Introduction
There are many economically promising plants that belong to the Cactaceae family. The most popular of which is Opuntia (Russell and Felker 1987). Opuntioid cacti, also known as prickly pear cacti, is characterized as a succulent plant with a shallow root system, swollen stem structure, thick and waxy cuticle, which allows water storage in its organs (Anderson 2001;Ogburn and Edwards 2010). These plants are energy efficient and are able to survive in drought due to their use of the Crassulacean Acid Metabolism (CAM) which allows plants to intake atmospheric CO2 during night time to minimize water loss (Erwin 2009;Taiz and Ziger 2010). These cacti have been cultivated in the Mediterranean region as well as in Southeast Asia and are used as ornamental plants, food crops and a source of raw materials for pharmaceutical (Butera et al. 2002;Kim et al. 2012).
Especially in Korea, prickly pear cactus is famously cultivated in Jeju Island where they are grown rapidly for its economic demand as a key component for healthy products such as chocolates, cookies, tea, jam, juice, vitamins and other supplementary drugs (Ahn 1998;Gonzalez-Stuart and Rivera 2019;Jung et al. 2013;Lopez 1995). Because of its importance in human consumption and medicine, breeders have tried to analyze its phenotypic and genetic characteristics to enhance its species such as that of ‘Baeknyeoncho’ (Opuntia ficus-indica) (In et al. 2006;Mazri 2018) and ‘Cheonnyeoncho’ (Opuntia humifusa) (Srikanth and Whang 2015) and increase its production through tissue culture (Al-Khayri et al. 2018;Pérez-Molphe- Balch et al. 2015).
Numerous studies have continued to identify its chemical composition in the different parts of these succulent species. ‘Baeknyeoncho’ has been studied in terms of chemical characterization of its chemical composition and anti-oxidant properties from its fruit (Butera et al. 2002;Gralati et al. 2003), flowers (Ammar et al 2012;Benayad et al. 2014), stems or cladodes (Lee et al. 2003;Ramirez-Moreno et al. 2013). Likewise, ‘Cheonnyeoncho’ species are also studied for similar chemical composition and antioxidant properties using their fruits (Cha et al. 2013), cladodes (Jun et al. 2013) and other plant parts (Jung et al. 2011).
These related studies pave the way for medical researchers to be able to use this data in order to determine its medical benefits. Several studies have been conducted identifying its effect and its ability to be a potential cure or prevention for numerous medical conditions such as gastric lesions (Kim et al. 2012), cardiovascular diseases (Han et al. 2012), chronic diseases, diabetes, obesity and cancer (Diaz et al. 2017), etc.
Currently, the most common source for its consumption and use as a raw material is the fruit. To this day, domesticated species has been researched and bred to produce enhanced fruits with respect to size, shape, texture, flavor and antioxidant benefits (Ingles et al. 2002;Reyes-Aguero and Rivera 2011;Tesoriere et al. 2004). Opuntia species fruits were found to have protein (4.70 - 11.6%), fat (0.69 - 1.25%), crude fiber (5.9 - 50.3%), ash (2.0 - 15.10%), as well as phenolic acids (33.40 - 56.80%) and flavonoids (16.80 - 19.40%) (Diaz et al. 2017).
Numerous studies show that antioxidant activities are affected by change in certain factors such as atmospheric gases (Piga et al. 2003;Teissedre et al. 1996), relative humidity (Shin et al. 2007), but among several factors temperature (Cordenunsi et al. 2005;Reblova 2012) has been more prominent. In respect to the demand of Opuntia, there are only a few studies that determine the difference of antioxidant activity towards its plant organs, species, and the season of harvest. Hence, this study aims to determine these factors affecting antioxidant activity among the two common species of Opuntia in Korea, which are ‘Baeknyeoncho’ and ‘Cheonnyeoncho’.
Materials and Methods
Plant Materials
Two species of the prickly pear cactus were used namely: ‘Baeknyeoncho’ and ‘Cheonnyeoncho’ (Fig. 1) which were taken from Jeju Island. These samples were taken to the Physio Lab, Department of Environmental Horticulture, Sahmyook University, Seoul.
Experimental Design
The study was conducted using a 5 x 2 factorial arrangement in Completely Randomized Design (CRD) for each species and antioxidant component investigated in this study. Each analysis was replicated five times. The respective parts that were harvested from each species served as Factor A which were the flower, mature and young stems, roots and fruit. The same plant parts were harvested for two separate seasons which served as Factor B, summer (August) and winter (December).
