Effects of Growth Regulators on Forced Sprouting and Growth of Ginger

Abstract

In Taiwan, the early harvesting of young ginger is a cultivation technique for domestic ginger, which can be harvested early by forced sprouting. At present, ethylene is used as the sprouting agent. Despite its favorable sprouting effect, the technique can still be improved. Experimental results revealed that 1% citric acid, 300-ppm and 450-ppm ethephon treatments effectively facilitated the formation of 2.4, 2.4, and 2.6 large buds (diameter of ≥ 1 cm), respectively, in the rhizome. The stems and leaves of the ginger seed rhizomes that were subjected to forced sprouting emerged from the soil one month after planting, and emergence rates of 46.7% and 83.3% were achieved using citric acid and ethephon, respectively. Although the fresh weight of semi-matured ginger rhizomes obtained by forced sprouting treatment was lower, the results can still provide a reference for the early harvest of young ginger in Taiwan. Among the examined gingers, semi-matured ginger that was subjected to 50-ppm GA₃ forced sprouting treatment exhibited the most favorable growth, and the average weight of its rhizomes reached 1,567 g, which was not significantly different from the weight obtained by conventional cultivation, but significantly greater than that obtained by other treatments. The GA₃ forced sprouting treatment was not very effective, but it had an excellent effect on improving the yield of semi-matured ginger rhizomes. In the future, this treatment will be conducive for the production of semi-matured or matured ginger.

Introduction

Ginger (Zingiber officinale Rosc.) is a perennial monocotyledonous plant of the Zingiberaceae family, which is native to India and Southeast Asia, and its main edible parts are its rhizomes (Xu, 2005). In Taiwan, the planting area of ginger ranged from 752 to 1,256 ha between 2011 and 2020. Ginger plays a crucial role in the vegetable farming industry in Central Taiwan, and it is mainly produced in Nantou County, which accounts for approximately 33.9% of the total ginger planting area in Taiwan. Cantonese ginger is the main cultivar; compared to other cultivars, it is more hypertrophic, contains more tillers and less fiber, and has a moderately pungent taste (Chang, 2003). In early ginger cultivation, ginger generally takes a longer time to sprout after planting because of its low temperature; moreover, uneven sprouting results in inconsistent growth, which causes subsequent problems relating to soil cultivation and management. To overcome the problems of a long sprouting time and uneven sprouting during the early harvesting of young ginger, the forced sprouting method is currently the most widely used approach to promote the early and consistent sprouting of planted ginger.

Furutani et al. (1985) and Evenson et al. (1978) used ethephon as a sprouting agent to increase the formation of endogenous ethylene and, consequently, enhance the sprouting of ginger. In addition, forced sprouting methods involving gibberellic acid (GA₃; Okwuowulu and Nnodu, 1988), high-concentration carbon dioxide (Chung, 2001; Yeh, 1999), low-temperature stratification (Xu, 2005), a ginger chamber (Ding and Yang, 2007), electric heating (Chen et al., 2007), covering materials (Chen, 2010), glacial acetic acid fumigation (Wu, 2010), and citric acid (Chen et al., 2014) were also found to enhance the sprouting of ginger.

In Taiwan, ethephon is widely used as the sprouting agent for ginger. It is a synthetic ethylene release agent that has the functions of regulating plant metabolism, promoting growth and development and facilitating fruit ripening, leaf aging, and dwarfing (Depaepe and Straeten, 2017). However, ginger seed rhizomes often produce an excessive number of buds after being subjected to forced sprouting with ethephon, and the resulting competition for nutrients among these buds weakens them, which is not conducive to subsequent growth. Given the need for early sprouting for ginger harvesting in Taiwan, the present study aimed to identify the optimal types and concentrations of the growth regulators used in the forced sprouting of ginger seed rhizomes. A sprouting technology suitable for commercial production and application was subsequently developed, and it can serve as a reference for the industry.

