2021 年 27 巻 4 号 p. 587-597
We determined the foliage yield, polyphenol content, and polyphenol yield of weekly greenhouse-cultivated major varieties of sweet potato with the aim to utilize the foliage as a polyphenol source. The two-year study was conducted using five varieties from May to November 2012, Kokei No. 14, Koganesengan, Murasakimasari, Tamaakane, and Suioh, and three varieties from May to October 2014, Kokei No. 14, Koganesengan, and Beniharuka. The foliage yield of Koganesengan and Beniharuka in 2014 was significantly higher than other varieties. The foliage polyphenol content was significantly higher in Koganesengan in both years compared to others. The foliage polyphenol yield of Koganesengan and Beniharuka in 2014 was significantly higher than others. Koganesengan is a variety whose leaves represent a major portion of the total foliage; and the highest polyphenol content was in its leaves. Therefore, Koganesengan has the potential to be an efficient source of polyphenol. Our study provides basic data for the stable use of sweet potato foliage as a polyphenol-rich material.
The foliage of the sweet potato, Ipomoea batatas L., is popular as a leafy vegetable in several East Asian, Southeast Asian, and African countries owing to its high nutritive value (Ishida et al., 2000; Antia et al., 2006; Sun et al., 2014a). When sweet potato stems become hardened, petioles, which are softer than the stems, are used for human consumption. Suioh (SO), a variety used for its foliage, with large leaves, long petioles, and good taste is bred in Japan (Ishiguro et al., 2004). Its leaves are rich in functional components, such as lutein and polyphenols, compared with other commercially cultivated vegetables (Ishiguro and Yoshimoto, 2006). The major polyphenols in the foliage of sweet potato are caffeic acid (CA) and caffeoylquinic acids (CQAs), including chlorogenic acid (ChA), 3,4-di-O-caffeoylquinic acid (3,4-DCQA), 3,5-di-O-caffeoylquinic acid (3,5-DCQA), 4,5-di-O-caffeoylquinic acid (4,5-DCQA), and 3,4,5-tri-O-caffeoylquinic acid (3,4,5-TCQA). These polyphenols have many functions, including antioxidant capacity, lowering of blood glucose level in a type 2 diabetes mouse model (Nagamine et al., 2014), inhibition of oxidation of low-density lipoprotein (LDL) that causes arteriosclerosis (Nagai et al., 2011; Taira et al., 2013), and inhibition of osteoclast formation and suppression of bone destruction in adjuvant-induced arthritic rats (Tang et al., 2006). In recent years, since ChA and CQAs have been found to have an anti-obesity effect (Thom, E. 2007), beverages such as coffee supplemented with polyphenol extract, thereby increasing the polyphenol content, are being sold. In addition, in anticipation that lifestyle-related diseases can be prevented or improved by diet, the development of processed foods containing polyphenols, which are expected to be effective, is steadily progressing. CA and CQAs were detected in all 1389 sweet potato genotypes investigated by Islam et al. (2002). However, as polyphenols (including CA and CQAs) are vulnerable to long-term heat treatment, their retention in sweet potato leaves after heat processing has been investigated; and the conditions for maintaining a high polyphenol content have been clarified (Sugawara et al., 2011; Sasaki et al., 2015a; Sun et al., 2014b; Sun et al., 2017). Thus, sweet potato leaves are a valuable healthy food material. Shimada et al. (2010) developed an efficient method for the large-scale extraction of polyphenols from sweet potato leaves, and this method has been employed for the industrial extraction of polyphenols from sweet potato in Japan.