Antioxidant Analysis
Different extraction and protocols to determine the levels of polyphenol, flavonoid, DPPH radical, ABTS radical, and anthocyanin were followed from previously published studies, and are indicated below. For content analysis, a modified Folin-Ciocalteau method was followed as indicated by the study of Hwang et al. (2015). The ballic acid was dissolved with distilled water and served as a standard solution. A 0.5 mL extract (1 mg·mL-1) was mixed with 0.5 mL of the polyin reagent and was lent to reacted at room temperature for 3 min. The specimen was prepared using 95% ethanol to make a 1mL extract of the specimen.
Polyphenol and Flavanoid Content Analysis
Na2CO3 at 2% was added to the sample extract in order to create a total volume of 1.5 mL and was mixed for 2 hours (Gutfinger 1981;Hwang et al. 2015). At 760 nm absorbance, the absorption degree (OD) was measured using a microplate reader (Infinite M200 Pro, Techan Group Ltd., San Jose, CA, USA). The total polyphenol content contained in the specimen is expressed as the Total Acid Equivalent (GAE) per gram of specimen through a standard curve of colic acid (6.25 - 100 μg·mL-1).
Similar methods were employed, but the total flavonoid content was obtained using quercetin (Sigma-Aldrich, USA) and was measured the absorbance at 415 nm (Chon et al. 2012;Lister et al. 1994).
DPPH Radical
DPPH (22-diphenyl-1-picrylhydrazyl) radical dissipation was measured according to the DPPH free radical removal method (Jang et al. 2015). After adding a specimen extract to the DPPH solution, it was dissolved in methanol. This was measured and marked with Trolox equivalent (TE) having 1 g in fresh weight. 1 mL of a specimen solution was added to a 3 mL of 60 μM 1-1-diphenyl-2-picryl-hydrazyl (DPPH) and was vortexed. After 15 min at room temperature, the absorbance was measured at 517 nm. After dissolving vitamin C (ascorbic acid) in EtOH, a standard solution was prepared. A 50 μM of DPPH solution was prepared in EtOH. The absorbance value of the DPPH solution was measured using the microplate reader. Antioxidant capacity (%) was obtained by using the formula: 100 - 100*(absorbance value of sample - blank absorbance value) / control's absorbance value - blank's absorbance value.
ABTS Radical
ABTS (2-azinobis-3-ethylbenzo-thiazoline-6-sulfonic acid) radical dissipation was measured by the method (Re et al. 1999). After mixing 2.45 mM potassium permulate and 7 mM ABTS solution in a ratio of 1:1(v/v), the ABTS solution was set aside for 12 - 16 hours and diluted with methanol. The absorbance was set at 734 nm and was measured after adding 3 mL of an ABTS solution to a 0.3 mL extract and leaving let it sit for 6 min. The results are expressed in TE and 1 g of fresh weight.
Anthocyanin Content Analysis
The total anthocyanin content was measured by the method of Pantelidis et al. (2007). It was calculated by using this formula: Contents of total anthocyanin (mg·g-1) = [(A × MW × 103 × dilution factor)]/ε. Where: A = (A 520-A 700) pH 1.0-(A 520-A 700) pH 4.5; ε = molar extension coefficient for cyaniding-3-glucoside (26,900); MW = molecular weight of cyanidin-3-glucoside (449.2 g·mol-1). The absorber 510 nm was measured and the buffer solution pH 1.0, pH 4.5 was used.
Data Analysis
Statistical analyses were conducted using Statistical Product and Service Solutions for Windows, version 16.0 (SPSS Inc., Japan). The data were analyzed using analysis of variance (ANOVA), and the differences between the means were tested using t-test and Duncan’s multiple range test (p < 0.05).
Results and Discussion
The results of the study are presented and expounded based on their cultivar, season and plant part, and their respective antioxidant activity. A comparison of means was likewise done. Discussions and literature that provide probable cause of the results are elucidated.
Opuntia Species
Table 1 presents the antioxidant activity of two species of Opuntia. Results indicated that species were highly affected (p < 0.001) by antioxidant activity of polyphenols, flavonoids, ABTS radical and anthocyanin. On the other hand, DPPH radical results were statistically non-significant (p > 0.659).
Previous studies suggest that antioxidant levels vary between species and cultivars (Allaith 2018). This was likewise observed for comparative studies of antioxidant levels between varieties for apples (Wpjdylo et al. 2008), garlic (Chen et al. 2013), onion (Lisanti et al. 2016), grapes (Singha and Das 2014) and purslanes (Lim and Quah 2007) among others. A similar study was conducted by Abdel-Hameed et al. (2014) of two prickly pear cactus species were analyzed for their antioxidant, phytochemical and nutritional properties. However, the species were not properly identified and were just tagged as red and yellow cactuses. Pertaining to the Korean species used in the study, ‘Baeknyeoncho’ possesses reddish colors, while ‘Cheonnyeoncho’ is prominently yellow. Similar results were also reported by the said study having red cultivars with higher total phenolic properties compared to the yellow cultivars.
