Effect of Media Composition and Cultivar on Yield and Spear Quality of White Asparagus under Forcing Culture Using Polyethylene Bags

Abstract

In Japan, white asparagus is produced by blanching with soil mounds or film-cover shading methods. However, with either method, white asparagus is produced only in the spring and not during the winter months, when labor and facilities can be more effectively utilized. Recently, a forcing-culture method for green asparagus using polyethylene bags was developed to reduce the workload required in green asparagus production during winter. In this study, we selected a medium composition that reduces bag weight while maximizing yield under a white asparagus forcing culture, as well as cultivars with superior yield and quality. In the first experiment, asparagus was cultivated under forcing culture in polyethylene bags using three different mixed-composition media: lightweight media and horticultural media (1:1, v/v), lightweight media and bark compost (1:1, v/v), and horticultural media and bark compost (1:1, v/v), comprising lightweight media with a bulk density of 0.45 g·mL⁻¹, horticultural media with a bulk density of 0.8–0.9 g·mL⁻¹, and bark compost of 0.4 g·mL⁻¹. The lightest bag (weighing 8.0 kg) contained a combination of lightweight medium and bark compost. No differences were observed between the three combinations in terms of asparagus growth, yield, or spear weight. In the second experiment, the cultivars ‘Taihowase’, No. 056, ‘Suguderu II’, and ‘Welcome’ were grown in a combination (1:1, v/v) of lightweight medium and bark compost. The spear weight of No. 056 was greater than that of ‘Welcome’. No differences in sugar content were observed among the four cultivars. The protodioscin content of ‘Suguderu II’ was higher than that of ‘Taihowase’, No. 056, and ‘Welcome’. These results indicated that forcing culture using bags is practical and that a combination of lightweight medium and bark compost is suitable for this culture method. The best asparagus cultivars among those tested were No. 056, which produced heavy young stems, and ‘Suguderu II’, which developed spears with a high protodioscin content.

Introduction

Asparagus (Asparagus offcinalis L.) is a popular health-promoting vegetable (Chin and Garrison, 2008; Maeda et al., 2005, 2010; Shao et al., 1996). Three types of asparagus are produced in Japan: green, white, and purple and green asparagus is the most common. However, the demand for white asparagus has recently increased, and as a result, its production in Hokkaido, Japan’s largest asparagus-producing region, is also increasing (Jishi and Miyamachi, 2019). The increasing demand for white asparagus is partly due to its distinctive bitter taste and partly because it contains more of the functional substance, protodioscin, compared with green asparagus (Maeda et al., 2012a, b).

In Japan, white asparagus is produced by blanching with soil mounds or film-cover shading methods. Maeda et al. (2012a) found that spears produced using the soil-mound method contained higher levels of the bitter component, protodioscin, than spears produced using the film-covered shading method and speculated that this was due to the influence of soil-borne stress. Protodioscin is a saponin that exerts several bioactive effects (Chin and Garrison, 2008). Bruckner et al. (2010) reported that sweetness and bitterness are important factors in the quality of white asparagus and that there are varietal differences in the bitter component, saponin. The soil mound method requires machinery to move the soil and harvesting is troublesome because young spears must be harvested before they sprout from the soil surface to prevent coloration. However, the film-cover shading method does not require the movement of soil and reduces harvest labor compared to the soil-mound method. However, with either method, white asparagus is produced only in the spring and not during the winter months, when labor and facilities can be effectively utilized. Jishi et al. (2008) proposed the use of shading film in rootstock planting, and using forcing cultures to produce white asparagus during winter. However, forcing culture has not expanded because of the workload required to dig up rootstocks from open fields and to replant them in plastic greenhouses (Jishi et al., 2015).

Recently, a forcing-culture method for green asparagus using polyethylene bags was developed to reduce the workload of green asparagus production during winter (Kohmura, 2021; Kohmura and Komoda, 2018; Kohmura et al., 2014, 2015, 2016). In these reports, various tests were conducted for green asparagus by adding chemical fertilizers to media easily available in Hiroshima Prefecture; however, there have been no reports on media composition and variety tests for white asparagus. In this study, we examined the effects of media composition and cultivar on the yield and spear quality of white asparagus under forced culture conditions using polyethylene bags.