Recently, the demand for sweet potato leaves as a functional food material and healthy vegetable has increased. Sweet potato plants are highly regenerative and its mature shoots can be harvested repeatedly (Ishiguro et al., 2004; Sasaki et al., 2015b; Suárez et al., 2020). Kobayashi et al. (2019) conducted a long-term cultivation study with the sweet potato line Kyukei 05303-3 (05303-3) under greenhouse conditions and reported that the harvesting and regeneration cycle of sweet potato foliage planted in spring could be repeated in the autumn. Further, it is noteworthy that seasonal variations in yield and polyphenol content of sweet potato were strongly influenced by temperature. However, there are no studies on the annual supply of foliage for major Japanese sweet potato varieties. The sweet potato line (05303-3) used by Kobayashi et al. (2019) is characterized by a very high polyphenol content; thus, it was deemed necessary to investigate whether the same tendency might be observed in popular sweet potato varieties with moderate polyphenol content. Notably, the content of functional ingredients in fresh vegetables influences their labeling. Further, it is important to document the seasonal variation of polyphenol content in raw materials, as large harvest time-dependent fluctuations in the content of functional components can be a serious disadvantage. Furthermore, the components of sweet potato foliage as a whole are often analyzed (e.g., leaves, petioles, and stems); however, the petioles are more frequently used as a food than the stems. Therefore, it is important to study them separately. Sweet potato leaves are in demand as a functional food and it is important to accumulate basic data to elucidate these characteristics in commercial varieties. In the present study, we examined the following major sweet potato varieties: Kokei No. 14 (K14) and Beniharuka (BH) for table use, Koganesengan (KS) for shochu spirit production and starch processing, Murasakimasari (MM) with purple flesh and Tamaakane (TA) with orange flesh for shochu spirit production, and SO for foliage use. As sweet potato varieties are characterized by leaf size and shape, and petiole and stem length, determining the yield and polyphenol content of each variety will yield important basic data with respect to the polyphenol materials available. These varieties were greenhouse-cultivated over a long period, and the differences in the annual yield and polyphenol content among these varieties were investigated to categorize the leaves of these major sweet potato varieties as a potential polyphenol source.
Plant materials In 2012 and 2014, sweet potato plants were cultivated in a plastic greenhouse (40 m in length, 5 m in width, and 3.2 m in height, covered with a 0.1-mm thick plastic sheet) at Kyushu Okinawa Agricultural Research Center, NARO (Miyakonojo, Miyazaki, Japan). The sweet potato varieties used in 2012 were K14, KS, MM, TA, and SO, and those used in 2014 were K14, KS, and BH. The greenhouse was ventilated by forced air at 35 °C under thermostat control. Sweet potato foliage was cultivated as described previously (Kobayashi et al., 2019). Stem cuttings, with 2–3 nodes, were planted to achieve 66.7 cuttings·m−2 (15 cm × 10 cm) in 2012 (planted on April 2) and 44.4 cuttings·m−2 (15 cm × 15 cm) in 2014 (planted on April 6) in a 1.5-m-wide nursery bed in the greenhouse. In 05303-3, the above two planting densities did not affect the dry matter yield (Kobayashi et al., 2019). A high planting density can lead to difficulties in harvesting; therefore, in 2014, the planting density was returned to 44.4 cuttings·m−2. Stem cuttings were used instead of seed storage roots to avoid variation in planting density, which result from variation in the number of sprouts per seed storage root. After 1 month of transplantation, the foliage of plants with more than seven nodes was harvested once a week from 9 to 10 am. For uniform sampling, the sprouts with more than seven nodes were harvested, leaving about one node on the ground for the next shoot. The harvested plants were washed with tap water and the leaf blades, stems, and petioles were separated from each other, then the fresh weight of the samples was measured. The foliage was harvested from May 11 to November 30 (30 weeks) in 2012 and from May 8 to October 30 (26 weeks) in 2014. These tests were set up in 3 iterations. After measuring the fresh weight, the samples were immediately freeze-dried and weighed. Then, the dry weight (DW) samples were milled into a powder and stored at -30 °C until used in determination of polyphenol content. Beginning in mid-August, Thiodicarb wettable powder was diluted 1000-fold and sprayed as an insecticide to control leaf-eating caterpillars.
The data on air temperature and sunshine duration in Miyakonojo were obtained from the weather data recorded by the Miyakonojo Observation Station, a part of the Automated Meteorological Data Acquisition System of the Japan Meteorological Agency.
Chemicals CA was purchased from Wako Pure Chemical Industries (Osaka, Japan) and ChA was obtained from Sigma Chemicals (St. Louis, MO, USA). 3,4-diCQA, 3,5-diCQA, 4,5-diCQA, and 3,4,5-triCQA were extracted and purified (> 97%) from sweet potato leaves (Kurata et al., 2011).
Polyphenol extraction and quantitative analysis Fifty milligrams of freeze-dried powder of the sweet potato sample (leaves, petioles, and stems) were added to 5 mL of 80% ethanol (v/v) and boiled for 5 min. After centrifugation at 3 000 rpm for 15 min, the supernatant was collected, and the total polyphenol content was measured using a partially modified Folin–Ciocalteu method (Coseteng and Lee, 1987); the polyphenol content was calculated as ChA equivalents (equiv.) using ChA as the standard substance.