Materials and Methods

Sprouting experiment on ginger seed rhizomes

Cantonese ginger was used as the material for the experiments conducted in the present study. The ginger seed rhizomes were purchased from Mingjian Township, Nantou County, Taiwan. After cleaning and selection, the seed rhizomes were cut into pieces that weighed 80 ± 5 g and placed under a shade at room temperature (25°C) for one day, after which the sprouting treatment was implemented on December 28, 2020. We modified the sprouting treatment method by Chen et al. (2014). The ginger seed rhizomes were soaked for 20 min in 50-, 100-, and 150-ppm GA₃ (Sigma-Aldrich, MO, USA) and in 150-, 300-, and 450-ppm ethephon (39.5%, Chia-Tai Enterprise, Taipei, Taiwan); 1% citric acid (Sigma-Aldrich) was used as the recommended treatment (Chen et al., 2014). The seed rhizomes that were subjected to control treatment were soaked in deionized water. After they were soaked, the ginger seed rhizomes were placed under a shade for 2 h at room temperature, after which they were soaked in a 50-ppm hypochlorous acid solution for surface sterilization. The seed rhizomes were then dried under a shade for 2 h and subsequently sealed in No. 10 polyethylene (PE) zipper bags, after which forced sprouting commenced in a dark environment at 25°C for 15 days. The ambient gas in the PE bag, which included ethylene, carbon dioxide, and oxygen, was analyzed every three days during the sprouting period. The sprouting of ginger buds was assessed based on whether 2 mm buds were observed. The total number of buds, the number of large (diameter ≥ 1 cm) and small (diameter < 1 cm) buds, and the length and diameter of buds were recorded 15 days after treatments. In addition, the total soluble sugar concentration (TSS) and starch content of the ginger seed rhizomes were analyzed. The ambient gas in PE bags was extracted for analysis using a syringe.

Changes in ambient gas during sprouting treatment

During the sprouting period, a GC-2014 gas chromatograph (Shimadzu, Kyoto, Japan) equipped with a thermal conductivity detector (TCD) was used to analyze the changes in the ethylene, oxygen, and carbon dioxide concentration levels in the PE zipping bags and breathable mesh bags every three days. Untreated gingers were used as the control group; the temperature of the column oven was set to 80°C, the temperature of the sample injection port was set to 120°C, helium was used as the carrier gas and the flow rate was 30 mL·min⁻¹.

Plant growth evaluation after forced sprouting of ginger seed rhizomes

The ginger seed rhizomes that received the optimal concentration for each treatment during the sprouting test and with no treatment as conventional cultivation were planted in an experimental field located in Mingjian Township (23°52'43.6"N 120°40'09.9"E) on January 14, 2021; 30 ginger seed rhizomes were planted for each treatment, and local practices (Tsai and Hsiao, 2012) were applied as the management model. The emergence rate, number of stems (leafy shoots), and the length of above-ground stems were recorded on the 30^th and 60^th days after planting, and these data were used as the basis to assess the effect of forced sprouting. The growth of the plants was assessed when they were harvested during the appropriate period for harvesting semi-matured ginger (9 months after planting). The following items were surveyed: plant height, plant width, number of stems, fresh weight of rhizome, length of rhizome, width of rhizome, as well as the number of sub-branches, maximum rhizome length, and maximum rhizome width of first- and second-generation offspring. The quality of the harvested gingers was assessed on the basis of their TSS, starch content, and crude fiber content.