Materials and Methods

Expermient 1 Effect of media composition on growth and yield of white asparagus in forcing culture using bags

The asparagus cultivar ‘Taihowase’ (Sanatech Seed Co., Ltd., Tokyo, Japan) was sown on April 11th, 2021 and potted up in 12 cm plastic pots on May 30th in a glasshouse at the Rakuno Gakuen field education and research center, controlled at a room temperature of 23 ± 5°C. The potted seedlings were overwintered outdoors on November 5th. After the seedlings were moved outdoors, the average outdoor temperatures ranged from 13.8°C–1.0°C. The seedlings overwintered in snow until April 1, 2022, due to snowfall on December 17, 2021. The potted seedlings were planted on April 1st in polyethylene bags (65 cm high × 45 cm wide, 0.1 cm wide) with seven drainage slits at the bottom. They were filled with 20 L of three types of media of three different mixed-composition media: lightweight media and horticultural media (1:1, v/v), lightweight media and bark compost (1:1, v/v), and horticultural media and bark compost (1:1, v/v), comprising lightweight media with a bulk density of 0.45 g·mL⁻¹, horticultural media with a bulk density of 0.8–0.9 g·mL⁻¹, and bark compost of 0.4 g·mL⁻¹. The horticultural medium used was Kumiai Horticultural medium containing 374 mg·kg⁻¹ nitrogen, 1,485 mg·kg⁻¹ phosphoric acid, and 242 mg·kg⁻¹ potassium, manufactured by Hokusan Ltd. (Hokkaido, Japan). Lightweight media was lightweight potting ace containing 440 mg·kg⁻¹ nitrogen, 1,300 mg·kg⁻¹ phosphoric acid, and 780 mg·kg⁻¹ potassium, manufactured by Kataoka Co-op Agri Co. (Tokyo, Japan). The bark compost used was Tokachi Bark No. 1, which is a fermented mixture of bark and cattle manure compost manufactured by Mori Sangyo Co., Ltd. (Hokkaido, Japan). The asparagus was grown in a glasshouse controlled at a room temperature of 23 ± 5°C until September 30th and outdoors from October 1st to November 25th. Irrigation was applied once every three days to keep the soil moist. On October 1st, when the asparagus was moved outdoors, the outdoor temperature was 21.7°C. Then, on November 25th, the outdoor temperature gradually decreased to 2.0°C. The asparagus were then grown in tunnels shaded with white silver (Tokankosan Co., Ltd., Tokyo, Japan) 0.08 mm thick and 100% shading, installed in a glasshouse controlled at room temperature of 15 ± 3°C from November 26th. The bag weight was investigated before irrigation, and plant height, number of stems 5 mm or more in diameter and maximum stem diameter on October 15th, and yield from December 5th to February 15, 2023 were measured. The yield was determined by examining the marketable yield and number of harvested stems, and the spear weight was calculated by dividing the market yield by the number of harvested stems. This experiment was conducted with three replicates of 10 plants per plot.

Expermient 2 Effect of cultivars on growth, yield, and spear quality of white asparagus in forcing culture using bags

Effect of cultivars on growth and yield of white asparagus in forcing culture using bags

Asparagus cultivars ‘Taihowase’, No. 056 (Rakuno Gakuen University breeding line), ‘Suguderu II’ (Sanatech Seed Co., Ltd.) and ‘Welcome’ (Sakata Seed Co., Ltd., Kanagawa, Japan) were grown in a glasshouse controlled at room temperature of 23 ± 5°C, and seeds were sown on October 4, 2021, potted up in 12 cm plastic pots on November 20th and grown until May 16, 2022. Seedlings were planted on May 17th in polyethylene bags containing 20 L of a 1:1 mixture of the lightweight medium and bark compost. Cultural practice was carried out as per Experiment 1. This examination measured plant height, number of stems 5 mm or more in diameter, and maximum stem diameter on October 15th and yield from December 5th to February 15th, 2023. The yield was determined by examining the marketable yield and number of harvested stems, and the spear weight was calculated by dividing the market yield by the number of harvested stems. The experiment was conducted in triplicate with eight plants per plot.

Effect of cultivar on spear quality of white asparagus in forcing culture using bags

