and hypocotyl and radicle growth of weeds (the latter in particular) at the highest dose, whereas it did not affect either the seed germination or seedling growth of the food crops. Turmeric essential oil can be an effective post-emergent bioherbicide against the tested weeds without phytotoxicity to crops.


Introduction
Human consumption of herbs and spices began in 5000 BC [1] and has continued until today due to the fact that these products are added to a great variety of food, especially ready-to-eat foods [2]. The world production of spices increased from 424.3 tons in 1961 to 2,413,284 tons in 2016 [3].
Herbs and spices offer a wide range of flavors that increase sensory variety in food and beverages without additional energy [4] while providing health benefits, due mainly to their antioxidant properties [5]. Several spices are dietary agents with anticancer properties due to containing compounds like curcumin, gingerol, anethole, or zerumbone, which are powerful inhibitors of nuclear factor κB (NF-κB), protein complex involved in DNA transcription [6].
Ginger (Zingiber officinale Rosc.) and turmeric (Curcuma longa L.), two powerful spices, have been widely used for both culinary and medical purposes. Ginger is an underground stem (rhizome) of a perennial herb and is used as a spice for pickles, candies, and as a preserve [7], while turmeric, popularly called "Indian saffron" [8], is also a dried rhizome of a herbaceous plant that imparts a distinctive flavor and orange color to food.

Chemical Composition of Ginger and Turmeric Essential Oils
Forty-one compounds in commercial ginger and turmeric essential oils accounting for 94.60% and 87.67% of the total composition, respectively, were identified by Gas Chromatography-Mass Spectrometry analysis. The components were clustered (Table 1) as homologous series of monoterpene hydrocarbons, oxygenated monoterpenes, sesquiterpene hydrocarbons, oxygenated sesquiterpenes, and others and listed according to Kovat's retention index calculated in GC on an apolar HP-5MS column. Sesquiterpene compounds represented the main phytochemical group found in both ginger and turmeric essential oils, of which sesquiterpene hydrocarbons (59.6 ± 0.3%) with seven compounds identified were the major set in ginger essential oil, while oxygenated sesquiterpenes (73.9 ± 1.4%) were the principal ones in turmeric essential oil with six components recognized (Table 1). It is well known that hydrocarbons and oxygenated sesquiterpenes not only have a higher structural diversity than monoterpene, but also contribute to a noteworthy extent to the special aroma and flavor of essential oils. [31] The sesquiterpene hydrocarbons α-zingiberene (24.9 ± 0.8%), β-sesquiphelladrene (11.9 ± 0.3%), ar-curcumene (10.7 ± 0.2%) and β-bisabolene (10.5 ± 0.3%), detected in lower percentages in turmeric essential oil (2.6 ± 0.1, 2.2 ± 0.0, 1.4 ± 0.1, and 0.6 ± 0.0%, respectively), were the main compounds in ginger essential oil. The results obtained were similar to those of recent research [32] in which zingiberene (16.3%), curcumene (12.4%), sesquiphellandrene (11.4%), and β-bisabolene (4.2%) were also found to be the major components of ginger essential oil from Ankara (Turkey) or with samples from Ecuador, in which α-zingiberene (17.4%) and β-sesquiphelladrene (6.7%) were between the main sesquiterpene hydrocarbons [33].
Zingiberene, the chief component of the Z. officinale essential oil here analyzed, is a monocyclic sesquiterpene hydrocarbon with natural antioxidant and cytotoxic activities: it is capable of protecting against H 2 O 2 -induced cytotoxicity and oxidative DNA damage in neuronal cells [35] as well as inhibiting the growth of lymphocytic cells in a dose-dependent manner [36]. Furthermore, high zingiberene content in tomato plants provides resistance against arthropod pests including spider mite (Tetranychus urticae) and whitefly (Bemisia tabaci) [37,38]. On the other hand, β-sesquiphelladrene, the main isomer of zingiberene and second main compound in the ginger essential oil here analyzed, has antiviral and antifertility effects [38] as well as anticancer potential by inducing apoptosis through mitochondrial pathways [39].
