Abstract
Mungbeans (Vigna radiata L.) are one of major legume crops in Asia as major sources of proteins and carbohydrates. Additionally, they provide functional substances such as vitamin and antioxidant compounds when consumed as bean sprouts. In this study, physical traits of mungbean sprouts, such as sprouts yield, thickness and length of hypocotyl, length of epicotyl, number of lateral roots, and length of root depending on four irrigation conditions (irrigation interval: 2 and 4 hours; irrigation time: 2 and 4 minutes) were measured for three cultivars (Dahyeon, Samgang, Sunhwa). In general, the length and thickness of hypocotyl increased gradually until day 3, followed by a decrease on day 4. Lateral roots and the length of root increased during the cultivation period. Sprouts yield was higher under short irrigation time than long irrigation time until day 3. Preferable traits including sprouts yield, the length of hypocotyl, and the thickness of hypocotyl were better under shorter irrigation time conditions (2 minutes). Traits of negative effect for consumers’ preference, such as number of lateral roots and length of epicotyl, were lower under longer irrigation interval conditions (4 hours). Samgang had the highest yield of sprouts. However, it also had the highest growth of epicotyl. Although Sunhwa had preferable physical traits of bean sprouts, it had the least yield of sprouts. Overall, Dahyeon cultivated under 4 hours (irrigation interval) & 2 minutes (irrigation time) condition had the most preferable traits for producing mungbean sprouts. This study provides information about irrigation conditions affecting quality-related traits to produce mungbean sprouts.
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Key words: Mungbean sprouts, Yield, Hypocotyl, Epicotyl, Root, Irrigation conditions
INTRODUCTION
Mungbeans (
Vigna radiata L.) are one of major legume crops in Asia. Mungbeans are high in protein (20-24%) and low in calories (
Tang et al. 2014). They contain more carbohydrates (50-60%) and phenolic compounds than other legumes (
Peng et al. 2008). Ethanol extracts of mungbean contain large amounts of vitexin and isovitexin known to possess antioxidant and anti-obesity effects (
Meenu et al. 2016;
An et al. 2020). Contents of these functional substances are affected by genetic factors and environmental conditions (
An et al. 2020).
In many Asian countries, mungbeans are mainly consumed as grains and sprouts. Mungbean sprouts grow fast indoors without being affected by weather. They have higher total phenol and flavonoid contents than seeds (
Kim et al. 2012). In addition, mungbean sprout extracts have higher inhibitory effects on nitric oxide production related to anti-inflammatory action than seed extracts (
Kim and Jeong, 2014). Likewise, the antioxidant activity of munbean sprouts is also higher than that of its seeds (
Jin et al. 2010).
For soybean sprouts, texture is one of the most important factors affecting choice of consumers. It is affected by the thickness of the hypocotyl, the number of lateral roots, and the ratio of hypocotyl to root (
Jeon et al. 2006). Generally, two irrigation methods are used to grow soybean sprout, water spraying and hydroponics (
Kang et al. 2004). Irrigation conditions such as irrigation method, water temperature, irrigation interval, and irrigation time are known to affect the growth of soybean sprout (
Kim et al. 2000a,
2000b). These irrigation conditions would also be important for the growth of mungbean sprouts. Although effects of irrigation methods (water spraying or hydroponics) on the growth of mungbean sprouts have been studied (
Kang et al. 2004), effects of irrigation conditions on the growth of mungbean sprouts remain unknown.
Thus, the objective of this study was to investigate effects of irrigation conditions (irrigation interval and irrigation time) on the growth of hypocotyl and roots and the formation of lateral roots of mungbean. We choose the three cultivars (Sunhwa, Samgang, and Dahyeon). Sunhwa is standard cultivar of mungbean and has high yield, Samgang is a typical cultivar used for mungbean sprouts and higher sprouts yield than Sunhwa (
Lee et al. 2004), and Dahyeon was used as sprouts in many component analyses studies (
Kim et al. 2009). Dahyeon has higher total phenolic and flavonoid compounds than other cultivar of mungbean sprouts (
Jin et al. 2010). Results of this study will provide important information about environmental factors affecting quality-related traits of mungbean sprouts and cultivation conditions suitable for producing mungbean sprouts.