Total soluble sugar concentration (TSS) and starch content analysis

The treatment of ginger seed rhizomes affects the sprouting number, and the stored nutrients in gingers affect the growth of their buds; thus, a carbohydrate analysis was conducted. After the ginger seed rhizome samples were subjected to freeze-drying and grinding, 0.01 g of the samples was finely weighed and added to a centrifuge tube containing 1 mL of deionized water. After the sample was oscillated in a water bath at 35°C for 3 h and subsequently centrifuged at 25°C and 13,000 × g for 10 min, the obtained supernatant was subjected to TSS analysis, and the residue was thoroughly rinsed with deionized water and dried for starch content analysis. For the TSS analysis, 0.2 mL of supernatant was added to a test tube containing 4.8 mL of deionized water and shaken evenly, after which 2 mL of the mixture was mixed with 0.1 mL of 90% liquid phenol and 6 mL of 98% H₂SO₄; the resulting mixture was then placed in a 80°C water bath for 30 min. A Bio-Rad Model 680 microplate reader (Bio-Rad Laboratories, CA, USA) was used to detect the absorbance of the mixture at 490 nm. For the starch content analysis, the residue was dried at 80°C, and 2 mL of deionized water was added. The mixture was then placed in a 100°C water bath for 15 min, after which it was removed and rapidly cooled in ice water; the mixture was then oscillated after 2 mL of 9.2 N HClO₄ was added and allowed to rest for 15 min (occasional oscillation was performed during this period). After the mixture was increased to 10 mL with deionized water, it was centrifuged at 10°C and 13,000 × g for 10 min. Subsequently, 1.9 mL of deionized water, 90% liquid phenol, and 6 mL of 98% H₂SO₄ were added to 0.1 mL of the supernatant, which was then placed in a water bath at 80°C for 30 min; its absorbance at 490 nm was similarly measured. The concentration levels of the samples were calculated by using a glucose standard solution (Sigma-Aldrich) as the standard curve.

Experimental design and statistical analysis

A completely randomized design was adopted for the experiment. Each treatment was repeated three times, and 10 plants from each repetition were examined. The data were subjected to Fisher’s least significant difference test (P < 0.05), which was performed using Costat 6.2 (CoHort Software, CA, USA) and used to identify significant differences among the various treatments.

Results

Forced sprouting of ginger seed rhizomes

The type of growth regulation treatment that was used significantly affected the sprouting of ginger seed rhizomes (Fig. 1). In the experiment, citric acid, ethephon, and GA₃ were used to promote the sprouting of ginger seed rhizomes. Both 1% citric acid, 300-ppm and 450-ppm ethephon were demonstrated to have effectively promoted the formation of large buds in rhizomes (bud diameter ≥ 1 cm); specifically, 1% citric acid, 300-ppm and 450-ppm ethephon yielded 2.4, 2.4, and 2.6 large buds, respectively, which were the highest numbers among the examined treatments (Table 1). In contrast, GA₃ treatment had no positive effect on the sprouting of large buds in the ginger seed rhizomes. Regardless of the concentration that was used during the experimental treatment, the number of large buds yielded ranged from 1.5 to 1.8, which was similar to the numbers obtained in the 150-ppm ethephon group (1.7 buds) and the control group (1.8 buds). For small buds (bud diameter < 1 cm), the number of small buds yielded by the control group, citric acid treatment group, and GA₃ treatment group was between 2.2 and 2.8. The ethephon treatment group yielded between 3.1 and 3.8 small buds, indicating that ethephon was effective in inducing the ginger seed rhizomes to form buds. However, an excessive number of small buds inhibits growth after planting; thus, the test results indicated that 1% citric acid and 450-ppm ethephon were optimal. The results pertaining to maximum bud length and width revealed that GA₃ treatment inhibited maximum bud length and width, which were 10.9–11.7 mm and 8.8–9.9 mm, respectively; these recorded lengths and widths were the lowest among the treatment groups. This indicated that GA₃ treatment had a limited effect on the sprouting of ginger seed rhizomes, failed to effectively induce the formation of large buds, and had little effect on bud quality. No statistical difference between the other experimental treatments and the control treatment was observed, and the maximum bud length and width ranged between 12.5 and 14.3 mm and between 10.2 and 11.4 mm, respectively (Table 1).

Fig. 1

State of ginger seed rhizomes after 15 days of forced sprouting using various growth regulators (Investigated on 12 Jan. 2021).