In this study, 5 cm lower-bottom cuts from 25 cm long spears, weighing 10–15 g, harvested between January 25th and 30th, 2023, were frozen, lyophilized, milled into a fine powder, and used for analysis. Sugars (fructose, glucose, and sucrose) were extracted from 50 mg of freeze-dried powder as follows: The freeze-dried powder in a 2.0-mL tube was preheated in boiling water for 20 min to inactivate any enzymes involved in sugar metabolism. Then, 1.0 mL of distilled water was added to the tube and the solution was heated in boiling water for 60 min. After centrifugation at 4°C for 10 min, the supernatant was filtered and applied to high performance anion-exchange chromatography with a pulsed amperometric detection (HPAEC-PAD) system using a DIONEX ICS-5000+ with a DIONEX CarboPac PA1 column (4 × 250 mm; Thermo Scientific, Waltham, MA, USA). The components were separated as described previously (Shiomi, 1993). Each peak was confirmed through separate analyses using standard solutions of glucose, fructose, and sucrose. The protodioscin content in the sample solution was determined using HPLC equipped with ELSD. HPLC analysis was conducted using a Shimadzu LC10A system (Shimadzu Co., Kyoto, Japan) equipped with a Model 300S ELSD (M&S Instruments Inc., Osaka, Japan) and a Waters X-Bridge C18 (4.6 × 250 mm) column (Nihon Waters K. K., Tokyo, Japan), according to the modified method described by Wang et al. (2003) and Maeda et al. (2012a). The mobile phase consisted of 0.1% trifluoroacetic acid (A) and acetonitrile (B). Analysis was performed by running each sample for 35 min at a column temperature of 40°C and using a linear gradient system at a flow rate of 1.0 mL·min⁻¹. The gradients were as follows: 0–5 min, 84% solvent A, and 16% solvent B; 25 min, 60% solvent A, and 40% solvent B; and 35 min, 40% solvent A, and 60% solvent B. The positioning time was 10 min. The electrical signal from the ELSD was fed into a personal computer and monitored and analyzed using an HPLC data integrator system (Chromato-PRO; Run Time Instruments Co., Kanagawa, Japan). The chromatogram peak of protodioscin was confirmed by injecting a commercial protodioscin standard solution (ChromaDex Inc., Los Angeles, CA, USA). The protodioscin content of the sample solution was calculated from the peak areas with reference to a calibration curve prepared using 0, 25, 50, and 125 ppm external standard solutions.

Statistical analysis

The results were analyzed by two-way ANOVA and a treatment mean comparison using Tukey’s honest significance test (P < 0.05) with Bell Curve for Excel software (Esumi Co., Tokyo, Japan).

Results and Discussion

Expermient 1 Effect of media composition on growth and yield of white asparagus in forcing culture using bags

The effect of media composition on bag weight in forcing culture is shown in Figure 1. The lightweight media and horticultural media mixture weighed 15.8 kg, lightweight media and bark compost mixture weighed 8.0 kg, horticultural media and bark compost mixture weighed 13.1 kg, and lightweight media and bark compost mixture was the lightest. Thus, the weight of the bag was influenced by the specific gravity of the mixed medium. The effects of media composition on the growth of asparagus in forced culture in bags are shown in Table 1. The plant height ranged from 195.2 to 203.8 cm, number of stems from 9.1 to 10.2, and stem diameter from 8.0 to 9.1 mm. No effects of the three media compositions on growth were observed. The effects of media on the yield of white asparagus in forced culture using bags are shown in Table 2. Harvested spears ranged from 7.6 to 9.4 spears/bag, yields ranged from 133.7 to 154.0 g/bag, and spears weights from 16.7 to 18.0 g, with no effect of the three media compositions on yield. Growth and yield were similar regardless of the composition of the medium, suggesting that a mixture of lightweight medium and bark compost, which is the lightest and easiest to work with, is suitable for forcing cultures in bags. Kohmura et al. (Kohmura, 2021; Kohmura and Komoda, 2018; Kohmura et al., 2014, 2015, 2016) used 90 g of chemical fertilizer and 180 g of bitter lime in a media mixed with cattle manure and crushed rice husks in a 1:1 ratio. The media used in this study already contained a fertilizer component, and there was no need for additional fertilizer; therefore, the media composition was considered to reduce workload compared with previous reports (Kohmura, 2021; Kohmura and Komoda, 2018; Kohmura et al., 2014, 2015, 2016). Forcing culture requires a great deal of labor to digest rootstocks from open fields and replant rootstocks in plastic greenhouses. However, forcing cultivation using bags does not involve the use of machinery and allows the use of seedling greenhouses for rice and other crops when they are not in use. In addition, by using lightweight media and bark compost as the medium for forcing culture, there was no rootstock digging, as has been the case with previous cultivation techniques, and the bags were lightweight, making them easy to move around. These findings suggest that forced culture using bags can be a labor-saving cultivation method with a lower workload.

Fig. 1

Effect of media composition on bag weight before irrigation. Error bar indicates standard error (n = 3). ^z Different letters indicate significant differences at P < 0.05 by Tukey test. ^y Lightweight media: Lightw, horticultural media: Hortic, bark compost: Bark.

Table 1

Effect of media composition on growth of asparagus in forcing culture using bags.

Table 2

Effect of media on yield of white asparagus in forcing culture using bags.