CA and CQAs were quantified by high-performance liquid chromatography (HPLC) following the method of Okuno et al. (2010). Briefly, the extract was filtered through a polytetrafluoroethylene membrane (0.20 µm; Advantec, Tokyo, Japan), and 5 µL of the filtrate was injected into the HPLC system and eluted as described below. The HPLC system consisted of a DUG-12A degasser, an SIL-10A XL auto-injector, a CTO-10A column oven, an SPD-M10A UV-VIS detector, an SCL-10A VP system controller, and two LC-10AD pumps (Shimadzu Co., Kyoto, Japan). The system was controlled by the LabSolutions (version 5.73) workstation (Shimadzu). The column used was YMC-Pack ODS-AM (75 mm × 4.6 mm i.d., 3 µm particle size; YMC Co., Ltd., Kyoto, Japan); the temperature of the column oven was set to 40 °C. CA and CQA derivatives were detected by measuring the absorbance of the sample at 326 nm. The mobile phase consisted 0.2% (v/v) formic acid (A) and acetonitrile (B) in water. The elution was performed at a flow rate of 1 mL·min−1, and a linear gradient of B as follows: 2–19% from 0 to 8 min; 19–52%, 8.01 to 16 min; 100% isocratic, 16.01 to 20 min; 100–2%, 20 to 20.01 min; and 2% isocratic, 20.01 to 26 min. The obtained polyphenols were then compared with the standards.
Statistical analysis The data were subjected to an analysis of variance. After determining significant differences, a multiple comparison test was performed using the Tukey–Kramer method to identify significant differences between groups. The level of significance between groups was set at 5%.
Varietal differences in the yield, polyphenol content, and polyphenol yield of sweet potato foliage Table 1 shows a comparison of the annual yield, average polyphenol content, and annual polyphenol yield of each part of the sweet potato foliage (leaves, petioles, and stems) among the selected varieties in 2012 and 2014. In 2012, SO presented the highest annual yield of leaves, whereas the yield of TA was significantly lower (p < 0.05). The annual yield of petioles was the highest in MM and the lowest in K14 and KS. KS showed a significantly (p < 0.05) lower annual stem yield than the other varieties, except MM. However, there were no significant differences in the foliage (Leaves + Petiole + Stem) annual yield. In a comparison of the polyphenol content of each part of the sweet potato foliage between varieties, the polyphenol content of leaves was highest in K14 and lowest in MM. In petioles, it was the highest in KS and lowest in SO; while in the stem, it was relatively high in KS and low in MM, TA, and SO. The polyphenol content of the foliage was divided into two groups, with K14 and KS being high and MM, TA, and SO being significantly lower (p < 0.05). The annual polyphenol yield (calculated from the yield and polyphenol content) was high in KS and low in MM and TA in the leaves. K14 showed significantly higher levels in the stem than other varieties. However, there were no significant differences between the varieties for the petioles and foliage.