Table 1

Effects of various growth regulators on the growth of ginger seed rhizomes after 15 days with forced sprouting.^z

The TSS and starch content analyses revealed that the total-soluble-solid content was significantly affected by the sprouting treatment of ginger seed rhizomes in which growth regulators were used; however, no obvious trend was observed with respect to the concentration levels used for each treatment. The total-soluble-solid content for each treatment ranged from 29.4 to 57.9 mg·g⁻¹. In particular, the 150-ppm ethephon treatment group had the highest total-soluble-solid content (57.9 mg·g⁻¹), whereas the 1% citric acid treatment group yielded the lowest (29.4 mg·g⁻¹). The starch content yielded by the treatment groups ranged from 63.4 to 148.1 mg·g⁻¹, with the 50-ppm GA₃ treatment group yielding the highest starch content and the control group yielding the lowest (Table 2).

Table 2

Effects of various growth regulators on carbohydrate content of ginger seed rhizomes after 15 days with forced sprouting.^z

Changes in ambient gas during sprouting of ginger seed rhizomes

The ethylene, oxygen, and carbon dioxide contents in the PE bags changed significantly after the ginger seed rhizomes were exposed to various growth regulators for forced sprouting (Figs. 2 and 3). The ethylene content only changed significantly in the PE bags containing samples treated with ethephon. In particular, on the sixth day of ethephon treatment, the ethylene content in the PE bags reached its highest values (i.e., 9.97, 24.35, and 34.76 ppm as the concentration of ethephon increased). By contrast, the ethylene content produced by other treatments in the PE bags was less than 1 ppm (Fig. 3). The oxygen content in the PE bags decreased significantly during the treatment period. The average oxygen content in the initial stage was 18.2 ppm. After the third day of storage in the PE bag, the average oxygen content for all treatments decreased significantly to 8.1 ppm; thereafter, it gradually stabilized, with the oxygen content under each treatment ranging from 6.77 to 11.4 ppm. Among the treatments, the control treatment had the highest oxygen content. The average oxygen content dropped significantly from 19.2 ppm during the initial stage of storage to 10.6 ppm on the third day of treatment and then remained at 11 ppm (Fig. 2). In contrast, for carbon dioxide content exhibited an opposite trend. An average carbon dioxide content of 2.1 ppm was recorded at the beginning of the storage period and it significantly increased to 4.8 ppm on the third day of treatment, remaining between 4.8 and 5.0 ppm with no significant change thereafter. The control group produced the lowest carbon dioxide content. The carbon dioxide content that it produced in the PE bags was 1.4 ppm during the initial stage of storage and this figure increased significantly to 5.6 ppm on the third day, subsequently decreasing to 3.7 ppm (Fig. 2).

Fig. 2

Changes in oxygen (top) and carbon dioxide (bottom) content in PE bags after forced sprouting of ginger seed rhizomes using various growth regulators.

Fig. 3

Changes in ethylene content in PE bags after forced sprouting of ginger seed rhizomes using various growth regulators.

In summary, the respiration rate of the ginger seed rhizomes was significantly increased after they received growth regulator treatment, which resulted in a reduction in oxygen content and accumulation of carbon dioxide content in the PE bags. Ethephon treatment significantly increased the ethylene content in the PE bags, which peaked on the sixth day of treatment and subsequently decreased. However, the association between the ethylene content in the PE bags and the sprouting of ginger seed rhizomes requires further clarification in the future.

