Assessment of genetic variability for grain nutrients from diverse regions: potential for wheat improvement

Domestication and selective breeding progression significantly contributed to the depletion of nutritional content and in minimizing the genetic diversity of crops (Ladizinsky 1998). The great “Green Revolution” diverted farmers’ interest from legumes to cereals and consequently, the yields of cereal grains especially of wheat increased tremendously. However, in this pursuit of filling the bellies, the objective of enhancing the nutritional value of grains lagged behind (Welch and Graham 2004). An analysis of the nutritional value, especially micronutrients in the CIMMYT-developed germplasm revealed a declining trend in the micronutrient levels of the varieties (Monasterio and Graham 2000; Ortiz-Monasterio et al. 2007; Velu et al. 2014).

Currently, over two billion people suffer from ‘‘hidden hunger’’, a term used to describe deficiency of micronutrients (FAO 2013). Hence, after a successful green revolution, a thriving “nutritional revolution” is a prerequisite (White and Broadley 2009). Human requires approximately fifty-one nutrients in adequate amount, including both macro and micronutrients for nourished and healthy lives (Welch 2008). Today, nutrient deficiencies are not only common in developing countries where diet is mainly dependent on cereals, but it is also prevalent in developed countries (Diaz et al. 2003; Whatham et al. 2008). Iron and zinc deficiencies affect about 40 and 33%, of the world population, respectively (Paesano et al. 2012; Wessells and Brown 2012). While both iron and zinc are limiting in the diets of Indians, zinc is limiting in the diet of Turkish people (Cakmak et al. 1999; Akhtar 2013; Anand et al. 2014). Zn is essential for a healthy immune system, growth, wound healing, reproduction, fertility and sense of taste and smell, whereas Fe plays a central role in the transportation of oxygen around the body (Berg et al. 2002; Gibson 2006). In infants and young people, reduced Fe levels can lead to impaired growth and behavioural problems (Baker and Greer 2010).

Undoubtedly, among the cereals, wheat is an imperative resource of energy throughout the world (FAOSTAT 2008; http://faostat.fao.org). Whole grain products of wheat are suggested to efficiently utilize its element rich aleurone layer and thus, providing extended good fitness (Slavin 2004). Moreover, wheat is parallel to soybean crop in protein supply to human and livestock as it is a staple diet in less developed countries (Shewry 2000). Hence, dietary significance of wheat proteins should be properly appreciated and attempts should be made to improve it. Billions of people depend on wheat for fulfilling their nutritional prerequisites because of its wide agronomic flexibility, ease of storage and the types of foodstuffs prepared by its flour. In such scenario, augmentation of nutritional quality of wheat products through breeding approaches can provide a sustainable solution against diseases. Selection of wheat varieties with natural elevated mineral content can easily facilitate this strategy.

Biofortification (referring to plant breeding based strategies for enhancement of micronutrients in the edible parts of the plants/crops) has been suggested among the top five solutions to the problem of micronutrient malnutrition at the Copenhagen consensus (2008). Additionally, Genetic Biofortification is considered as a potential resolution to mineral malnutrition (Singh et al. 2005; Uauy et al. 2006). In 2005, EU Healthgrain Project has been initiated to exploit the nutritional components of cereal grains including wheat (van der Kamp et al. 2014). Under this project, Shewry (2009) screened 150 bread wheat lines along with some other wheat species and cereals for grain protein content and reported 12.9–19.9% variation in wholemeal. In 2006, a fundamental initiative was taken by Harvestplus for the estimation of genetic variability of element content in six crops and to facilitate varietal development in the breeding programs of these crops (Bouis 1996; Cakmak et al. 1996; Anglani 1998; Graham et al. 2001; Gregorio 2002; Welch and Graham 2004; Welch 2002; White and Broadley 2009) (http://www.harvestplus.org). Existence of a large variation in element and grain protein content in various wheat and its related species has been documented in numerous studies (Cakmak et al. 2004; Oury et al. 2006; Morgounov et al. 2007; Škrbić and Onjia 2007; Peleg et al. 2008; Shi et al. 2008; Ficco et al. 2009; Zhao et al. 2009; Chatzav et al. 2010; Zhao et al. 2011; Harmankaya et al. 2012). High variability is reported for both iron and zinc concentration in wild emmer wheat (Peleg et al. 2008; Gomez-Becerra et al. 2010), followed by durum wheat (Velu et al. 2011; Badakhshan et al. 2013) and lower levels of variability in bread wheat (Graham et al. 1999; Oury et al. 2006). However, still persistent efforts are necessary to search innovative wheat genetic resources augmented with essential minerals and protein.

India and Turkey are renowned for their major role in global wheat production holding second and tenth position in the world (FAO 2014). Detection of the varieties with balanced nutrient and protein content from these two countries can facilitate the wheat biofortification programs worldwide. The knowledge of nature and magnitude of genetic variability in the population is of immense value for planning efficient breeding programme to improve the yield potential of the genotypes. The selected wheat germplasm resources from different countries may be utilized in the future to attain more nutritionally rich wheat food. Therefore, in the present study an effort has been made to determine (1) genetic diversity for grain protein and macro/microelements in 121 Indian and 29 Turkish bread wheat varieties, (2) the correlations among grain nutrients and their association with protein content, (3) relationship between eco-geographical origin of involved bread wheat varieties and their nutrient concentrations, (4) to understand the level of crucial macroelements (Ca, K, Mg, Na, P and S), microelements (Cu, Zn, Fe and Mn) and protein content in Indian and Turkish bread wheat varieties.