However, different freezing rates and thawing methods can significantly affect the composition of ginger essential oil: gingerol (3.6%) and zingerone (18.3%), the main spicy compounds of fresh ginger, reached maximum percentages when ginger was thawed by an infrared method (gingerol, 7.3%) or after thawing ginger using an infrared-microwave (zingerone, 38.3%) method [40]. These results indicated that the essential oil here analyzed and employed in phytotoxic assays was not obtained from ginger rhizome by infrared or infrared-microwave methods.
On the other hand, ar-turmerone (38.7 ± 0.8%), β-turmerone (18.6 ± 0.6%), and α-turmerone (14.20 ± 0.86%), which were not found in ginger oil, were the leading components of turmeric essential oil. The rest of the sesquiterpenes did not reach 1% in either essential oil analyzed (Table 1). These results coincide with those of previous studies in which ar-turmerone, α-turmerone, and β-turmerone were also found to be the leading compounds in turmeric essential oil [41]. However, similarly to ginger essential oil, other studies have reported changes in the chemical composition of turmeric essential oil depending on the biological raw material (fresh or dried) employed, with ar-turmerone (24.4%), α-turmerone (20.5%) and β-turmerone (11.1%), or ar-turmerone (49.1 ± 3.5%) and β-turmerone (16.8 ± 0.4%) [42] in fresh C. longa rhizome and ar-turmerone (21.4%) and the sesquiterpene hydrocarbons α-santalene (7.2%) and ar-curcumene (6.6%) in turmeric essential oil obtained from dry rhizome [42]. Higher percentages of the sesquiterpene hydrocarbons ar-curcumene (7.8%), zingiberene (4.2%), and β-sesquiphelladrene (22.8%) were found in turmeric essential oil obtained by hydrodistillation from C. longa leaves [43], confirming the GC-MS analysis [44] that our essential oil was obtained from fresh rhizomes by hydrodistillation. The therapeutic potential of ar-turmerone has been extensively studied due to its numerous beneficial effects such as anti-inflammatory and cytotoxic effects in the treatment of various neurodegenerative disorders [45,46]. Regarding pest control, ar-turmerone has also been observed to protect against insect and mite infestation; consequently, it has been incorporated into packaging material in order to avoid pest penetration of packaged products [47]. Specially, ar-turmerone has been observed to be highly toxic against maize weevil (Sitophilus zeamais) and fall armyworm (Spodoptera frugiperda) at low doses [48].
Recent studies [49] showed that essential oils containing 1,8-cineole are toxic against the tick species Rhipicephalus (Boophilus) microplus, and neral and geranial have exhibited anti-inflammatory activity through significant and similar inhibition of the gene NLRP-3 inflammasome-mediated IL-1β secretion, showing use as functional food ingredients [50].

Seed Germination and Seedling Growth Inhibition of P. oleracea, L. multiflorum, E. crus-galli, C. selloana, and N. glauca with Ginger and Turmeric Essential Oils
As several studies have indicated that essential oils may be promising herbicides [51], the effects of ginger and turmeric essential oils were tested (Tables 2 and 3 and Figures 1 and 2) against the seed germination and seedling growth of P. oleracea, L. multiflorum, E. crus-galli, C. selloana, and N. glauca.   Figure 1C). A noteworthy reduction in radicle development was achieved in C. selloana, which experienced a decline percentage of 75.26% at the highest dose (1 µ L/mL) assayed (Table 3, Figure 1D); finally, a significant inhibition of 48.32% of the radicle growth of N. glauca was observed at the highest dose (1 µ L/mL) applied (Table 3, Figure 1E). Furthermore, other Zingiber spp. have also shown phytotoxicity against different weeds; for instance, Z. zerumbet Smith, with zerumbone (74.82%) as its major compound, affected the seedling growth of Philaris minor Retz. in a concentration-dependent manner, achieving inhibition of both the hypocotyl and radicle development at 1000 ppm and showing less or no effect on the germination of seeds of Triticum aestivum L. [53].