Plant growth assessment after sprouting of ginger seed rhizomes

In this experiment, the ginger seed rhizomes were planted in fields after being subjected to forced sprouting treatment with 1% citric acid, 50-ppm GA₃ and 450-ppm ethephon. They were managed using the conventional approach (Tsai and Hsiao, 2012), and the subsequent plant growth was monitored. The results indicated that samples that were subjected to forced sprouting treatment using 1% citric acid and 450-ppm ethephon experienced significant growth; specifically, these samples rose up through the soil one month after planting (Fig. 4), and the emergence rates for citric acid and ethephon treatments were 46.7% and 83.3%, respectively. The average numbers of stems achieved by using 1% citric acid and 450-ppm ethephon were 0.8 and 1.7, respectively, while the average stem lengths were 12 and 24.3 cm, respectively. These results were significantly greater than those achieved by the other treatment groups (Table 3). By comparison, in the control group (deionized water), 50-ppm GA₃ group, and conventional cultivation group (no forced sprouting), no emergence was observed one month after planting (Fig. 4). Two months after planting, the emergence of ginger seed rhizomes was observed in all treatment groups (Fig. 5). In the control group (deionized water), 1% citric acid, 50-ppm GA₃, 450-ppm ethephon, and conventional cultivation groups, the emergence rates were 46.7%, 100%, 20%, 100%, and 76.7%, respectively. The average numbers of stems were 0.8, 2.3, 0.2, 3.6, and 1.4, respectively and the average lengths of the stems were 32, 41.3, 5, 44.0, and 29.7 cm, respectively (Table 4). The stems of the ginger seed rhizomes that were subjected to forced sprouting using 1% citric acid and 450-ppm ethephon emerged one month earlier (Fig. 4), and the emergence rates reached 100% two months after planting.

Fig. 4

Emergence of ginger seed rhizomes subjected to forced sprouting one month after being planted in a field (Investigated on 13 Feb. 2021). Control group (deionized water, A), 1% citric acid (B), 50-ppm GA₃ (C), 450-ppm ethephon (D), and conventional cultivation groups (E).

Table 3

Growth of ginger seed rhizomes one month after planting using various growth regulators.^z

Fig. 5

Emergence of ginger seed rhizomes subjected to forced sprouting two months after being planted in a field (Investigated on 15 Mar. 2021). Control group (deionized water, A), 1% citric acid (B), 50-ppm GA₃ (C), 450-ppm ethephon (D), and conventional cultivation groups (E).

Table 4

Growth of ginger seed rhizomes two months after planting using various growth regulators.^z

However, the growth regulators produced varying results with respect to subsequent plant growth and rhizome development (Fig. 6). During the harvest period for semi-matured ginger (9 months after planting), the ginger seed rhizomes that were subjected to forced sprouting with GA₃ produced rhizomes with a fresh weight of 1567.5 g, which was greater than that achieved in the other treatment groups and similar to that achieved by conventional cultivation (1568.6 g; Table 5). In contrast, the fresh weights of the rhizomes produced in the control group, 1% citric acid treatment and 450-ppm ethephon treatment were 1002.4, 1060.5, and 1206.3 g, respectively, which were lower than those yielded by GA₃ and conventional cultivation treatments (Table 5). In addition, the use of growth regulators for forced sprouting also affected the stem height, number of sub-branches, and maximum rhizome length of the first-generation offspring. For plant height, the forced sprouting treatment in which GA₃ was used produced a plant height of 147 cm, which was greater than that achieved by the other treatments. For number of sub-branches, the treatments in which GA₃ or ethephon was used yielded the most sub-branches in the first-generation offspring (1.6 and 1.7 for GA₃ and ethephon, respectively). For the maximum rhizome length of the first-generation offspring, GA₃ treatment yielded the optimal result (7.9 cm; Table 6).

Fig. 6

Effects of forced sprouting using various growth regulators on the appearance of semi-matured ginger rhizomes nine months after planting (Investigated on 13 Oct. 2021). Control group (deionized water, A), 1% citric acid (B), 50-ppm GA₃ (C), 450-ppm ethephon (D), and conventional cultivation groups (E).