| 2012 | Annual yield (g DW · m−2 · y−1) | Average polyphenol content (g · 100g−1 DW) | Annual polyphenol Yield (g · m−2 · y−1) | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Variety | Leaves | Petiole | Stem | Foliage | Leaves | Petiole | Stem | Foliage | Leaves | Petiole | Stem | Foliage |
| Kokei No. 14 | 757.7±44.0ab (53.0) |
280.6±27.6c (19.6) |
391.3±48.3a (27.3) |
1428.5±119.2 (100.0) |
6.3±0.2a | 1.7±0.0b | 3.4±0.2b | 4.6±0.2a | 46.6±2.3ab | 4.4±0.4 | 12.4±1.3a | 63.3± 3.9 |
| Koganesengan | 873.1±35.9ab (62.6) |
257.0± 5.3c (18.4) |
263.8± 5.4b (18.9) |
1393.9± 45.9 (100.0) |
6. 1±0.2ab | 1.9±0.0a | 3.8±0.2a | 4.9±0.1a | 52.9±0.7a | 4.7±0.1 | 9. 1±0.5b | 66.7± 0.8 |
| Murasakimasari | 849.8±63.9ab (54.3) |
377.7±40.9a (24.1) |
336.1±34.5ab (21.6) |
1563.5±139.0 (100.0) |
5.4±0.4c | 1.6±0.0bc | 2.7±0.0c | 3.9±0.3b | 44.0±3.9b | 5.6±0.6 | 8.4±0.9b | 58.0± 5.1 |
| Tamaakane | 751.1±73.5b (53.9) |
289.6±24.5bc (20.8) |
353.5±34.1a (25.4) |
1394.1±131.6 (100.0) |
5.8±0. 1abc | 1.8±0.1ab | 2.5±0.1c | 4.1±0. 1b | 42.4±4. 1b | 4.9±0.2 | 8.5±0.6b | 55.7± 4.9 |
| Suioh | 904.0±50.0a (54.5) |
357.4±21.2ab (21.3) |
400.2±11.7a (24.1) |
1661.6± 82.4 (100.0) |
5.6±0.2bc | 1.4±0.1c | 2.6±0.1c | 4.0±0.1b | 50.0±3.5ab | 4.9±0.8 | 9.5±0.6b | 64.4± 4.9 |
| 2014 | Annual yield (g DW · m−2 · y−1) | Average polyphenol content (g · 100g−2 DW) | Annual polyphenol Yield (g · m−2 · y−1) | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Variety | Leaves | Petiole | Stem | Foliage | Leaves | Petiole | Stem | Foliage | Leaves | Petiole | Stem | Foliage |
| Kokei No. 14 | 822.9± 51.6b (55.1) |
300.1±31.5b (20.1) |
369.2±34.4b (24.7) |
1492.2±113.7b (100.0) |
5.3±0.3 | 1.6±0.0b | 2.9±0.0b | 4.0±0.2b | 41.8±1.8b | 4.3±0.5b | 9.8±0.8 | 55.9± 2.6b |
| Koganesengan | 1152.8±113.4a (60.5) |
388.8±50.2b (20.4) |
363.3±33.3b (19.1) |
1905.0±196.7a (100.0) |
5.8±0.4 | 1.8±0.0a | 3.3±0.1a | 4.5±0.2a | 64.3±8.8a | 6.2±1.1ab | 10.9±1.5 | 81.4±11.0a |
| Beniharuka | 1201.4± 61.2a (54.9) |
513.5±48.0a (23.5) |
473.1±35.7a (21.6) |
2188.0±138.3a (100.0) |
5.1±0.2 | 1.7±0.1ab | 2.7±0.1b | 3.8±0.1b | 59.3±5.6a | 8.0±1.0a | 11.9±1.1 | 79.1± 6.8a |
Values are means ± SD (n = 3).
Means within rows followed by different lowercase letters are significantly different (p < 0.05).
The varieties of leaves, petioles, and stems to foliage composition is shown as percentage figures in parentheses. These values ware calculated from the average weekly dry-matter yield.
In 2014, we examined two varieties (K14 and KS) with a high polyphenol content for the whole foliage in 2012, and BH, the planting area of which has recently expanded (Table 1). The annual leaf yield of K14 was significantly (p < 0.05) lower than other varieties. The annual yield of petioles and stems was the highest in BH. The foliage yield was high in KS and BH and significantly lower in K14 (p < 0.05). In comparing the polyphenol content between varieties, the polyphenol content of the leaves was not significantly different, but that of the petioles and stems was significantly higher in KS than in the other varieties (p < 0.05). The polyphenol content in the foliage was relatively high in KS and significantly lower in K14 and BH. The annual yield of polyphenols was high in KS and BH for leaves and high in BH for petioles. For the foliage, KS and BH were significantly higher than K14 (p < 0.05).
The average dry matter weight (%) of the leaves, petioles, and stems of each variety in 2012 and 2014 is shown in parentheses in Table 1. Most of the varieties had a leaf ratio of around 55%, whereas KS showed a value of more than 60% in both years, indicating that it is a leafy variety. The ratio of petioles was around 20%, except for MM and BH, whose values were 24.1% and 23.5%, respectively. The ratio of stems ranged between 18.9% (in KS) and 27.3% (in K14).