MATERIALS AND METHODS
Cultivation of mungbean
In this study, three cultivars (Dahyeon, Samgang, and Sunhwa (VC1973A)) of mungbeans were used to determine irrigation conditions for mungbean sprouts production. The cultivation of mungbean sprouts was performed on the water spraying method using a sprout cultivator (Sundotcom, ST001A) designed to control temperature in the cultivator, water temperature, irrigation interval, and irrigation duration (
Supplementary Fig. S1). The cultivation was performed according to previous reports (Kim
et al. 2000;
Gan et al. 2016) with slightly modifications. Briefly, mungbean seeds were rinsed three times with distilled water. Then 50 seed were soaked in 25 mL of distilled water in a dark condition at 37℃ for 17 hours using an incubator (JEIO TECH. ISS-4075R). Germinated seeds were then planted on water culture sponge (Ju-entirex) and moved into the sprout cultivator. Cultivation of sprouts was carried out for four days with four different conditions: irrigation interval of 2 or 4 hours and irrigation time of 2 or 4 minutes at 30 ± 2℃.
The 100-seed weight and fresh weight of mungbean sprouts were measured daily for four days. Physical traits such as thickness and length of hypocotyl and root length were measured using ImageJ (
Schneider et al. 2012). Set the length of standard for 2 cm on the ruler of pictures as straight line. And then, measured the length and thickness of mungbean sprouts as freehand line. The thickness of hypocotyl was measured the thickening part between the hypocotyl and root. The length of root was measured to finish of root from thickening part. The length of hypocotyl was from thickening part to cotyledon, and the length of epicotyl was from cotyledon to finish of epicotyl. Lateral roots over 1 mm were counted. The yield of sprouts was measured as fresh weight/seed weight × 100(%).
Of 50 sprouts, 20 were randomly selected and used to measure physical traits (thickness, hypocotyl length, epicotyl length, number of lateral roots, and root length) for each cultivar. Two-way analysis of variance (ANOVA) was performed at a significance level of 0.05 (
Stahle and Wold 1989). Duncan’s Multiple Range test was conducted using RStudio (
RStudio Team 2021).
RESULTS
Dynamic changes in physical traits of mungbean sprouts according to irrigation conditions
In this study, mungbean sprouts were cultivated using four different irrigation conditions (2 hours/2 minutes, 2 hours/4 minutes, 4 hours/2 minutes, 4 hours/4 minutes). with two different irrigation intervals (2 and 4 hours) and two different irrigation durations (2 and 4 minutes). Sprout yield, length and thickness of hypocotyl, the number of lateral roots, length of roots, and length of epicotyl were measured for four days. Yields of sprouts were gradually increased under all conditions during the cultivation period (
Table 1).
Irrigation intervals of 2 and 4 hours showed similar yields of mungbean sprouts, although the 2 hours irrigation interval condition had higher yield than the 4 hours irrigation interval condition on day 4 (
Fig. 1).
Under two different irrigation durations (2 and 4 minutes), yield increased gradually. The yield of irrigation duration for 2 minutes was higher than that of irrigation duration for 4 minutes on day 2 and day 3 (
Fig. 1). Irrigation duration affected the growth of mungbean sprouts more than irrigation interval. The 2 minutes irrigation duration condition promoted the development more than the 4 minutes irrigation duration condition regardless of irrigation interval. Among cultivars, Samgang showed the most yield of sprouts while Sunhwa had the least yield of sprouts during the cultivation period (
Table 1).
The length of hypocotyl increased gradually under all irrigation conditions during cultivation periods (
Fig. 2a). Epicotyl appeared on day 3. Its length increased rapidly on day 4 (
Fig. 2b). Thickness of hypocotyl also increased for three days. It then decreased on day 4 (
Fig. 2c). Under all irrigation conditions for all cultivars, the length and thickness of hypocotyl increased for three days. They then decreased on day 4. Since day 3, the emergence of epicotyl and the vigorous growth slowed the development of hypocotyl.