Table 5

Plant growth and rhizome yield of semi-matured ginger nine months after planting with forced sprouting of ginger seed rhizomes using growth regulators. ^z

Table 6

Rhizome differentiationof semi-matured gingers nine months after planting with forced sprouting of ginger seed rhizomes using growth regulators.^z

As for the rhizome quality of semi-matured ginger, the forced sprouting treatments in which citric acid or GA₃ was used increased the TSS levels of the rhizomes; specifically, the citric acid and GA₃ treatments yielded 44.4 and 44.5 mg·g⁻¹ of soluble sugar, respectively. However, their TSS levels were still lower than those obtained by conventional cultivation (51.8 mg·g⁻¹). Low TSS yield levels were produced by the control group (25.1 mg·g⁻¹) and ethephon treatment group (27.1 mg·g⁻¹; Table 7). The citric acid and conventional cultivation treatments yielded high starch content levels (83.9 and 73.7 mg·g⁻¹, respectively), while the control group and GA₃ treatment group yielded starch content levels of 64.7 and 69.0 mg·g⁻¹, respectively; the ethephon treatment produced the lowest starch content (45.3 mg·g⁻¹). These findings are similar to those pertaining to total soluble solids. The ethephon treatment yielded the highest crude fiber content (17.4%), whereas the citric acid treatment produced the lowest (13.4%); the crude fiber content obtained by the other treatments ranged between 14.9% and 16.5%.

Table 7

Carbohydrate and crude fiber content of semi-matured ginger rhizomes nine months after planting with forced sprouting of ginger seed rhizomes using various growth regulators.^z

These results indicated that the application of ethephon for the forced sprouting of ginger reduced the time required for sprouting, but had no significant effect on the growth of rhizomes in the subsequent stage. Furthermore, it reduced TSS and starch content levels and increased crude fiber content levels. Citric acid treatment improved the quality of rhizomes for the production of semi-matured ginger; however, the rhizomes produced by this treatment had the lowest fresh weight (1060.5 g). This indicated that citric acid and ethephon were unsuitable for producing semi-matured ginger. The use of GA₃ for forced sprouting did not promote the early sprouting of ginger seed rhizomes; however, it improved the yield of semi-matured ginger, producing rhizomes with an average weight of 1,567 g, which was similar to that yielded by conventional cultivation.

Discussion

Forced sprouting techniques can be applied to effectively improve the sprouting of early ginger, which is time consuming and inconsistent (Tsai and Hsiao, 2012). Because ginger is a tropical crop with a preference for warm climates, it germinates at a temperature of 18°C or higher. The optimal temperature for growing ginger is between 20 and 30°C, and the growth of ginger stops at a temperature of 13°C or lower. Ginger rhizomes are dormant in the winter and sprout when the temperature increases the following year (Kandiannan et al., 1996). Evenson et al. (1978) reported that soil temperature is crucial for the sprouting and initial growth of root crops. When ginger is planted at a soil temperature of 25°C, the number of days required for sprouting is fewer that that at 20°C. Conversely, when ginger is planted at a soil temperature of 30°C, it requires fewer days for sprouting than at 35°C. Ai et al. (2005) reported similar findings in their study. The planting period significantly affected the yield of ginger rhizomes. At a low cultivation temperatures, planting ginger too early affects sprouting, whereas planting too late affects the ginger rhizome yield because of the shortened growth period. In addition, the cultivation of ginger at a temperature of between 22°C and 25°C allows for the production of strong buds. Although a cultivation temperature of 28°C or higher can lead to faster sprouting, the resulting buds are usually weak. A temperature of 20°C or lower leads to slow sprouting of the propagule, thereby increasing the time required for cultivation. This indicates that ambient temperature is crucial for the sprouting and rhizome size of ginger.

In the present experiment, the forced sprouting treatments were performed using a modified version of the method used by Chen et al. (2014). The ambient temperature for forced sprouting was changed to 25°C to avoid growth of an excessive number of small buds due to the high cultivation temperature (Ai et al., 2005). The forced sprouting treatment of gingers with 1% citric acid and 450-ppm ethephon induced the formation of 2.4 and 2.6 large buds (Table 1); these results are consistent with those reported by Evenson et al. (1978) and Chen et al. (2014). Evenson et al. (1978) demonstrated that the forced sprouting of ginger seed rhizomes at 35°C for 24 h or the soaking of seed rhizomes in 250-ppm ethephon for 15 min significantly induced sprouting. In particular, the ethephon treatment was more effective in inducing bud formation and producing more root systems. Chen et al. (2014) used 1% citric acid for forced sprouting, and this treatment induced the formation of an average of 2.2 large buds and produced buds measuring up to 4.5 cm long.