Seasonal variations in sweet potato foliage yield by variety The average weekly DW foliage yield for different months is shown in Fig. 1. All varieties grew vigorously in summer, and the yield tended to decrease toward autumn. We also investigated the yield of foliage parts (leaves, petioles, and stems), which showed almost the same tendency of seasonal fluctuation as the whole foliage. In 2012, the maximum yield was observed in July, and thereafter the yield tended to decrease. The varieties were divided into three types based on the maximum yield: the varieties that maintained a high yield in September and October (SO), varieties whose yield rapidly reduced after August (KS and TA), and intermediate varieties whose yield gradually reduced (K14 and MM). In 2014, the yield peaked in June or July and declined thereafter. The average yield of K14 from May to July 2014 was lower than that of other varieties, but it was equal to or higher than that of other varieties in 2012. In 2014, the yield of various varieties except K14 increased. The annual variation in BH was considered to be of the intermediate type.
Varietal and seasonal changes in weekly dry matter yield of sweet potato leaves, petioles, and stems of five varieties in 2012, and three varieties in 2014.
Seasonal changes in total polyphenol content of different varieties The average weekly polyphenol content in the foliage at different months is shown in Fig. 2. In all the varieties, the trend of the polyphenol content was opposite to that of the yield, and it tended to decrease in summer and increase in spring and autumn. In 2012, the polyphenol content was highest in May, and the increase in polyphenols in the stems of K14 and KS was remarkable after September. In 2014, all KS and BH parts in May and October were characterized by a high polyphenol content. The maximum difference in polyphenol content between varieties in 2012 was observed in November when KS was 140.5% of SO, and the minimum difference was observed in May when KS was 121.4% of TA. In 2014, the maximum difference between varieties was observed in October, when KS was 137.5% of K14, and the minimum difference was observed in June when KS was 102.2% of BH. In May, the polyphenol content was higher in 2012 than in 2014 for K14: 4.7, KS: 3.7 g ChA equiv·100 g−1 DW.
Varietal and seasonal changes in the polyphenol content in sweet potato leaves, petioles, and stems per week in 2012 and 2014. Total polyphenol content is indicated as g chlorogenic acid equivalent - 100 g−1 dry weight.
Seasonal changes in total polyphenol yield of different varieties The average weekly polyphenol yield in the foliage at different months is shown in Fig. 3. These were estimated by multiplying the weekly yield of each variety by the content of each polyphenol. In all results, leaf polyphenol yields were more than twice the combined petiole and stem values. It was clarified that the contribution of leaves to the total polyphenol yield was very high. As shown in Figs. 1 and 2, the tendency of seasonal fluctuations in yield and polyphenol content was opposite, so it was expected that polyphenol yields would not be subject to seasonal fluctuations. However, the polyphenol yield of K14 was low in May for both years. Variety differences in polyphenol yields in 2012 showed high values for KS in May and June, and high values for K14 and SO in October and November. These results indicate that seasonal variation in polyphenol yields is more affected by yield than expected and is also affected by high polyphenol content in spring. Therefore, with a few exceptions, polyphenol yields in June were shown to be high in both years.
Varietal differences in weekly polyphenol yield per square meter in 2012 and 2014.
Varietal differences in caffeoylquinic acid content of sweet potato in 2012 and 2014 Table 2 shows the varietal differences in the content of CQAs, which are the main polyphenols in sweet potato. The content of CA and 3,4,5-TCQA was too low to be expressed in g·100 g−1 DW (data unavailable). The content of 3,5-DCQA was the highest in the foliage parts (leaves, petiole, and stem) of all varieties in both years. The content of CQAs decreased in the following order: 3,5-DCQA > ChA ≥ 3,4- or 4,5-DCQA > CA or 3,4,5-TCQA. The content of total CQAs decreased in the following order: leaf > stem > petiole; this order was the same for all varieties in both years. Moreover, the approximate amount of total CQAs was determined as follows: leaf : stem : petiole = 4 : 2 : 1. Therefore, the yield of leaves is highly important for its use as a raw material for CQA extraction. In 2012, the content of CQAs in the leaves was K14 > KS > SO > TA > MM, and in 2014, it was KS > K14 > BH. In the stems and petioles, the content of each CQA was lower than that in the leaves; therefore, the differences among the varieties were not noticeable. The annual content of CQAs in K14 changed considerably, but it was relatively high and remained stable in KS. The amount of total CQAs in the total polyphenol content is shown as CQA (%); it was observed that more than 50% of the polyphenols were CQAs, except in the petioles of MM.
