1.1 Background to the Study
Cowpeas (Vigna unguiculata) are an important native African legume crop, whose seeds are sold in local urban and rural markets (Ortiz, 2007). It is a popular staple food that belongs to the family Fabaceae and the genus Vigna. It is generally called beans in tropical Africa. It has many local names in Nigeria; Ewa in Yoruba, Waka in Hausa and Agwe in Igbo language. It is referred to as blackeye pea, also known as northern Mexico pea, lobia in north India and niebe in West Africa. (Singh et al., 2010). Cowpea is grown extensively in sixteen (16) African countries with Africa producing two-third of the world production. Nigeria produced 2.1 million tons of cowpea making it the world largest producer, followed by Niger (650,000 tons) and Mali (110,000 tons) (IITA, 2004). Other major Africa producers include Burkina Faso, Ghana and Kenya. In Nigeria, cowpea is the most important legume crop grown (Fatokun and Singh, 1987). The genus Vigna contains about seventy (70) species of different growth habitats (Blackhurst and Miller, 1980). Cultivated species are usually variable because of artificial selection under diverse environments and cowpeas are no exception. Varieties are prostate, erect, semi erect, or climbing the pods is coiled round crescent or linear. The Peduncles are 5cm to more than 50cm long.
Cowpea is of major importance to livelihoods of relatively poor people in less developed countries of the tropics including South Sudan, especially where animal protein is not easily available for the family. It is an important crop in the agriculture of African countries for provision of nutritious food, provision of high quality feed for animals, provision of organic matter to the soil, provision of cash, as cover crop, fixation of atmospheric oxygen and addition of nitrogen to the soil (Ngalamu, 2015). Despite the economic importance of the crop, cowpea growth and productivity have been hampered by soil pollution with trace metals and other contaminants.
Pollution of environment by toxic metals arises as a result of various industrial activities and has turned these metal ions into major health issue (Waisberg et al. 2003). The behavior of heavy metals in the soil and plant system varies and depends on the type of element, in which form it occurs, its concentration in the sludge, the soil properties, and plant species (Singh and Agrawal, 2008; Nogueira et al., 2010). When soil pH is neutral or alkaline, heavy metals are usually inert in the soil under low mobility forms (Hayes and Traina, 1998). This is especially true for Cr with pH values above 5.0 where Cr is in the trivalent form (Cr3+), which is more stable, and has a low solubility and mobility (Alcântara and Camargo, 2001). Heavy metal accumulation in plants can occur without any toxicity symptoms and decrease crop yield (Rao and Shantaram, 1996).
Heavy metals when released into the soil, constitute havoc to life forms by reducing their growth, development, agricultural productivity and yield as well as affecting the natural ecological functions (Odum, 1981) which leads to environmental degradation, health hazards and destruction of crop plants (Agbogidi et al., 2013; Ikhajiagbe et al., 2013). Concerns about environmental hazards have stimulated research to evaluate the effect of pollutants on the biosphere.
Among the numerous trace metal pollutants, Cadmium, an element with no known beneficial biological function is of major concern (Egharevba, 2010). Of all toxic heavy metals, cadmium (Cd) ranks the highest in terms of damage to plant growth and human health. Moreover, its uptake and accumulation in plants poses a serious health threat to humans via the food chain (Shah and Dubey 1998). The presence of excessive amounts of Cd in soil commonly elicits many stress symptoms in plants, such as reduction of growth, especially root growth, disturbances in mineral nutrition and carbohydrate metabolism (Moya et al. 1993) and may thus strongly reduce biomass production (Joh, Ahmad, Gadgil & Sharma, 2008).
Lead (Pb) is one of the most abundant toxic metals in the earth crust. Exposure to lead in the environmental and occupational settings continues to be a serious public health problem (WHO, 1995). Elevated lead in soils may compromise soil productivity and even a very low concentration can inhibit some vital plant processes, such as photosynthesis, mitosis and water absorption with toxic symptoms of dark leaves, wilting of older leaves, stunted foliage and brown short roots (Mohan and Hosetti 1997, Patra et al., 2004)
1.2. Literature Review
Metals are crucial to the metabolism of cells at low concentrations but are toxic at high concentrations (Marschner, 1995). There are ninety (90) metals in which fifty-three (53) are naturally occurring. (Weast, 1984). Among these metals, Fe, Mo and Mn are important as micronutrients, while Zn, Ni, Cu, Co, Va, and Cr are toxic elements with high or low importance as trace elements, Ag, As, Hg, Cd, Pb and Sb have no known function as nutrients and seem to be more or less toxic to plants and microorganisms (Niess, 1999).