Turmeric essential oil, with the exception of the radicle elongation of P. oleracea, significantly inhibited both hypocotyl and radicle growth of the selected weeds at all doses (0.125, 0.25, 0.50, and 1 µ L/mL) assayed. The hypocotyl development was reduced without significant differences between doses applied to reach percentages of 56.55% (P. oleracea), 40.45% (L. multiflorum), 39.33% (E. crusgalli), 97.83% (C. selloana), and 86.23% (N. glauca) ( Table 3). The radicle elongation of L. multiflorum and E. crus-galli was significantly reduced at all doses of turmeric essential oil, reaching 36.74% and 44.41%, respectively, at the highest dose tested. C. selloana was again the most sensitive species to turmeric essential oil with percentages of radicle growth inhibition of 77.32%, 81.44%, 86.08%, and 99.74% at the doses of 0.125, 0.25, 0.50, and 1 µ L/mL, whereas N. glauca reached a percentage of 51.42% at the highest dose applied.  Turmeric essential oil had no phytotoxic effects on the seed germination of P. oleracea, L. multiflorum, E. crus-galli, and N. glauca at all doses (0.125, 0.25, 0.50, and 1 µL/mL) assayed; however, significant inhibition of the seed germination of C. selloana was achieved in a dose-dependent manner, reaching 81.71% of reduction at the highest dose (1 µL/mL) tested (Table 2).
Previous studies showed that P. oleracea, L. multiflorum, and E. crus-galli were sensitive to winter savory (Satureja montana L.), which exerted a total inhibitory effect on the seed germination of the three weeds at all doses (0.125, 0.25, 0.50, and 1 µL/mL) tested, and peppermint (Mentha piperita L.), which completely inhibited the seed germination of L. multiflorum and significantly affected the seed germination of P. oleracea and E. crus-galli at the highest dose (1 µL/mL) applied [52].
Regarding ginger essential oil, although there was no significant inhibitory effect on the seed germination of E. crus-galli and N. glauca, a remarkable decrease in the seed germination of P. oleracea, L. multiflorum, and C. selloana was observed at the highest dose-reductions of 45.35%, 46.67%, and 43.91%, respectively-in relation to the control (Table 2). Ginger and turmeric essential oils are not suitable as a potent pre-emergent treatment in the control of P. oleracea, E. crus-galli, and L. multiflorum because other essential oils such as oregano essential oil with carvacrol (60.4 ± 0.1%), p-cymene (15.5 ± 0.0%), and γ-terpinene (5.2 ± 0.0%) or winter savory essential oil with carvacrol (43.3 ± 0.1%) and thymol (23.2 ± 0.1) as main compounds can completely inhibit the germination of these three weeds at all doses (0.125-1 µ L/mL) applied [52,54]. These essential oils have similar herbicidal potential to Thymus mastichina essential oil with 1,8-cineole (49.5 ± 0.4%), linalool (5.7 ± 0.0%), and α-terpineol (5.6 ± 0.0%), which showed significant effects in seedling length depending on the weed and dose [54]. In addition, turmeric essential oil could be used as a bioherbicide in the control of the invasive species C. selloana. Their use as promising postemergent alternatives will depend on the phytotoxicity of these essential oils in food crops.

Seed Germination and Seedling Growth Effect of Ginger and Turmeric Essential Oils in Tomato, Cucumber, and Rice
Seed germination of tomato, cucumber, and rice was not affected at any dose (0.125, 0.25, 0.50, and 1 µ L/mL) applied of ginger essential oil (Table 4). Table 4. In vitro seed germination and hypocotyl and radicle growth of tomato (To) cucumber (C) and rice (R) with ginger (G) and turmeric (T) essential oils.  In the seedling evolution, ginger essential oil caused a significant dose-dependent inhibition of the hypocotyl development of P. oleracea, L. multiflorum, C. selloana, and N. glauca, reaching high reduction percentages of 82.74%, 66.85%, 73.68%, and 63.77%, respectively, at the highest dose (1 µL/mL) in comparison to the control (Table 3). However, no significant reduction in E. crus-galli hypocotyl growth was observed at any dose assayed (0.125, 0.25, 0.50, and 1 µL/mL) with respect to the control (Table 3, Figure 1C).

* Dose
Ginger essential oil also considerably influenced the radicle progress of the five selected weeds. The radicle development of P. oleracea was significantly reduced by 57.22% and 86.06% relative to the control after the application of ginger essential oil at 0.5 and 1 µL/mL, respectively; this was similar to L. multiflorum, whose radicle enlargement was decreased at these doses between 60.23% and 72.36% (Table 3, Figure 1A,B). The radicle elongation of E. crus-galli significantly declined at these doses between 39.95% and 50.61% (Table 3, Figure 1C). A noteworthy reduction in radicle development was achieved in C. selloana, which experienced a decline percentage of 75.26% at the highest dose (1 µL/mL) assayed (Table 3, Figure 1D); finally, a significant inhibition of 48.32% of the radicle growth of N. glauca was observed at the highest dose (1 µL/mL) applied (Table 3, Figure 1E).