| 2012 | ChA | 3,4-DCQA | 3,5-DCQA | 4,5-DCQA | Total CQAs | CQA(%) | |
|---|---|---|---|---|---|---|---|
| Leaves | Kokei No.14 | 1.0±0.3 | 0.6±0.2 | 2.8±0.5 | 0.4±0.1 | 4.7±1.0 | 74.6 |
| Koganesengan | 0.9±0.3 | 0.7±0.2 | 2.0±0.6 | 0.3±0.1 | 3.9±1.0 | 63.9 | |
| Murasakimasari | 0.6±0.3 | 0.4±0.1 | 1.4±0.3 | 0.2±0.1 | 2.7±0.8 | 50.0 | |
| Tamaakane | 0.9±0.3 | 0.4±0.1 | 1.7±0.3 | 0.2±0.1 | 3.3±0.7 | 56.9 | |
| Suioh | 0.7±0.3 | 0.3±0.1 | 2.2±0.5 | 0.2±0.1 | 3.4±0.9 | 60.7 | |
| Petiole | Kokei No.14 | 0.2±0.1 | 0.2±0.2 | 0.4±0.2 | 0.1±0.1 | 0.9±0.5 | 53.5 |
| Koganesengan | 0.3±0.1 | 0.3±0.1 | 0.4±0.2 | 0.1±0.0 | 1.1±0.5 | 57.8 | |
| Murasakimasari | 0.2±0.1 | 0.2±0.1 | 0.3±0.1 | 0.1±0.0 | 0.6±0.4 | 40.3 | |
| Tamaakane | 0.3±0.1 | 0.3±0.1 | 0.4±0.1 | 0.1±0.1 | 1.0±0.3 | 55.7 | |
| Suioh | 0.2±0.1 | 0.2±0.1 | 0.3±0.2 | 0.1±0.1 | 0.8±0.5 | 55.3 | |
| Stem | Kokei No.14 | 0.4±0.2 | 0.2±0.1 | 1.4±0.5 | 0.3±0.2 | 2.3±0.8 | 68.6 |
| Koganesengan | 0.4±0.2 | 0.2±0.1 | 1.5±0.6 | 0.3±0.2 | 2.5±1.0 | 65.8 | |
| Murasakimasari | 0.3±0.1 | 0.1±0.0 | 1.0±0.3 | 0.2±0.1 | 1.6±0.5 | 59.9 | |
| Tamaakane | 0.3±0.1 | 0.1±0.0 | 1.0±0.3 | 0.3±0.1 | 1.8±0.5 | 70.2 | |
| Suioh | 0.4±0.2 | 0.2±0.1 | 0.9±0.3 | 0.2±0.1 | 1.8±0.7 | 68.3 | |
| 2014 | ChA | 3,4-DCQA | 3,5-DCQA | 4,5-DCQA | Total CQAs | CQA(%) | |
| Leaves | Kokei No.14 | 0.6±0.4 | 0.6±0.2 | 1.8±0.4 | 0.4±0.2 | 3.5±0.8 | 65.3 |
| Koganesengan | 0.6±0.3 | 0.9±0.3 | 2.0±0.5 | 0.5±0.2 | 4.0±1.2 | 69.1 | |
| Beniharuka | 0.6±0.4 | 0.5±0.2 | 1.7±0.4 | 0.3±0.2 | 3.3±1.0 | 64.2 | |
| Petiole | Kokei No.14 | 0.2±0.1 | 0.2±0.1 | 0.4±0.2 | 0.1±0.0 | 1.0±0.4 | 61.2 |
| Koganesengan | 0.2±0.1 | 0.3±0.1 | 0.4±0.2 | 0.1±0.1 | 1.0±0.4 | 53.6 | |
| Beniharuka | 0.2±0.1 | 0.3±0.1 | 0.4±0.2 | 0.1±0.0 | 0.9±0.4 | 54.8 | |
| Stem | Kokei No.14 | 0.4±0.1 | 0.2±0.1 | 1.2±0.3 | 0.3±0.1 | 2.0±0.5 | 70.0 |
| Koganesengan | 0.3±0.1 | 0.2±0.1 | 1.3±0.4 | 0.3±0.2 | 2.3±0.8 | 68.5 | |
| Beniharuka | 0.4±0.2 | 0.3±0.1 | 1.1±0.3 | 0.2±0.2 | 2.0±0.7 | 73.9 |
Values are means ± SD (n = 3).
