Phenolic Compounds, Antioxidant Activity and Mineral Contents in Grains of Phaseolus lunatus L. and P. coccineus L. Landraces

Beans are among the most cultivated legumes and contribute greatly to food and socioeconomic resources worldwide. The largest area planted with beans is found in Asia, followed by Africa and America, with 18.9, 8.5 and 7.2 million ha, respectively; beans are planted not only in developing countries but also in countries where they are often consumed and imported (10.4 million tons) (FAO, 2021). From a global economic perspective, bean production is primarily focused on the cultivation of P. vulgaris L., P. lunatus L. and P. coccineus L. The wild forms of these species are distributed from Mexico to Central America (Mesoamerica), a region considered the origin, domestication and diversification center and they can be distinguished from different landraces preserved by small farmers (Soleri et al., 2013). Phenolic compounds perform various biological functions during plant growth and development, where biosynthesis is influenced by biotic and abiotic stress conditions, genetic factors and genetic-environmental interactions (Yang et al., 2018). A bean grain consists of the embryo, cotyledons and seed coat, each with different chemical compositions. For example, in the seed coat of P. vulgaris and P. coccineus, the polyphenol, flavonoid and anthocyanin contents were higher than those in cotyledons (Capistrán-Carabarin et al., 2019). Therefore, there is interest in determining the composition of the seed coat to study biological activity and applications in human health (Yang et al., 2018).
       
P. vulgaris has received widespread attention for its bioactive and nutritional compounds and antioxidant activity (Alcázar-Valle et al., 2021). Previous studies have focused mainly on improved varieties of P. vulgaris, followed by P. coccineus (Quiroz-Sodi et al., 2018) and finally P. lunatus (Seidu et al., 2015; Palupi et al., 2022). Therefore, evaluating the variation in bioactive compounds and nutritional-nutraceutical content of Phaseolus seeds will help guide genetic improvement programs by providing nutritional assessments. The objective of this study was to compare the phenolic compounds, mineral contents and antioxidant activity of landraces of P. lunatus and P. coccineus based on samples of populations cultivated in indigenous communities of Oaxaca, Mexico and with reference to improved varieties of P. vulgaris.

Germplasm evaluated
 
The evaluated collection consisted of four populations of P. lunatus from communities of Oaxaca and Yucatan and four populations of P. coccineus collected in Oaxaca and Puebla, Mexico, all during 2018 and 2019. A commercial variety, Michigan, two experimental varieties and a landrace of P. vulgaris were used as controls. All experimental activities and laboratory analyses were performed at the CIIDIR-Oaxaca research center from Instituto Politecnico Nacional, Mexico.
 
Evaluation of phenolic compounds and antioxidant activity
 
The sample preparations were performed according to García-Díaz et al., (2018) and Capistrán-Carabarin et al., (2019). The polyphenols in the seed coat, cotyledons and whole grains were determined according to the methods described by Singleton and Rossi (1965), where the absorbance of the reaction was measured at 750 nm with a spectrophotometer (Shimadzu UV-1800, Kyoto, Japan) and the results are reported in mg gallic acid equivalents per gram of dry weight (mg GAE g^-1 dw). The flavonoid contents were evaluated based on the methods described by Zhishen et al., (1999), where the absorbance was measured at 510 nm in a spectrophotometer and the results are reported as mg equivalents of catechin per gram of dry weight (mg EC g^-1 dw). The monomeric anthocyanin content was determined in the seed coat and whole grains based on the method described by Giusti and Wrolstad (2001) and the values obtained were expressed as mg equivalents of cyanidin-3-glucoside per gram of dry weight (mg C3G g^-1 dw).
 
Antioxidant activity by DPPH and FRAP
 
The free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) was evaluated following the procedure described by Brand-Williams et al., (1995), where the absorbance of the reaction was measured at 517 nm using a spectrophotometer and the results are expressed as equivalent micromoles of Trolox per gram of dry weight (μmol TE g^-1 dw). In the FRAP method, the iron reduction capacity was measured as described by Benzie and Strain (1999), where the absorbance reaction was evaluated at 593 nm by a spectrophotometer and the results are expressed as equivalent micromolar of Trolox per gram of dry weight (μmol TE g^-1 dw).
 