A number of authors have emphasized on the neglect of wheat nutritive value in successful breeding programs relying on yield increment (Welch and Graham 1999; Cakmak 2008; Cakmak et al. 2010). Although several authors contributed towards the determination of elemental content in bread wheat grains (Dikeman et al. 1982; Peterson et al. 1983; Monasterio and Graham 2000; Šramková et al. 2009), still more studies are required to illustrate variations in the elemental content and nutritive importance of wheat grains (Stallknecht et al. 1996). Focusing the issue, results of present study showed a considerable variation in elemental and protein content of wheat genotypes characterizing Indian and Turkish origin.

Series of macroelement and microelement concentrations of our bread wheat genotypes were in line with range obtained in several previous studies (Zhao et al. 2011). Most of the Turkish genotypes used in our study were similar to those involved by Harmankaya et al. (2012) and revealed similar elemental content. In addition, one of the varieties used in our study, Gerek_79 was included in previous grain nutrients analysis conducted by Chatzav et al. (2010). Although, there were slight differences among the values, but all the nutrient and protein contents could be compared. Similar range of nutrient concentrations of same genotypes in different experiments/studies directs towards varietal impact of wheat cultivars. Several fold variations among genotypes can be attributed to their genetic origins along with the harvest timings and agricultural practices.

Several genes and physiological processes including nutrient uptake, their mobilisation, absorption in plant system and finally, storage in seeds highly contribute towards the correlation between grain nutrients and protein content (Bouis and Welch 2010). However, Cakmak et al. (2004) have focused on different genetic factors controlling nutrient efficiency and their accumulation in grains, their presence in different organs of plants may be affecting the entire mechanism of nutritional enhancement.

Positive correlation between Fe and Zn concentrations and strong association of grain iron and protein content of wheat grains in our experiment was concurrent with some previous studies performed on bread wheat (Morgounov et al. 2007; Zhao et al. 2009). This pointed towards the possibility of simultaneous improvement of both the nutrients. These associations can be ascribed to co-segregating alleles for different grain nutrients and co-localization of quantitative trait loci (QTLs). Several QTLs controlling Fe, Z and protein alliance are found in emmer wheat, double haploid populations and diploid wheat (Uauy et al. 2006; Morgounov et al. 2007; Peleg et al. 2009; Velu et al. 2014), however QTL information on grain Zn, iron and protein content in bread wheat is limited (Shi et al. 2008). In our study, contrary to previous experiments, wheat grain zinc and protein content were not found to be strongly correlated. Though, there were numerous genotypes like HW_5202, MACS_6222, 30KR8, UP_2696, Atilla with high zinc and protein content and showing positive correlation between the two, but total correlation estimated for all the genotypes was not strongly correlated. As zinc and protein content in wheat genotypes are considered to segregate together (Uauy et al. 2006; Peleg et al. 2009), this non-association may be linked to origin and may need further confirmation.

Not only, Fe and Zn with each other, but grain Cu and Mn also showed strong positive correlation with them. This can be linked to a major QTL on chromosome 5 controlling high Fe, Zn, Cu and Mn content in T. monococcum genotypes (Ozkan et al. 2007).

Positive correlation of sulphur with Fe and Zn content is of great significance due to the crucial role of sulphur containing aminoacids as promoters enhancing the Zn bioavailability (Welch and Graham 2004). This association is also crucial due to the involvement of S based aminoacid methionine in phytosiderophores production in cereals facilitating Fe and Zn mobilization in soil as well as in plants (Cakmak et al. 2010). Additionally, it has been illustrated that even marginal increase in sulphur supply leads to increased grain Zn and Fe concentrations (McDonald and Mousavvi Nik 2009). Positive association among Mg, P and Mn in our study was in accordance with the results obtained in the study conducted by Morgounov et al. (2007) on common wheat.

It is worth noting that macronutrient concentration in wheat is crucial for micronutrient balance, transport and bioavailability and their correlation should be considered. In our study, grain zinc was positively correlated with phosphorus. Co-localization of QTLs for grain Zn content with the QTLs for P content can be responsible for directing the positive association and will assist maintaining the wheat grain Zn and Phosphorus content simultaneously (Shi et al. 2008). Due to the accumulation of a major part of total P in the form of phytic acid, consequently phytate, Zn bioavailability is supposed to be affected (Stangoulis et al. 2007). Accumulation of grain Zn in aleurone layer and embryo, where phytate resides seems to contribute towards strong correlation between wheat grain Zn and Phosphorus content. These types of genetic, physical and physiological processes should be taken into account during nutrient enhancement targeted breeding programs.

Simultaneous elemental and protein composition of Indian and Turkish wheat genotypes has been studied for the first time. As per the results obtained, nutritional profile distinguished the genotypes from the two countries. However, close association of some of the varieties justified the success of the global wheat exchange programs. Most of the associations between different nutrients were in accordance with the previously conducted nutritional analyses and in favour of the systematic interactions with in the plant system. However, non-correlation between Zn concentration and grain protein content pointed towards its regulation by some other genetic factors or processes. It was a crucial correlation required to be understood. Although several QTLs controlling grain nutrient content in diploid and tetraploid wheat have been identified, information on QTLs associated with high nutrient concentration in bread wheat is still limited. We suggest some wheat varieties with promising nutrient concentrations for future biofortification programs. Combined associations of several elements open the doors for their simultaneous improvement in breeding systems, however, individual concentration and effect of each component should be considered separately. Outcomes of present study will be supportive to plant breeders, food-processing industries and people consuming whole grain wheat products. Wheat classification based on their geographic origin employing elemental and protein analyses along is a competent approach that may indirectly support the target of removing malnutrition from the world in coming future.