Major traits among cultivars and irrigation conditions were compared for day 3 when hypocotyl developed the most and before epicotyl emerged. For cultivar Sunhwa, the length of hypocotyl had the highest value at all conditions (
Fig. 3b). Dahyeon showed the highest length of hypocotyl of 13.5 ± 0.7 cm under 2 hours/2 minutes condition. Samgang and Sunhwa had the highest length of hypocotyl of 12.7 ± 0.4 and 15.1 ± 0.6 cm, respectively, under 4 hours/2 minutes condition
(
Fig. 4b). For all cultivars, the length of hypocotyl was increased under 2 m condition than that under 4 minutes condition (
Fig. 3b,
4b). These results indicate that irrigation time affects the growth of hypocotyl more than irrigation interval.
For the thickness of hypocotyl, Samgang had the thickest hypocotyl of 0.3 ± 0.01 cm under irrigation interval of 2 hours. Sunhwa had the thickest hypocotyl of 0.28 ± 0.02 cm under 4 hours/2 minutes condition and 0.26 ± 0.01 cm under 4 hours/4 minutes condition (
Fig. 3a). In general, irrigation condition of 2 hours/4 minutes appeared to lead to the highest value of thickness for all cultivars. The 2 hours irrigation interval condition resulted in higher thickness than the 4 hours irrigation interval condition, indicating hypocotyl grows thicker under frequent irrigation conditions (
Fig. 4a).
For the length of epicotyl, the irrigation duration of 2 minutes resulted in longer epicotyl than the 4 minutes condition, similar to results of hypocotyl (
Fig. 4c). Epicotyl developed more with 2 hours of irrigation interval than with 4 hours of water interval. The 2 hours/2 minutes condition showed the highest value of epicotyl for all cultivars. Samgang had the longest epicotyl of 2.4 ± 0.19 cm. Under 4 hours/4 minutes condition, epicotyl developed the least in all cultivars. Dahyeon had the lowest value of epicotyl at 1.4 ± 0.17 cm (
Fig. 3c,
4c). Overall, Samgang had the highest value of the length of epicotyl under all conditions (
Fig. 3>c).
The length of root and the number of lateral roots kept increasing during the cultivation period (
Supplementary Fig. S2). Samgang had the longest root length except under 4 hours/2 minutes condition. Under 4 hours/4 minutes and 2 hours/2 minutes conditions, Sunhwa and Samgang had longer roots than Dahyeon. Under 2 hours/4 minutes condition, the length of root had no significant differences among three cultivars (Dahyeon, 9.1 ± 1.2 cm; Samgang, 9.3 ± 1.3 cm; Sunhwa, 9 ± 1.6 cm) (
Fig. 3e). Sunhwa and Samgang developed longer roots under 4 hours/4 minutes and 2 hours/2 minutes conditions than under 4 hours/2 minutes and 2 hours/4 minutes conditions. However, root length of Dahyeon was not affected by water conditions (
Fig. 4e). For the number of lateral roots, Dahyeon had smaller value at all conditions except for that under 2 hours/2 minutes condition. Samgang had the most number of lateral roots in general except that under 2 hours/4 minutes condition. Sunhwa had the most number of lateral roots at 8.5 ± 1.1 under 2 hours/4 minutes condition (
Fig. 3d,
4d).
DISCUSSION
Mungbeans are important resources of proteins and carbohydrates in many developing countries in Asia (
Tang et al. 2014). Mungbeans contain higher concentrations of various phenolic compounds such as vitexin and isovitexin than other legume crops (
Peng et al. 2008;
Meenu et al. 2016;
An et al. 2020). These compounds are known to possess antioxidant and anti-obesity activities.