Experiment 2: Effect of cultivars on growth, yield and spear quality of white asparagus in forcing culture using bags

Effect of cultivars on growth and yield of white asparagus in forcing culture using bags

The effects of cultivar composition on the growth of asparagus in forced culture in bags are shown in Table 3. The plant height ranged from 144.1 to 170.6 cm, number of stems from 11.1 to 12.9, and stem diameter from 6.4 to 7.5 mm. No effect of cultivar on growth was observed. The spears of the four asparagus cultivars harvested in forcing culture using bags are shown in Figure 2. No differences in head tightness or spear coloration were observed among the four cultivars. The effects of the cultivar on the yield of white asparagus in forcing culture using bags are shown in Table 4. Harvested numbers of spears ranged from 7.2 to 10.5 spears/bag, yields ranged from 110.0 to 172.3 g/bag and there was no effect of cultivar on yield. The spear weights ranged from 14.3 to 18.1 g, with No. 056 being the heaviest and ‘Welcome’ the lightest. Kohmura et al. (2014) reported that forcing culture of ‘Welcome’ asparagus using rootstock grown in a bag had more spears number, but lighter spears and lower yields, than ‘Winderu’. In this study, ‘Welcome’ was significantly lighter than No. 056 and tended to have a lower yield, which was consistent with a previous report (Kohmura et al., 2014). As there is currently a high demand for thick asparagus in Japan, No. 056 would be a more appropriate cultivar than ‘Welcome’ to meet this demand.

Table 3

Effect of cultivar on growth of asparagus in forcing culture using bags.

Fig. 2

Spears of four asparagus cultivars harvested in forcing culture using bags.

Table 4

Effect of cultivar on yield of white asparagus in forcing culture using bags.

Effect of cultivars on spear quality of white asparagus in forcing culture using bags

The effects of the cultivar on the sugar content of white asparagus in forced culture using bags are shown in Table 5. In the forcing culture using bags, glucose ranged from 65.0 to 82.5 mg·g⁻¹ DW, fructose from 186.2 to 262.1 mg·g⁻¹ DW, and sucrose from 15.5 to 22.4 mg·g⁻¹ DW; with no difference in sugar content between the four cultivars. Maeda et al. (2012a) reported that there were no significant differences in the sugar content of spears grown using the soil-mound and film-cover methods. These findings suggest that sugar content may not vary significantly in white asparagus, not only between the two cultivation methods but also in different cultivars. The effect of cultivars on the protodioscin content of white asparagus in forced culture in bags is shown in Figure 3. The protodioscin content was 1.60 for ‘Taihowase’, 1.72 for No. 056, 7.54 for ‘Suguderu II’, and 1.64 for ‘Welcome’, with the protodioscin content of ‘Suguderu II’ being significantly higher than the other three cultivars. This result is consistent with a report by Bruckner et al. (2010), showing that the saponin content of white asparagus varies among cultivars. Maeda et al. (2012b) reported that spray treatment of white asparagus in a film-cover culture with 500 ppm methyl jasmonate increased the protodioscin content to 7.0 mg·g⁻¹ DW. In this study, the protodioscin content of ‘Suguderu II’ was 7.54 mg·g⁻¹ DW. These results suggest that asparagus with high protodioscin content can be produced by selecting a cultivar that can be grown without any chemical treatment.

Table 5

Effect of cultivars on sugar content of white asparagus in forcing culture using bags.

Fig. 3

Effect of cultivars on protodioscin content of white asparagus in forcing culture using bags. Error bar indicates standard error (n = 3). ^z Different letters indicate significant differences at P < 0.05 by Tukey test.

Conclusion

In this study, the effect of media composition on growth and yield of white asparagus was investigated and no significant variation was observed using three different media compositions: lightweight media and horticultural media (1:1, v/v), lightweight media and bark compost (1:1, v/v), and horticultural media and bark compost (1:1, v/v). This indicates that the lighter mixture of lightweight media and bark compost could be used for growing white asparagus in polyethylene bags with no compromise in terms of growth and yield in forcing culture using the film-covered shading method. This could make the process easier and lower production costs compared to the soil-mound method as it can make use of idle greenhouses during winter, does not require large machinery and consumes fewer resources than the conventional cultivation method. This forced culture using bags is considered easier for the elderly and women because the bags are lightweight and easy to move. The second experiment in this study with four cultivars found that No. 056 had the heaviest spears, thus being more suitable to meet market demand, while ‘Suguderu II’ had the highest protodioscin content (a functional component) compared to ‘Taihowase’ and ‘Welcome’ suggesting that No. 056 and ‘Suguderu II’ would be well suited to meet white asparagus demand in Japan. In future, the creation of new demand for white asparagus produced during the winter months will be considered.

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