Changes in Plant Growth, Nutrient Dynamics and Accumulation of Flavonoids and Anthocyanins by Manipulating the Cropping Systems Involving Legumes and Cereals- a Review (Review Article) (Report‪)‬

Introduction The recent global increases in food and fuel prices have increased the pressure on the agricultural production systems and have caught the attention of many scientists. Previous FAO reports on the state of global food insecurity have shown that about 800 million people in developing countries have insufficient food to eat (FAO, 2000). In sub-Saharan Africa, the food crisis is chronic even though high proportions (70-85%) of Africans are active in agriculture (Borlaug, 1991). For example, Africa produced only 5.3% of the world's total cereal crop yield and many reports show that food imports into Africa have increased in the past decade (FAO, 2000; von Braun and Paulino, 1990; World Bank, 1989). According to the World Bank the increase rate of cereal yield in Africa was as low as 0.7% over the years, as opposed to the growth rates of 1.2-2.3% in other developing regions of the world (AGRA, 2007). The above mentioned trends are due to low soil fertility, low grain yield, poor [N.sub.2] fixing cultivars and cultural practices, severe pests and disease infestations (Boserup, 1981; Cooper et al., 1996; Sanchez et al., 1997). To reverse these trends and increase production of these crops, concerted efforts are needed by various key players. These must be targeted at improving soil fertility; identifying high yielding and [N.sub.2] fixing genotypes which are predominantly common in Africa and developing cultural practices which may confer resistance to insect pest and enhance yield stability. This may be achieved by altering plant densities and cropping systems. Cowpea (Vigna unguiculata L. Walp.) is among the indigenous African grain legumes grown extensively throughout Africa. It is the most important food legume, fodder and cover crop (Padulosi and Ng, 1990; Jackai and Adalla, 1997). In addition, to its early maturity potentials, it is versatile in adaptation, drought tolerance, and has a broad range of local genetic diversity. Nutritionally, cowpea grain is rich in protein (20.5-31.7%), carbohydrates (56.0-65.7%); fat (1.1-3.0%), fiber (1.7 4.5%) and moisture (6.2-8.9%) (Onwuliri and Obu, 2002). The green leaves and young pods of cowpea contain up to 35% protein and are eaten as vegetables. Cowpea also contains other essential nutrients, such as Ca, Fe, nicotinic acid and thiamine (Platt, 1962). Similar to other grain legumes, cowpea has been shown to contain several other important phytochemicals rich in health-related properties (Anderson et al., 1999). Some of the known health promoting phytochemicals in cowpea include phytosterols, saponins, isoflavone, phenolic compounds and antioxidants (Narasinga, 1995; Warrington et al., 2002). Likewise, compounds such as flavonoids, anthroquinones, anthocyanidins and xanthones commonly present in these legumes, possess remarkable antioxidant activity (Siddhuraju et al., 2002). Diets rich in polyphenolic compounds have been associated with longer life expectancy due to their richness in health-related properties such as anticancer, antiviral, anti-inflammatory activities, effects on capillary fragility and ability to inhibit human platelet aggregation (Stampfer et al., 1993; Deshpande et al., 1996; Hertog and Hollman, 1996). In this regard, increased dietary intake of natural flavonoids and anthocyanins through these legumes may greatly correlate very well with increased health benefits mentioned above. Cowpea has the potential for high grain yields of up to 3,000 kg.[ha.sup.-1] (Rusoke and Rubaihago, 1994). However, cowpea grain yields vary widely and are in the average of 200-400 kg.[ha.sup.-1] in Uganda (Sabiti et al., 1994), 200-300 kg.[ha.sup.-1] in Nigeria (Alghali, 1992), 400-1,000 kg.[ha.sup.-1] in Cameroon (Langyintuo et al., 2003), 50-300 kg.[ha.sup.-1] in Niger (Sivakumar et al., 1996) and from 1,100-1,400 kg.[ha.sup.-1] in Ghana (Adjei-Nsiah et al., 2008). This implies that farm yields of cowpea ranges between 1.7% and 46.7% of its potential. The observed low y