Gan et al. (2016) have reported that amounts of phenolic contents are increased as mungbean sprouts grow. The antioxidant capacity of mungbean sprouts is elevated in accordance with increasing polyphenolic contents (
Lee et al. 2007). In addition to these bioactive compounds, mungbean sprouts are also abundant in essential amino acids, dietary fiber, and vitamins that can provide benefits to human health. Therefore, consumption of mungbean sprouts has increased globally. Their market has consistently grown according to dietary trends worldwide (
www.Industryresearch.biz, accessed on 23 August 2021).
In soybean sprouts, phenolic and isoflavone compounds under various environmental conditions such as temperature, relative humidity, and irrigation methods have been well investigated (
Gan et al. 2016;
Syukri et al. 2018;
Silva et al. 2020). Among these factors, irrigation condition is one of key factors that can regulate the yield and development of sprouts. The length and thickness of hypocotyl are important physical traits affecting consumers’
preference for both mungbean and soybean sprouts. The ratio of hypocotyl to total length of bean sprouts is increased when the irrigation time is increased or when the irrigation interval is less frequent. However, oxidation activities of sprout extract are enhanced when irrigation time is increased. The oxidation activity has shown a quadratic regression with whole sprout length and hypocotyl length (
Kim et al. 2000b). As reported by
An et al. (2020), bioactive compounds of mungbean sprouts are significantly affected by environmental variations, including cultivation regions and seeding dates (
An et al. 2020). Despite such benefits of mungbean sprouts, effects of irrigation conditions on their development and accumulation of phenolic contents have not been reported yet. Because physical traits of mungbean sprouts are highly associated with the accumulation of bioactive compounds known to be a very important factor affecting customers’ preference, we investigated changes in physical features of mungbean sprouts under different irrigation conditions using three different cultivars during the post-germinations stage.
The yield of mungbean sprouts was not significantly different between two irrigation intervals (2 hours and 4 hours). However, the yield of sprouts was affected by irrigation time or duration, with the 2 minutes condition showing higher yield than the 4 minutes condition (
Table 1).
Kim et al. (2000b) have found that the yield of soybean sprouts is increased as the irrigation interval is shortened. However, mungbean sprout shows a high yield with a short irrigation time regardless of the irrigation interval (
Kim et al. 2000b). For soybean sprouts, the length of hypocotyl is affected more by irrigation interval than by irrigation time. However, mungbean sprout was affected by irrigation time more than by irrigation interval (
Fig. 4b). These differences might affect yields of these two bean sprouts.
The quality of soybean sprout is affected by traits such as lengths of hypocotyl and root, thickness, and number of lateral roots (
Jeon et al. 2006). It is known that consumers prefer a thicker hypocotyl, fewer lateral roots, and lower ratio of root to hypocotyl. The epicotyl is not preferred in general. These traits of mungbean sprouts are considered to be important. Therefore, this study was carried out until cultivation day 3 when hypocotyl developed the most with the least epicotyl (
Fig. 2,
Supplementary Fig. S3).
Dahyeon and Samgang are known to have high yields of sprouts (
Lee et al. 2004,
Kim et al. 2009). In the present study, Samgang showed the highest yield of sprouts. It is due to its faster development of epicotyl and later roots than the other two cultivars (
Fig. 3c and 3d), which are negative features of mungbean sprouts. Dahyeon had less lateral roots and epicotyls. Although Sunhwa had high values of various traits as Dahyeon, it had significantly less yield of sprouts than Dahyeon (
Table 1,
Fig. 3), because Sunhwa (100-seed weight, 6.6 g) had larger and heavier cotyledon than Dahyeon (100-seed weight, 5.2 g). Overall, since consumers prefer small cotyledon of mungbean sprouts, Dahyeon is considered as the most suitable cultivar for producing mungbean sprouts among the three cultivars tested in this study (
Table 1,
Fig. 3c,
3d).
The development of trait including yield of sprouts, length of hypocotyl, and thickness of hypocotyl appeared suitable tendency for the production of mungbean sprouts under short irrigation time conditions (
Table 1,
Fig. 4a and
4b). In addition, traits with negative effects on consumers’ preference, such as number of lateral roots and length of epicotyl, showed lower development under long irrigation interval conditions (
Fig. 4c,
4d). Therefore, the 4 hours/2 minutes condition was the most suitable for cultivating mungbean sprouts in this study.