Cadmium is a non-essential element that negatively affects plant growth and development. Among the heavy metals, Cadmium is recognized as an extremely significant pollutant due to its high toxicity and large solubility in water (Das et al., 1997). Contamination of soils by Cadmium has become a serious worldwide problem (Cai and Braids, 2001; Zhou and Song, 2004). Uptake of metals into plant roots is a complex process involving transfer of metals from the soil solution to the root surface and inside the root cells. As soon as Cadmium enters the root, it can reach the xylem through an apoplastic and/or a symplastic pathway (Salt et al., 1995a), complexed by several ligands, such as organic acids and/or phytochelatins (Salt et al., 1995b). Cadmium toxicity can induce complex metabolic changes in plants with respect to physiological and biochemical aspects. Cadmium disturbs physiological metabolisms in plant like teanspiration, photosynthesis, respiration, nitrogen assimilation, etc. It is attributable to chloroplast damage (Baszynski et al., 1998), affects chloroplast function or CO2 fixation (Krupa and Baszynski, 1995), disrupt action of metals on photo system efficiency (Chugh et al., 1997) and chlorophyll content (Larsson et al., 1998). Additionally, Cadmium excess in the environment inhibited stomatal opening (Perfus-Barbeoch et al., 2002), stomatal conductance (Dunand et al., 2002), plant water balance (Zhou and Qui, 2005), photo activation of photo system II by competitive binding to the essential Ca2+ site (Faller et al., 2005), the activity of photo systems I and II (Kupper et al., 2007), the activity of photosynthesis enzymes (Mobin and Khan, 2007), carotenoid content (thapar et al., 2008) etc.
Lead is known to be one of the most abundant toxic metals in the biosphere. It is one of the potentially toxic heavy metal pollutants with no known niological function and its concentrations are rapidly increased in agricultural soil (Chaney and Ryan, 1994; Hamid et al., 2010). As many of the Lead pollutant are indispensable for modern human life, soil contamination with lead is not likely to decrease in the near future (Yang et al., 2000). Lead is considered a general protoplasmic poison, which is cumulative, slow acting and subtle.
Lead is available to plants from soil and aerosol sources. Soil characteristics like low pH, low density of phosphorus (P) and abundance of organic ligands can increase Lead absorption by plants (Kabata-Pendias and Pendias, 2000). Various studies of lead toxicity showed two possible conflicting observations. The first is that Lead is highly toxic to many organisms under certain conditions. Secondly, even though very large concentration of Lead is present in localized plant environment and even associated in or on plants, Lead induced toxic effects are reported only in a few cases (Khan and Frankland, 1983). According to Oliver and Naidu (2003), plants show different reactions against Lead toxicity. Some of them are sensitive and the others have more tolerance. Toxicity of Lead depends mainly on the solubility of Lead compounds. Silva, Araujo, Nunes, Melo and Singh (2013) studied heavy metals in cowpea after many sludge compost amendments. They observed that repeated soil amendment with industrial waste can affect the accumulation of chemical elements, mainly heavy metals in plants. They concluded that only Cr accumulation was significant (P<0.05) in the cowpea shoots after three (3) years of Tannery Sludge Compost (TSC) amendment but it was not translocated to the grains. Also, accumulation increased as TSC rates were applied.
1.3. Statement of the Research Problem
Over the years, researchers have intensified their efforts to determine the effect heavy metals on morphological, anatomical, physiological, biochemical and the productivity of plants. Wahid et al., (2007) studied plant response to Cadmium and Lead in cowpea especially mungbean (Vigna radiata), but there is no detailed document on the effect of both heavy metals (Cadmium and Lead) on the gradual changes on cowpea (Vignus unguiculata).
Researchers have documented the level of tolerances of the crop to cadmium and lead pollution separately. However, there is little information on the tolerance and response of the crop to combined contamination of cadmium and lead in the soil. In nature, these metals occur together with other pollutants and pose synergistic or inhibitory effect on growth and productivity.
1.4. Justification of the Study
There is no doubt that agriculture sector plays an important role in the economy and food industry of a country. Many kinds grains especially cowpea (Vigna unguiculata), are being produced from this sector, which are making major share in the export industry. But this is being affected by pollution which is as a result of several human activities such as use of insecticides, pesticides and sludge. Productivity problems and biodiversity have also been observed through the use of fertilizers. There is a need to maximize the agricultural production to overcome the increasing demand of food. Nowadays, farmers are using new techniques to increase the crop productivity and quality, due to the knowledge of pollution, especially of heavy metals such as cadmium and lead. There should be a primary focus to strengthen the regulatory programs to prevent the agricultural pollution and its drastic effects on crop yield and quality.
The aim of this study is to determine the combined effects of cadmium and lead on the growth and biochemical functions of cowpea.
1.6. Objectives of the Study
The objectives of this study are to investigate the combined effects of cadmium and lead on the:
- growth parameters of cowpea
- concentration of photosynthetic pigments in cowpea; and
- proline concentration in cowpea under the stress conditions imposed by the metals.