Malnutrition is one of the challenges facing Africans. It was recorded that in the 1980’s and 1990’s in sub-Saharan Africa, the rate of mortality due to protein malnutrition ranged between 25-35% on average (Rutherford and Mahanjane, 1985; Gernaat et al., 1998). In the current decade, sub-Saharan Africa is faced with the highest level of diseases associated with malnutrition in the world, as a fraction of the population, and this is increasing, whereas it is decreasing for the rest of the world (FAO, 2008). South Africa is also faced with the challenge of malnutrition, especially in the rural, tribal areas. Hence it is important to assess the available genetic pool of sorghum for nutritional quality traits such as their protein and amino acid digestibility, profile and content. Assessment of the nutritional profile of plant genotypes is essential to reduce malnutrition by breeding for improved content and composition of minerals, proteins and vitamins in crops such as sorghum (Welch and Graham, 2004; Feil et al., 2005). Knowledge of genetic and nutritional diversity can impact on conservation of sorghum genetic resources and the breeding of improved varieties (Simionuc et al., 2002).
Genetic variation in protein, mineral composition, total starch and its components was also observed among Ethiopian sorghum landraces (Shegro et al., 2012). Shegro et al. (2013) again reported nutritional diversity among a total of thirty one sorghum landraces from Western Ethiopia. Nguni et al. (2012) reported genetic and nutritional diversity among the sorghum accessions from Malawi, Tanzania, and Zambia. The authors assessed grain iron and zinc, total protein, and starch contents among the accessions, and used ten SSR markers to estimate genetic diversity. Considerable variation in minerals (Fe and Zn) was observed among cultivars, breeding lines and selected sorghum accessions (Hariprasanna et al., 2014). The results show that there is hope for enrichment of micronutrients in sorghum in order to combat malnutrition.
Mokrane et al. (2010) found differences among Algerian sorghum genotypes in protein and amino acid concentrations. Variation in amino acid profile and storage protein content was also reported among the commercial sorghum, MASSA 03, and nine ICRISAT high-lysine genotypes form India (Vendiamatti et al,. 2008). Furthermore, when white sorghum hybrids
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from Foggia in Italy and Kansas, USA were evaluated for chemical composition, protein and lipid content, and total amino acid, the protein content was higher in Foggia, southern Italy, than in Kansas, USA, although the grain quality was comparable between sorghums grown in this two regions (Pontieri et al., 2010). Therefore, sorghum can serve as source of essential amino acids and also potential parents with high amino acid concentrations can be selected for further breeding improvement.
1.8.1 Methods of protein and amino acid composition analysis
The amino acid composition of sorghum refers to the levels of various amino acids present in the protein fraction of grain. The amino acid profiles can be assessed by various methods such as chromatography procedures including gel filtration, ion exchange chromatography, preparative IEF, and hydrophobic interaction chromatography. However, near-infrared spectroscopy (NIR) is the most effective and non-destructive technique for analysis of quality traits such as protein and amino acid levels relative to the other techniques (Brauteseth, 2009).
1.8.1.1 Near-infrared spectroscopy
The use of near infrared spectroscopy (NIR) in the analysis of amino acid compositions has been reported in various studies (Olesen et al., 2011). Near-infrared spectroscopy is a technique that was first developed in the 1950’s (Barton et al., 2002). It was reported in the early 1960 as a non-destructive method that can be utilized in various ways. This technique functions with wavelengths between 750-2600 nm in which overtones and combinations of vibrations of numerous functional groups (-OH, -CH, -NH, -SH, etc) can be excited and detected. Hence, it can give information about structural and physical characteristics of biological compounds (Alexandrakis et al., 2008). It is fast, cheap, accurate, and can identify multiple chemical components in sample composite matrices. It offers an uncomplicated sample presentation. Hence, it is useful for speeding up selections in breeding programmes aiming to increase quality traits and decrease toxins.
The NIR has been used for analysis of many traits in various crops (Fontaine et al., 2001).
Hacisalihoglu et al. (2010) NIR to measure the levels starch, protein, and seed dry matter in common bean. The starch and protein parameters have been estimated by NIR in potatoes.
Starch content was 90% while total protein content was 62% (Haase, 2006). Schultz et al.
(2005) analyzed carotenoids in plants using the NIR. Pedro and Ferreira (2005) reported various levels of total and insoluble solids, lycopene and β-carotene in tomato fruit. The NIR technique was also reported to be useful in the analyses of starch, ash, cellulose, total nitrogen, and total sugars in the roots and tubers such as cassava, taro, yams and
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sweetpotato (Lebot et al., 2009). The NIR technique can be used in the analysis of trans fatty acids of various ground cereal products (Kim and Kays, 2009). Amino acid composition can be determined in soybean using NIR (Kovalenko et al., 2006). In wheat, the NIR has been calibrated for amino acids and showed useful for explaining variation of about 70-98%
(Fontaine et al., 2002). Delwiche et al. (2011) identified waxy starch in wheat using NIR analysis (high levels of amylopectin relative to amylose).
Several studies have shown the efficiency of NIR analyses when applied to sorghum.
Figueiredo et al. (2006) used NIR to measure amylose, protein, and lipid contents, endosperm texture, and hardness in cultivated sorghum core collections in whole and ground grain. Hicks et al. (2002) compared whole and ground grain NIRS calibrations of sorghum genotypes and hybrids for starch, lipid, and protein content, together with protein digestibility, in two sites. NIR analysis has been reported to provide accurate and efficient measurements in the analysis of protein for nutritional value and in the labelling of seeds of a number of cereal cultivars (YoungYi et al., 2010). Fontaine et al. (2002) used NIR to evaluate protein content and amino acid composition of milled sorghum grains. The protein and starch levels were also determined in sorghum lines and single hybrids for nutritional value and the protein, which ranged from 9.43-17.7% (Pepó et al., 2011). Roberts et al.
(2011) analyzed sweet sorghum bagasse for gross calorific value, in vitro true digestibility and crude protein using NIR. The correlation coefficients were above 0.9 for all analyses.
Although this technique has potential for the analysis of quality traits, there is still limited information on the application of NIR in sorghum landraces for breeding purposes, in particular, in terms of developed, open source calibration models, which is essential if plant breeders are to make use of the technology.
1.8.1.2 High performance liquid chromatography
High performance liquid chromatography (HLPC) is one of the methods used for analysis of chemical constituents of plants, including proteins and amino acids. The technology is widely used because of its reliability, the high level of reproducibility, and the low detection limits that HPLC offers (Breithaupt, 2004). However, this method requires a long process of sample preparation, and extraction of the desired pigments, which may be attached to other fractions in the plants that can mask the authentic content (Schulz et al., 2005).The use of HPLC has been reported in the analysis of sorghum (Taylor et al., 2007). Loerger et al.
(2007) studied the variation in proteins of vitreous and flowery sorghum endosperm using HPLC methods. Mokrane et al. (2009) characterized the primary sorghum proteins, kafirins, using a combination of SDS-PAGE, SE-HPLC, and RP-HPLC, as found in various sorghum genotypes and concluded that sorghum could be an excellent source of amino acids and
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protein, if the problems of amino acid profile, and digestibility of proteins, could be solved. El Nourf et al (1998) classified sorghum kafirins in relation to cross linking behaviour. Dykesa et al. (2011) reported other quality traits in lemon-yellow sorghum genotypes grown in two locations. Although several studies have reported the use of HLPC for analysis of protein and amino acid composition, the HPLC method can be time consuming and expensive when large collections require analysis for quality traits in plant breeding programmes.
1.9 Participatory rural appraisal: farmer production constraints and variety