Furthermore, other Zingiber spp. have also shown phytotoxicity against different weeds; for instance, Z. zerumbet Smith, with zerumbone (74.82%) as its major compound, affected the seedling growth of Philaris minor Retz. in a concentration-dependent manner, achieving inhibition of both the hypocotyl and radicle development at 1000 ppm and showing less or no effect on the germination of seeds of Triticum aestivum L. [53].
Turmeric essential oil, with the exception of the radicle elongation of P. oleracea, significantly inhibited both hypocotyl and radicle growth of the selected weeds at all doses (0.125, 0.25, 0.50, and 1 µL/mL) assayed. The hypocotyl development was reduced without significant differences between doses applied to reach percentages of 56.55% (P. oleracea), 40.45% (L. multiflorum), 39.33% (E. crus-galli), 97.83% (C. selloana), and 86.23% (N. glauca) ( Table 3). The radicle elongation of L. multiflorum and E. crus-galli was significantly reduced at all doses of turmeric essential oil, reaching 36.74% and 44.41%, respectively, at the highest dose tested. C. selloana was again the most sensitive species to turmeric essential oil with percentages of radicle growth inhibition of 77.32%, 81.44%, 86.08%, and 99.74% at the doses of 0.125, 0.25, 0.50, and 1 µL/mL, whereas N. glauca reached a percentage of 51.42% at the highest dose applied.
Ginger and turmeric essential oils are not suitable as a potent pre-emergent treatment in the control of P. oleracea, E. crus-galli, and L. multiflorum because other essential oils such as oregano essential oil with carvacrol (60.4 ± 0.1%), p-cymene (15.5 ± 0.0%), and γ-terpinene (5.2 ± 0.0%) or winter savory essential oil with carvacrol (43.3 ± 0.1%) and thymol (23.2 ± 0.1) as main compounds can completely inhibit the germination of these three weeds at all doses (0.125-1 µL/mL) applied [52,54]. These essential oils have similar herbicidal potential to Thymus mastichina essential oil with 1,8-cineole (49.5 ± 0.4%), linalool (5.7 ± 0.0%), and α-terpineol (5.6 ± 0.0%), which showed significant effects in seedling length depending on the weed and dose [54]. In addition, turmeric essential oil could be used as a bioherbicide in the control of the invasive species C. selloana. Their use as promising post-emergent alternatives will depend on the phytotoxicity of these essential oils in food crops.

Seed Germination and Seedling Growth Effect of Ginger and Turmeric Essential Oils in Tomato, Cucumber, and Rice
Seed germination of tomato, cucumber, and rice was not affected at any dose (0.125, 0.25, 0.50, and 1 µL/mL) applied of ginger essential oil (Table 4). Table 4. In vitro seed germination and hypocotyl and radicle growth of tomato (To) cucumber (C) and rice (R) with ginger (G) and turmeric (T) essential oils. Phytotoxic effects observed at 1 µL/mL of ginger essential oil in P. oleracea (45.35%) and L. multiflorum (46.67%) ( Table 2)-weeds commonly affecting tomato crops [55]-were not reproduced in tomato germination, but, unfortunately, both hypocotyl and radicle development were significantly inhibited (Table 4, Figure 3A). These results agree with those of previous work in which seed germination of soybean was not inhibited by the aqueous extract of ginger rhizome at the doses assayed, whereas the hypocotyl and radicle length were reduced at the higher doses applied [56]. On the other hand, neither seed germination nor the hypocotyl growth of cucumber and rice were affected by ginger essential oil at any dose (0.125, 0.25, 0.50, and 1 µL/mL) assayed. The radicle elongation of cucumber was decreased in a dose-dependent manner up to a percentage of 21.44% at the highest dose (Table 4, Figure 3C). Slight differences in the radicle lengths of rice among the measurements were observed, but the data are not presented due to the difficulty of accurately measuring curved radicles (Figure 4).
Values are mean of five replications ± standard error after 14 days of incubation. Means followed by different letters in the same row indicate that are significantly different at p < 0.05 according to T3 Dunnet and Tukey tests. *Dose: µ L/mL; Hyp: Hypocotyl (mm); Rad: Radicle (mm).