Ethephon is a synthetic organophosphorus ethylene release agent. Ethylene is an endogenous phytohormone that plays a crucial role in regulating numerous developmental processes such as fruit ripening, leaf shedding, tuber sprouting, and the withering of flowers (Depaepe and Straeten, 2017). Exogenous ethylene has various effects on the sprouting mechanism of tuber crops. An early study on potatoes and onions demonstrated that storage at room temperature (approximately 20°C) with ethylene application promotes sprouting (Vacha and Harvey, 1927). However, several studies have also demonstrated that exogenous ethylene treatment can inhibit the sprouting of potatoes, onions, and sweet potatoes (Bufler, 2009; Cheema et al., 2013; Prange et al., 2005). A study on ginger sprouting revealed that the use of ethephon or daminozide can promote the sprouting of ginger (Furutani and Nagao, 1986). Rhizome sprouting can be accelerated by storing ginger at 22°C and using ethephon (Paull et al., 1988). However, the mechanism by which exogenous ethylene promotes the sprouting of ginger is unclear. Lv et al. (2021) suggested that ginger seed rhizomes germinate because of an increase in reducing sugar content or the accumulation of reactive oxygen species; they also noted that an increase in the respiration rate of tuber crops during storage promotes bud sprouting, leading to changes in the content levels of reducing sugar and starch during storage. Furthermore, Lv et al. (2021) studied ginger storage and confirmed that 1-methylcyclopropene could inhibit sprouting, reduce the respiration rate, reduce the sugar content of rhizomes, and increase starch content. A study on sweet potatoes also reported that the application of 1-methylcyclopropene treatment during sprouting reduced the content of reducing sugars (Cheema et al., 2013). The present study also achieved similar results; specifically, the use of 150-ppm ethephon for forced sprouting yielded a total of 4.8 buds, which was superior to that yielded by the control group (4.0 buds; Table 1). Furthermore, the use of 150-ppm ethephon also resulted in a significant increase in TSS level relative to the other treatments. Similar results were obtained with the high-concentration ethephon treatment and the resulting TSS level ranged between 39.1 and 41.2 mg·g⁻¹, which was lower than that achieved by the 150-ppm treatment, but more favorable than that achieved by the other treatments (Table 2). In addition, the application of ethephon treatment for forced sprouting led to more favorable results in terms of the total number of buds that were produced, suggesting that the use of ethephon to promote ginger sprouting is associated with the accumulation of TSS. However, the TSS content of the 300-ppm and 450-ppm ethephon treatments was lower than that of the 150-ppm ethephon treatment, which may be related to the total number of buds formed by ginger seed rhizomes. Previous studies have shown that the concentration of soluble sugars in potato tubers was higher during sprouting than dormancy, and the length and dry weight of the sprouting showed a highly positive and significant association with the content of reducing and total sugars (Morales-Fernández et al., 2015; Suttle, 2004). In this experiment, the 150-ppm ethephon treatment resulted in a total of 4.8 buds (large buds plus small buds), which was lower than the 6.2 and 5.8 buds for the 300-ppm and 450-ppm ethephon treatments. The 150-ppm ethephon treatment was able to maintain higher TSS content, probably due to the lower number of total sprouting buds, which reduced TSS consumption. In the future, this can be confirmed by continuous analysis of the TSS content of ginger during forced sprouting treatment.