Determination of minerals
 
The mineral contents were evaluated following the methodology described by Martínez-Martínez et al., (2019). The quantification of micro- and macronutrients (Fe, Zn, Mn, P, Ca, Mg, K, Na and S) was performed by inductively coupled plasma-optical emission spectrometry (ICP-OES Thermo Scientific iCAP 6500 DUO, United Kingdom) based on multielement standards (High Purity® Standards, USA) and the results are expressed in mg of the element per 100 g of dry weight (mg 100 g^-1 dw).
 
Statistical analysis
 
A database was compiled to evaluate the phenolic compounds, minerals and antioxidant activity in the seed coat, cotyledons and whole grains of each population from each spicie using three replicates per sample analyzed. Analysis of variance was performed using a completely random linear model assuming nesting of populations within species. In addition, multiple comparisons of means were performed by Tukey’s test (p£0.05).

Variation in the contents of phenolic compounds and antioxidant activity
 
The analysis of variance revealed significant differences in polyphenols, flavonoids, anthocyanins and antioxidant activity in seed coat, cotyledons and whole grains among species (Fs > 217.0; p£0.01) and within species or between populations (Fs > 31.0; p£0.01). Among species, differential patterns were observed; for example, the phenolic compound contents and antioxidant activity were consistently lower in P. vulgaris than in P. lunatus or P. coccineus, except for anthocyanins and flavonoids in cotyledons. Most polyphenols and flavonoids and antioxidant activity were found in the seed coat, with the highest levels in P. lunatus, followed by P. coccineus and P. vulgaris; similarly, the concentrations of polyphenols and flavonoids in whole grains were highest in P. coccineus, followed by P. lunatus and P. vulgaris. In all cases, the highest concentrations of compounds and antioxidant activity were in the seed coat compared with whole grains and cotyledons (Table 1).
 

       
In whole grains of P. coccineus, Alvarado et al., (2019) observed a lower content of polyphenols (<1.68 mg GAE g^-1) but recorded higher values of anthocyanins in whole grains with a black and purple color (>1.07 mg C3G g^-1). P. vulgaris and P. coccineus had higher contents of phenols and anthocyanins in the seed coat and cotyledons, while the values observed for P. lunatus were like those determined by Alcázar-Valle et al., (2021) (Table 1).
 
The differences in phenolic compounds, antioxidant activity and mineral concentrations in grains among and within P. lunatus, P. coccineus and P. vulgaris differed slightly from the patterns found by Alcázar-Valle et al., (2021). For example, they did not find differences between P. coccineus and P. lunatus and in this study, the differences were evident in the seed coat and whole grains. The differences between values and response patterns are due, in part, to differences in the genotypes/landraces evaluated and genotype-environment interactions, as indicated by García-Díaz et al., (2018) for P. vulgaris.
       
In this study, the improved varieties of P. vulgaris used as controls had lower concentrations of polyphenols in the seed coat, cotyledons and whole grains than the populations of P. lunatus and P. coccineus; however, for the anthocyanin contents in the seed coat and whole grains, the behavior was reversed. Alcázar-Valle et al., (2021) observed the same pattern in their comparison of the same three species. In the samples of P. coccineus evaluated by Alvarado-López et al., (2019), the variation in polyphenols and flavonoids ranged from 1.29 to 2.07 mg GAE g^-1 and from 1.08 to 1.61 mg QE g^-1, respectively and in this study, the variation ranged from 17.0 to 34.1 GAE g^-1 and 1.13 to 2.32 mg EC g^-1, indicating that the populations evaluated have the potential to be used in plant breeding (Table 2).
 

 
Antioxidant activity
 
The highest concentrations of polyphenols, flavonoids and anthocyanins were recorded in the seed coat, followed by whole grains and finally cotyledons in the three species evaluated (Table 2). This pattern was also observed for antioxidant activity evaluated by the DPPH and FRAP methods (Table 3). In this case, the populations with high antioxidant activity in the seed coat were PL-018-4 (P. lunatus) and PC-022 (P. coccineus). Populations PL-014 and PL-015-1 exhibited excellent antioxidant activity in their cotyledons but low values in their whole grains. The improved varieties of P. vulgaris had lower antioxidant activity in their seed coat, cotyledons and whole grains, while the whole grains of P. coccineus had excellent antioxidant activity. This finding indicates that a larger seed size may result in a greater amount of seed coat and major antioxidant activity. Capistrán-Carabarín et al., (2019) recorded equivalent results and this specie offers food complementarity for its consumers but also is affected by changes during processing (Modgil et al., 2016).
 