The total amount of CQAs present in the total polyphenol content is shown as CQA (%).
Relationship between polyphenol content and environmental conditions of leaves harvested weekly from May to October The correlation between polyphenols and cultivation environmental factors was investigated in the leaves, which contained the highest amount of polyphenols (Table 3). In order to compare the two years using the same environmental factors, the sampling period was set from May to October. As a result, some varieties showed a weak positive correlation between polyphenol content and sunshine duration; however, the correlations were not stable because the results changed from year to year. We focused on three factors for temperature: average temperature, lowest temperature, and highest temperature, and analyzed the correlation between each of them and the polyphenol content. The value of the correlation coefficient r was higher at the lowest temperature compared with that at the highest temperature, and a significant negative correlation was found for all varieties at the lowest temperature. In particular, KS had a significant correlation (P < 0.01) at all temperatures, and a strong negative correlation was observed with a correlation coefficient r = -0.727 (2012) at the lowest temperature. These results indicatethat the polyphenol content in sweet potato leaves is negatively correlated with temperature rather than sunshine duration, especially at the lowest temperature. In both years, K14 was shown to have a low correlation between polyphenol content and environmental factors.
| Correlation coefficient (r) for polyphenol content | |||||
|---|---|---|---|---|---|
| Variety | Average air temperature (°C) | Highest temperature (°C) | Lowest temperature (°C) | Sunshine duration (h) | |
| 2012 (n=25) | Kokei No.14 | −0.354 | −0.248 | −0.411* | 0.381 |
| Koganesengan | −0.704** | −0.603** | −0.727** | 0.392 | |
| Murasakimasari | −0.413* | −0.290 | −0.472* | 0.453* | |
| Tamaakane | −0.589** | −0.478** | −0.630** | 0.404* | |
| Suioh | −0.604** | −0.545** | −0.609** | 0.261 | |
| 2014 (n=26) | Kokei No.14 | −0.335 | −0.256 | −0.418* | 0.404* |
| Koganesengan | −0.547** | −0.392* | −0.640** | 0.552** | |
| Beniharuka | −0.576** | −0.459* | −0.646** | 0.465* | |
* and ** indicate significant at p < 0.05, and p < 0.01, respectively.
The annual foliage yield of 05303-3 under long-term greenhouse cultivation was reported to be equivalent to that of spinach and spinach mustard (komatsuna) (Kobayashi et al., 2019). A similar foliage yield was observed in the popular varieties examined in the present study (Table 1). As the dry matter yield is an important factor affecting polyphenol materials, common varieties can be efficiently used as a polyphenol raw material. In this study, the leaves had the highest content of polyphenols in all varieties. This is in agreement with the findings of previous studies on the high polyphenol content in leaves (Jang and Koh, 2019; Jeng et al., 2015; Kobayashi et al., 2019). In addition, among the varieties, the difference in foliage yield (16.1% in 2012 and 31.8% in 2014) was larger than that of the foliage polyphenol content (20.4% in 2012 and 15.6% in 2014) (Table 1). It was found that it is necessary to consider the effect of the dry matter yield, which fluctuates more widely than the polyphenol content, as the polyphenol yield is the amount obtained by multiplying the polyphenol content by the yield. In addition, the seasonal variations under long-term greenhouse cultivation were commonly observed among the varieties; that is, the foliage yield increased in summer, and the polyphenol content increased in spring and autumn (Figs. 1, 2). This result concurs with that reported previously (Kobayashi et al., 2019). However, in the present study, the foliage yield of some varieties (TA and KS) was reduced after August, and the yield of polyphenols in these varieties was substantially reduced in the latter half of the cultivation period. SO, which has a good taste and large leaves, without a high polyphenol content, seems to be effective for use from summer to autumn because the foliage yield was maintained after August. However, in late August, leaf-eating caterpillars emerged and grew quickly. Therefore, it is necessary to control the caterpillars with insecticides at the most appropriate time to protect leaf and polyphenol yields.