Mungbean sprouts are receiving attention in the food market globally due to their nutritional benefits and excellent texture. This study was conducted to determine effects of different irrigation conditions on growth of mungbean sprouts. According to irrigation conditions, significant differences in development were found among cultivars. Many distinctions were also detected even for the same cultivar. To date, breeding of mungbean has been concentrated on the increase of seed productivity and resistance to pests despite popular demand for high-quality mungbean sprouts. Our results offer appropriate irrigation conditions to improve the physical quality of mungbean sprouts without using any artificial growth hormone or chemicals. Moreover, growth and development of bean sprouts are associated with the accumulation of phenolic compounds that are responsible for their antioxidant activities. Therefore, the present study presents a promising strategy to improve polyphenols contents and growth of mungbean sprouts.
Supplemental Materials
ACKNOWLEDGEMENTS
This paper was supported by research funds for newly appointed professors of Gangneung-Wonju National University in 2020.
Fig. 1Measurements of yields of mungbean sprouts for four days according to watering conditions. Average values are shown for three cultivars according to watering conditions. Circle indicates watering interval of 4 hours. Triangle indicates watering interval of 2 hours.
Fig. 2Measurement of growths of mungbean sprouts for four days. (a) Length of hypocotyl, (b) length of epicotyl, (c) thickness of hypocotyl. Watering conditions, including watering interval and irrigation amount, are indicated by colors. Triangle indicates average value per day. One-way analysis of variance (ANOVA) and post-hoc Duncan test were carried out. Statistical significance was indicated by lowercase letters (P < 0.05). Each column represents three mungbean cultivars (Dahyeon, Samgang, and Sunhwa).
Fig. 3Measurement of physical traits of mungbean sprouts. (a) Thickness of hypocotyl, (b) length of hypocotyl, (c) length of epicotyl, (d) number of lateral roots, and (e) length of root. Watering intervals of 2 hours and 4 hours are indicated by 2 h and 4 h, respectively. Irrigation amount or duration of 2 minutes and 4 minutes are indicated by 2 m and 4 m, respectively. Statistical analysis was conducted by Duncan test (P < 0.05).
Fig. 4Measurement of physical traits of mungbean sprouts. (a) Thickness of hypocotyl, (b) length of hypocotyl, (c) length of epicotyl, (d) number of lateral roots, and (e) length of root. Watering intervals of 2 hours and 4 hours are indicated by 2 h and 4 h, respectively. Irrigation amount or duration of 2 minutes and 4 minutes are indicated by 2 m and 4 m, respectively. Statistical analysis was conducted by Duncan test (P < 0.05).
Table 1Yields of mungbean sprouts according to watering conditions and cultivation day (Unit: %).
Table 1
|
Watering condition (hours/minutes) |
Cultivar |
Cultivation day |
|
1 |
2 |
3 |
4 |
|
4/2 |
Dahyeon |
382.4 |
685 |
1,037.50 |
1,260.20 |
|
Samgang |
666.3 |
903.9 |
1,178.60 |
1,349.30 |
|
Sunhwa |
334.8 |
514.3 |
853.1 |
950 |
|
4/4 |
Dahyeon |
408.7 |
651.9 |
867.3 |
1,019.20 |
|
Samgang |
681.8 |
756.2 |
996.4 |
1,160.70 |
|
Sunhwa |
279.1 |
525 |
762.1 |
949.3 |
|
2/2 |
Dahyeon |
400.3 |
693.5 |
1,076.90 |
1,204.10 |
|
Samgang |
487.6 |
978.6 |
1,202.60 |
1,794.50 |
|
Sunhwa |
395.5 |
514.8 |
791.6 |
951.4 |
|
2/4 |
Dahyeon |
382.4 |
566.7 |
808.3 |
1,217.00 |
|
Samgang |
408.3 |
952.4 |
1,311.00 |
1,770.00 |
|
Sunhwa |
309 |
389.3 |
599.7 |
984.4 |
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