‘Baeknyeoncho’ indicated higher contents of polyphenol (7.04 μg·mL-1) and anthocyanin (0.26 μg·mL1). Studies of Briedis et al. (2003) suggests that there is a correlation between high antioxidant activity with high polyohenol content. Intake of food with high polyphenol content promotes better nutrient absorption, improves metabolism, and decreases inflammation in both human and animal models (Bolca et al. 2003;Cheynier et al. 2015).
‘Cheonnyeoncho’ species had a higher flavonoid (7.34 μ g·mL-1) and ABTS radical (6.53 μg·mL1) content. Despite having no significant differences, DPPH radical was found to be higher in ‘Cheonnyeoncho’ species compared to those of ‘Baeknyeoncho’. This results was also similar to those of the studies of Lee et al. (2006). It was found that the increase of ABTS and DPPH radicals were observed with higher flavonoid content in Chinese mulberries. This may be attributed to their same chemical property of hydrogen- or electron-donation to flavonoids. Flavanoiids are compounds that are known to fight off allergens, pathogens, biruses and prevents cardiac problems, eye disorders, cancers and even Alzhimer’s (Shahidi and Ambigaipalan 2015;Vodnar et al. 2017).
Season
An important factor that majorly affects plants, its mechanisms, and as well as the important compounds produced within (Went 1953). Investigating how seasonal harvesting of plants has been found to be very important especially in high-valued crops wherein certain chemical compositions or compounds are centers of interest.
Differences in seasonal harvest significantly affected the antioxidant levels of Opuntia (p < 0.000). A comparison of means suggested that the different seasons were highly different for antioxidants polyphenol, flavonoid, DPPH radical and anthocyanin (Table 2a). Non-significant differences between seasons were observed from only those of ABTS radical. Higher antioxidants were significantly taken from those harvested during the summer season. Even those for ABTS radical that was found to be non-significant showed a higher antioxidant level during the summer season compared to those of the winter season. Results were similar when antioxidant activities were analyzed between groups of Opuntia species (Table 2b).
Studies showed that this was particularly observed through several crops on their antioxidant activity (Reblova 2012). Almeselmani et al. (2006) explained that antioxidant levels increase during high-temperature stresses, like the summer season, in plants as a protective mechanism to withstand damage to plant cells. This was evident in the study of Wang and Zheng (2001) with strawberries, wherein plants that were grown in cooler temperatures had the lowest phenolic acid, flavonoids, and anthocyanins while those of high-temperature growing conditions yielded the highest phenolic content and radical absorbance capacity. Aside from the growing conditions, temperature during extraction temperatures has the same effect as observed among grapes (Spigno et al. 2007).
Plant Parts
Different plant parts of succulents are edible and may be used for different purposes. As an integral ingredient and raw material for several commercial products, it would be best to identify the highest component of beneficial antioxidants and their activity within different plant parts of Opuntia species. Hence, different plant parts were analyzed for their antioxidant activity.
Results revealed that the different plant parts were significantly affected by the antioxidant activity (p < 0.001) (Table 3a) regardless of the species. Among these plant parts, flowers were found to have the highest antioxidant activities while those of the fruits were found to have the lowest antioxidant activities except for the DPPH and ABTS radical. It could be noted that there was a higher antioxidant activity level for those of the stems (young and mature) compared to the fruits. Likwise, it was observed that within Opuntia species ‘Baeknyeoncho’ and ‘Cheonyeoncho’ had similar results (Table 3b). Despite having the least antioxidant activity, fruits are surprisingly more commonly and commercially used among plant part of prickly pear cactuses for processing foods and other by-products (de Cortazar and Nobel 1992;FAO 2013). These flowers with high antioxidant contents may be used for raw materials if functional foods, medicine, flavorings and key ingredients for commercial products. Studies of Cavaiuolo et al. (2013) reported that flowers are natural sources of bioactive compounds and the high concentration of antioxidants sparks from the high flower pigments, such as carotenoids and polyphenols, on their petals.
Aside from the flowers, the use of stems has also proved to be better source of antioxidant activity in contrast with the fruits. Studies on grape stems had similar results to those of prickly pear cactus where their stems had significant amounts of polyphenolic compounds like phenolic acids and flavonoids (Souquet et al. 2000). Because Opuntia species are succulents, most of the most important physiological processes are found within their cladodes. These stems do not primarily grow for support, but they are enlarged storage units for the whole plant (Andersen 2001). Hence, it may not be suprising that antioxidants are found within these organs. Several crops are found to have high antioxidant levels in their leaves such as that of Cassua spp. (Kaur et al. 2006;Kumaran and Karunakaran 2007;Siddhuraju et al. 2002), peony (Li et al. 2009), Nasturium (Garzon and Wrolsyan 2009) among others.