       
The variation in the concentration of phenolic compounds and antioxidant activity are regularly associated with the presence or absence of intense colors in the seed coat, such as black, red, or purple. In addition, the profile of phenolic compounds in every fraction of the grain is influenced by genotype, environment and environment-genotype interactions (Alvarado-López et al., 2019; García-Díaz et al., 2018) and in this study, it was evident that the species also affects the grain composition and antioxidant potential.
 
Mineral contents
 
Significant differences were observed in the mineral contents among species (Fs>2.5; p£0.05, 0.01) and among populations within species (Fs>1.0; p£0.01) for all the elements evaluated except Fe and Zn between species and Na between populations. The differences between species present three integral and complementary patterns in mineral micro- and macroelements: for the concentrations of Ca, P, S, Na and Cu, P. lunatus <P. coccineus£P. vulgaris; for the concentrations of K and Mg, P. vulgaris£P. lunatus < P. coccineus; and for the concentration of Mn, P. coccineus < P. vulgaris <P. lunatus. In this study, P. lunatus presented similar contents of Fe and Zn as P. coccineus and P. vulgaris, but the concentrations of the other six mineral elements were significantly lower in P. lunatus (Table 4). However, the values of K and P observed for P. lunatus were higher than those observed by Palupi et al., (2022), Jayalaxmi et al., (2016) and Seidu et al., (2015) and the values of P were similar to those reported by Giami (2006) for the same species, although methodological differences may have influenced these results. The species analyzed showed that the variation due to genotype and growth environment or crop location has a significant effect on the grain composition, as was reported for P. vulgaris (Bulyaba et al., 2020; Ribeiro and Maziero, 2022).
 

       
In the comparison of populations, the contents of Ca, K, Mg, Zn and Mg in the landraces of P. lunatus and P. coccineus were significantly higher than those in the improved varieties of P. vulgaris (control). A similar pattern was observed by Celmeli et al., (2018) comparing improved modern varieties and landraces and the first two species are potential sources of genes for a plant breeding program or for direct consumption, according to the methods suggested by Ribeiro and Mezzomo (2020). Zn and Fe are essential mineral elements for human health and were exceptionally high in the PL-005 and PL-014 populations of P. lunatus, the PC-022 population of P. coccineus and the PV-003 population of the control species P. vulgaris, with 3.88 to 4.49 and 5.62 to 7.38 mg 100 g-1 Zn and Fe, respectively (Table 5). These values are lower than the Fe content reported by Seidu et al., (2015) and Palupi et al., (2022) in P. lunatus but within the ranges of Fe and Zn in P. vulgaris reported by Herrera-Hernández et al., (2018).
 

       
The evaluated populations of P. lunatus and P. coccineus had lower contents of Ca and P than the evaluated populations of P. vulgaris. The contents of K and Mg were greater in a population of P. lunatus (PL-005) and one of P. coccineus (PC-022) than in P. vulgaris and this pattern was also observed for Zn and Fe. PL-005 and PC-022 have excellent concentrations of K, Mg, S, Na, Zn and Fe and the traditional variety PV-001 (P. vulgaris) has excellent concentrations of Ca, Na, Zn, Fe, Mn and Cu (Table 5).

The present study focused on P. lunatus and P. coccineus as promising sources of nutritional and nutraceutical compounds with high antioxidant activity levels and mineral contents. Significant differences were recorded between P. lunatus and P. coccineus relative to P. vulgaris (control) in the contents of total polyphenols, flavonoids, macroelements and microelements (except for Fe and Zn) and antioxidant activity and there were also significant differences and high variability between populations evaluated for all the compounds (except Na). The populations of each species are complementary in terms of the nutrition they provide when whole grains are consumed. The highest concentrations of phenolic compounds and highest levels of antioxidant activity were observed in the seed coat, followed by whole grains and cotyledons, indicating that the seed coat is an important source of bioactive compounds for nutraceutical purposes.