Among the varieties, the highest polyphenol yield from foliage was obtained in KS in both years. This result seems to be due to the high yield of leaves and the high polyphenol content regardless of the year. Assuming that the KS yield is 100%, the lowest was for TA in 2012 at 83.5% (Table 1). The polyphenol content in K14 seems to have a large inter-year difference. In 2014, the foliage polyphenol yield of K14 was 68.7% that of KS, owing to the low yield and low polyphenol content of K14. Due to a substantial varietal difference, selecting appropriate varieties is important for producing polyphenol materials. Moreover, because the content of polyphenols in leaves was approximately two times higher than that in the petioles and stems (Table 1), a variety with a high proportion of leaves would be effective for obtaining even higher polyphenol yields. KS presented a consistently high leaf composition ratio in both years (Table 1); therefore, it is expected that the yield of polyphenols will be higher than that in other varieties. It was also assumed that KS is a suitable material for industrial use.
Cultivation environmental factors that increase the polyphenol content were analyzed for each variety (Table 3). The results show that K14 was less sensitive to environmental factors than other varieties. In addition, with the exception of K14, there were differences in susceptibility among varieties, but it was shown that the polyphenol content responded to temperature rather than the duration of sunshine. This result supports the findings of our previous report (Kobayashi et al., 2019). However, the present study clarified that the lowest temperature has a higher negative correlation than the average temperature for all varieties. This suggests that there is an optimal temperature range for polyphenol synthesis in sweet potato leaves. Therefore, it may be expected that the yield of polyphenols will be further increased by cultivation under industrial conditions, i.e., under automated temperature control. Therefore, it is necessary to consider the cultivation temperature conditions with the highest polyphenol yield in the selected varieties in the future.
As the composition of the main CQAs present was almost the same in leaves, petioles, and stems, it was not possible to obtain the unique CQAs characteristics in each foliage component as we had expected. However, 3,4,5-TCQA was found in small amounts only in the leaves. The composition of CQAs changed due to the increase in the total amount of CQAs. In 2012, 3,5-DCQA and ChA in the leaves of K14 were higher than in 2014, with insignificant changes in the composition of other CQAs. When the amount of CQAs increases, it seems reasonable to infer from the hydroxycinnamoyl synthesis pathway that ChA and 3,5-DCQA increase first (Moglia et al., 2014; Liu et al, 2020). The total CQA content in the leaves, petioles, and stems of MM showed a particularly low CQA (%) content, suggesting that it contains more polyphenols than CQA (e.g., anthocyanins and flavonoids) compared to other varieties (Table 2). We plan to investigate whether this also applies to other purple-fleshed varieties. Here, except for MM, CQA (%) showed almost the same content in the leaves (60%), petioles (55%), and stem (70%). Therefore, the six varieties tested in this study (K14, KS, MM, TA, SO, and BH) can be used as raw materials for polyphenols, mainly CQAs. However, since CQA (%) differs depending on the variety, it is necessary to select a variety with a high CQA (%) when considering the extraction of high-concentration or pure CQAs.
In recent years, the processing and distribution of coffee extracts (Castroa et al., 2018) and artichoke extracts (Showkat et al., 2019), which mainly contain CQAs, have become widespread, and sweet potato foliage is expected to be used as a polyphenol source. In Japan, the planting share of different sweet potato varieties in 2016 was as followsi): KS ranked 1st at 21.0%, K14 ranked 3rd at 12.2%, BH ranked 5th at 10.1%, MM ranked 10th at 0.7%, and TA ranked 13th at 0.4%; thus, these varieties are abundantly cultivated. KS is a stable polyphenol material, and BH has a high annual yield; therefore, we hope to utilize these results in the future to promote the effective use of sweet potato foliage under long-term cultivation by continuing the use of nursery greenhouses.
Acknowledgments The authors gratefully acknowledge Dr. Atsushi Oyanagi and Dr. Masaru Tanaka (NARO) for helpful suggestions on the manuscript, and Ms. Ayumi Yonetsu and Ms. Mariko Shirakawa for technical assistance.
Conflict of interest There are no conflicts of interest to declare.
Kyukei 05303-3
3,4-DCQA3,4-di-O-caffeoylquinic acid
3,4,5-TCQA3,4,5-tri-O-caffeoylquinic acid
3,5-DCQA3,5-di-O-caffeoylquinic acid
4,5-DCQA4,5-di-O-caffeoylquinic acid
BHBeniharuka
CAcaffeic acid
ChAchlorogenic acid
CQAcaffeoylquinic acid
DWdry weight
HPLChigh-performance liquid chromatography
K14Kokei No. 14
KSKoganesengan
MMMurasakimasari
SOSuioh
TATamaakane