Phytotoxic effects observed at 1 µ L/mL of ginger essential oil in P. oleracea (45.35%) and L. multiflorum (46.67%) ( Table 2)-weeds commonly affecting tomato crops [55]-were not reproduced in tomato germination, but, unfortunately, both hypocotyl and radicle development were significantly inhibited (Table 4, Figure 3A). These results agree with those of previous work in which seed germination of soybean was not inhibited by the aqueous extract of ginger rhizome at the doses assayed, whereas the hypocotyl and radicle length were reduced at the higher doses applied [56]. On the other hand, neither seed germination nor the hypocotyl growth of cucumber and rice were affected by ginger essential oil at any dose (0.125, 0.25, 0.50, and 1 µ L/mL) assayed. The radicle elongation of cucumber was decreased in a dose-dependent manner up to a percentage of 21.44% at the highest dose (Table 4, Figure 3C). Slight differences in the radicle lengths of rice among the measurements were observed, but the data are not presented due to the difficulty of accurately measuring curved radicles (Figure 4).   Promising results were obtained with turmeric essential oil against the food crops tested. Neither seed germination nor the hypocotyl growth of tomato, cucumber, and rice were significantly affected by the application of turmeric essential oil at any dose (0.125, 0.25, 0.50, and 1 µ L/mL) ( Table 4, Figures 3B and 3D). The radicle elongation of tomato, cucumber (Table 4), and rice ( Figure 4) was also not affected by turmeric essential oil. Previous studies have also reported the harmlessness of Promising results were obtained with turmeric essential oil against the food crops tested. Neither seed germination nor the hypocotyl growth of tomato, cucumber, and rice were significantly affected by the application of turmeric essential oil at any dose (0.125, 0.25, 0.50, and 1 µL/mL) (Table 4, Figure 3B,D). The radicle elongation of tomato, cucumber (Table 4), and rice ( Figure 4) was also not affected by turmeric essential oil. Previous studies have also reported the harmlessness of turmeric essential oil versus other food crops: for instance, chickpea, in which no adverse effect was observed in either seed germination or seedling growth [57]. However, other Curcuma spp. like C. zedoaria have been shown to inhibit the seed germination of lettuce and tomato in a dose-dependent manner (0.00%, 0.25%, 0.50%, 0.75%, and 1.00%) as well as to delay their growth, damaging the root in particular. These results are due to the different chemical compositions of these essential oils, with ar-turmerone (38.7 ± 0.8%), β-turmerone (18.6 ± 0.6%), and α-turmerone (14.2 ± 0.9%) being the main compounds in the turmeric essential oil here analyzed (Table 1), and 1,8-cineole (15.8%) and epi-curzerenone (18.2%) being those in C. zedoaria essential oil [29].
It is interesting to note that at the highest dose assayed, turmeric essential oil was able to significantly reduce the hypocotyl development of P. oleracea (Table 3) as well as both the hypocotyl and radicle growth of L. multiflorum, E. crus-galli, C. selloana, and N. glauca, without the phytotoxic effects shown by other essential oils such as rosemary (Rosmarinus officinalis L.), winter savory (Satureja hortensis L.), and bay (Laurus nobilis L.) in tomato [58].

Gas Chromatography-Mass Spectrometry Analysis
GC-MS analysis was carried out using a 5977A Agilent mass spectrometer and a gas chromatograph (Agilent 7890B) apparatus equipped with an Agilent HP-5MSi (30 m long and 0.25 mm i.d. with 0.25 µm film thickness) capillary column (95% dimethylpolysiloxane/5% diphenyl). The column temperature program was 60 • C for a duration of 5 min, with 3 • C/min increases to 180 • C, then 20 • C/min increases to 280 • C, which was maintained for 10 min. The carrier gas was helium at a flow rate of 1 mL/min. Split mode injection (ratio 1:30) was employed. Mass spectra were taken over the m/z range 30-650 with an ionizing voltage of 70 eV. The resulting individual compounds were identified by MS and their identity was confirmed by comparison of their Kovat's retention index calculated using co-chromatographed standard hydrocarbons relative to C 8 -C 32 n-alkanes and mass spectra with reference samples or with data already available in the NIST 11 mass spectral library and in the literature [59].