Gibberellins can be used as plant growth regulators to regulate growth and influence various developmental processes, including stem elongation, sprouting, the breaking of seed dormancy, flowering, sex expression, and the inhibition of leaf and fruit senescence. In the present experiment, the use of GA₃ as a forced sprouting agent did not effectively promote the sprouting of ginger seed rhizomes; specifically, only a 20% emergence rate at two months after the seed rhizomes were planted in the field, which was significantly lower than the rates achieved by the other treatments (Table 4). This finding was similar to that reported by Furutani and Nagao (1986), who discovered that GA₃ treatment inhibited the flowering and sprouting of ginger. However, the application of forced sprouting treatment with 50-ppm GA₃ significantly increased the rhizome yield for semi-matured ginger (Table 5). Pariari et al. (2018) discovered that, for ginger cultivation, the application of 150-ppm GA₃ by foliar spraying at 90 and 120 days after planting significantly increased rhizome yields (rhizome growth was assessed using plant height as a crucial parameter). In the present experiment, the plant height achieved by the application of GA₃ treatment was 147 cm, which was significantly taller than that achieved by the other treatments; the rhizome produced by GA₃ treatment had a fresh weight of 1,567 g, heavier than those produced by the other treatments. The rhizome with the second highest height and second heaviest fresh weight was produced by the 450-ppm ethephon treatment. This indicated that the correlation between ginger plant height and rhizome growth in the present study is consistent with the findings of Pariari et al. (2018). Similar results were obtained in the study of Sengupta et al. (2008), in which spraying 150-ppm GA₃ significantly increased rhizome yield and plant height, and the rhizome yield was also positively correlated with the plant height. Exogenous GA treatment also increased tuber yield of potatoes (Javanmardi and Rasuli, 2017; Sarkar, 2008). Application of GA may stimulate the plant to form more stolons (more possible positions for tuber formation) and increase foliage, supplying the necessary assimilates to support stolons and further tuber growth (Javanmardi and Rasuli, 2017). In the present experiment, the exogenous application of GA treatment significantly increased the height of ginger plants, and may have also provided more positions for rhizome formation, thereby increasing the fresh weight of rhizomes per plant. Alternatively, the plant height increased so the leaves could capture more light, allowing the plant to accumulate additional photosynthetic compounds. However, the relationship between exogenous GA treatment and the growth and development of ginger rhizomes in this experiment needs to be further verified.

Ginger is a non-climacteric crop. In the present experiment, gingers were paced in PE bags after being subjected to forced sprouting. Ethylene was barely detected in the treatment groups in which ethephon was not used (Fig. 3); the level of oxygen decreased significantly after the third day of packing to 10.6 ppm, whereas the level of carbon dioxide increased significantly to 4.8 ppm (Fig. 2), forming an environment with low oxygen and high carbon dioxide content. A previous study demonstrated that a single day of 3% carbon dioxide treatment was sufficient to promote ginger sprouting, which could be further enhanced by increasing the CO₂ concentration and the length of the treatment (Yeh, 1999). In the present experiment, the ginger subjected to forced sprouting was packaged in PE bags to simulate the heat preservation effect achieved by using a plastic cloth to cover a cultivation field. The airtight environment within the PE bags was used to create an environment with a high treatment concentration of carbon dioxide that forced the ginger to sprout, thereby improving on the inconsistent results obtained by conventional ginger sprouting.

Conclusion

In the present experiment, 1% citric acid or 450-ppm ethephon were used for forced sprouting, and the ginger was packaged in PE bags to effectively promote the formation of large buds (bud diameter ≥ 1 cm) on the rhizomes. Emergence was observed at one month after the ginger seed rhizomes were planted in the field. Although the effect of 1% citric acid and 450-ppm ethephon treatment on young ginger cultivation was not investigated in this experiment, it was found that the fresh weight of semi-matured rhizomes obtained by forced sprouting treatment was lower, indicating that citric acid and ethephon treatments need to be improved, but the results still can provide a reference for early harvest young ginger in Taiwan. GA₃ forced sprouting treatment was not very effective, but it had an excellent effect on improving the yield of semi-matured ginger rhizomes. In the future, this treatment can be applied to the production of semi-matured or matured ginger.

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