Common bean (Phaseolus vulgaris L.) is predominantly a self-pollinated crop originating in Latin America and mostly cultivated in the tropics and subtropics as well as in the temperate regions of the world (Gepts and Bliss 1988; Zeven, 1997; Zeven et al., 1999). Common bean represents 50% of the grain legume consumed worldwide, and is thus considered as the most important legume crop (McConnell et al., 2010). This crop is grown for its dry beans, and immature green pods. Being a good and cheap source of protein (20‐28%), minerals like Fe (70 mg/kg) and Zn (33 mg/kg), energy (32%), and fiber (56%) can solve malnutrition and hunger-related problems. Large variability among bean genotypes for micronutrient densities is known to exist (Beebe et al., 2000; Tryphone and Nichumbi-Msolla, 2010). In countries where daily bean consumption is high, it provides significant amount of proteins, calories and micronutrients helping to avoid the consequences of malnutrition and hunger (Valdemiro and Whitaker, 1982). Common bean is considered as an important functional food because of its high levels of chemically diverse components (phenols, starch, vitamins, fructo-oligosaccharides) which give protection against oxidative stresses, cardiovascular diseases, diabetes, metabolic syndrome and many types of cancers (Camara et al., 2013). Loss of crop diversity and extinction of genetic resources have led to a deterioration of nutritional quality of food crops (Gouveia et al., 2011; Singh, 2001). Micronutrients like Fe and Zn play an important role in the metabolism of both plants and animals. Iron deficiency anemia (IDA) and other micronutrient deficiencies affect large number of people worldwide and in many parts of Africa and Latin America it is the top health concern especially among the poor (Graham et al., 2001). Worldwide, over three billion people are affected by IDA while zinc deficiency in the human diet is probably almost as widespread as iron deficiency (Frossard et al., 2000). IDA causes loss in work productivity and complications in child birth while zinc deficiency causes stunting and reduces immunity against disease causing organisms (Welch and Graham 1999, 2004). Keeping these facts in view, in this study we evaluated the common bean genotypes for seed Fe, Zn & protein contents in order to identify superior genotypes that can act as a genetic resource for mining alleles/QTLs contributing for higher amounts of Fe and Zn. These genes/QTLs can further be introgressed in desired background through molecular breeding for enhancing nutritional value of common bean varieties.

We further analyzed seeds for their protein content, which ranged from 7.2% to 31.6% with an average content of 20.30%. Highest protein content was observed in KS6 (31.6%) whereas genotype K12 had the least protein content of 7.2%. Table 1 and Figure 1 represent the mean values of protein content observed in seeds of 51 diverse common bean genotypes. Similar results have been reported in previous studies, although variation between the content may be due to the environmental factors, geographical location, and growing season. In earlier reports, the variation in the seed protein content was observed ranging from 17.4% to 29% (Sood et al., 2003; Silva and Iachan, 1975; Sgarbieri et al., 1979; Márquez and Lajolo, 1981, Durigan & Sgarbieri, 1985; Durigan et al., 1987 & Tezoto & Sgarbieri, 1990). Protein content of 36 North American bean cultivars evaluated by Koehler et al., (1987) also ranged from 19.6 to 32.2%. A wide range in micronutrient and protein contents among 51 genotypes in this study indicates the existence of extensive genetic variation which can be explored for enhancing nutritional value of common bean varieties. Further, we grouped these 51 genotypes on the basis of their mean values (Table 2) to cluster them based on particular range of micronutrient (Fe and Zn) and protein content.

Table 1 Variation among Fe, Zn, and protein content in diverse common bean genotypes

Table 2 Distribution of 51 common bean genotypes on the bases of their mean value range of Fe, Zn, & protein

Figure 1 Mean±SE of mineral (Fe & Zn) and protein concentrations in seeds of common bean and their correlation with each other

Table 3 Correlation among Fe, Zn, & protein

Dry seeds (10~20) from each genotype were finely powdered in a grinder. 250 mg of this powder was cold digested in 5 mL concentrated nitric acid over-night, followed by digestion in 5 ml of diacid mixture containing nitric acid: perchloric acid, in 10:4 proportion (Jackson 1973). The resulting clear solution was diluted to 25 mL using double distilled water. The concentration of Fe and Zn was determined using ICPOES (Thermo Fischer 600). The concentration values were further converted and expressed in mg100g^-1 using the following formula.

Zeven A.C., Waninge J., Van Hintum T., and Singh S. P., 1999, Phenotypic variation in a core collection in common bean (Phaseolus vulgaris L.) in The Netherlands, Euphytica, 109: 93-106
http://dx.doi.org/10.1023/A:1003795805956