Aluminium tolerance: a determinant factor to cowpea Vigna unguiculata (L.) Walp. (Fabales: Fabaceae) productivity

Alternative approach to mitigate the negative consequences of aluminium toxicity on cowpea Vigna unguiculata (L.) Walp. (Fabales: Fabaceae) productivity cannot be overemphasized. The effects of aluminium toxicity on some morphological parameters of five cowpea accessions were investigated with the aim of determining the threshold of tolerance for the crop. Five cowpea accessions were collected from the International Institute of Tropical Agriculture (IITA.), Ibadan, Nigeria. The seedlings were raised in perforated plastic pots filled with 10 kg of top soil and treated till maturity with 50 μm, 100 μm, 200 μm of AlCl3 while those irrigated with tap water served as the control (0 μm). Variations were observed among accessions and treatments as plant height was accession dependent in contrast to stem girth, number of branches, root growth and shoot growth. Suppression of root growth among the accessions were attributed to more carbon allocation to the shoot at the expense of shoot growth leading to chlorosis, necrosis and reduced photosynthetic capacity among the accessions. Accession 5 was adjudged the best among the accessions based on the response to aluminium treatment. However, further research on the mechanism of tolerance especially at the molecular level is highly recommended.


Introduction
Aluminium stress is a major constraint to crop production on acid soils, in view of the fact that 40% of the world's arable land is acidic. Aluminium stress remains a major hurdle for increasing world food production, especially in developing tropical and subtropical regions where increase in food production is needed. Aluminium stress reduces crop yield through root growth inhibition and impairment in nutrient and water uptake . Restriction of plant growth by excess aluminium could either be due to direct inhibition of nutrient uptake or disturbance of root cell functions. Because root cell function is disrupted, cell elongation and division is impeded thereby root growth is restricted such that ability of plant to explore soil volume for water and nutrient is reduced .
The exert mechanisms by which certain plants tolerate high levels of aluminium is still debated. Several hypotheses have been suggested, that aluminium tolerant plants either prevent excess aluminium absorption by the roots or detoxify aluminium after it has been absorbed, they have higher rates of root growth, thereby uptake of water and nutrient is greater. They usually contain high level of organic acids that help them chelate and detoxify aluminium within the plant, example of these organic acids are; oxalate, malate and citrate (Riveiro et al., 2001). Several efforts had been made to maximize yield of cowpea, however this had being largely hindered by adverse effect of biotic stresses such as leaching and poor cultivation practices. These effects cause a huge loss due to low yield and failure of the crop to establish in some cases. Alternative approach towards efficient and cost effective means of production of cowpea is therefore very desirable (Riveiro et al., 2011).
Mechanisms of aluminium tolerance are classified as those that prevent aluminium ions from entering the root apical cells (i.e., apoplastic mechanisms) or that detoxify internal aluminium (i.e., symplastic mechanisms) Kochian et al., 2004). In symplastic mechanisms, aluminium enters the cytoplasm and is detoxified once inside the cell by complexation with organic compounds . Several compounds can form stable complexes with aluminium inside the cell, including organic acids such as citrate, oxalate, malate (Foy, 1988;Taylor, 1988;Miyasaka, 1998), andproteins (Suhayda andHaug, 1985). Free Al 3+ or aluminium complexes with chelating agents can be transported to cell vacuoles, where they are stored without causing toxicity (Kochian et al., 2004).
The present study investigated the aluminium toxicity on the productivity of cowpea Vigna unguiculata (L.) Walp.
(Fabales: Fabaceae). This serve to provide information on the relationship between aluminium stress and some aspect of primary metabolic activities of V. unguiculata.

Experimental location and set up
The study was conducted at the screen house of Plant Science and Biotechnology Department, Adekunle Ajasin University, Akungba Akoko, Ondo State, Nigeria (7° 371N latitude, 5° 44' E Longitude, and 100 m above the mean sea level; see Figure 1). The soil was air dried, sieved (in 2 mm sieve) and then 3 kg of the sample was weighed and poured into plastic polythene pots each with holes of approximately 3 mm bored at the bottom to enhance drainage and prevent waterlogging during the course of the experiment. The soil was then treated with different levels of AlCl 3 which are 0 µm (control), 50 µm, 100 µm, 200 µm, respectively. Each treatment was replicated five times with single plant replicate per pot, and were arranged on the screen house bench in a completely randomizes form.  Plant height was measured from the base of the stem to apical bud, using meter rule while stem girth was measured using digital Vernier caliper at 5 cm from the base of the stem. The plants were carefully uprooted after soaking the soil with water to prevent root damage. The roots were washed, counted and their length measured. The leaves, nodes and number of branches were counted. Fresh plant parts were weighed fresh and after drying in an oven at 80 °C to constant weight.

Statistical analysis
All data were statistically analyzed using the statistical package for Social Sciences (SPSS Version 20.0). Statistical means were separated using Tukey Honest Significant Difference (HSD) test at 95% level of significance.

Results and discussion
Suppression of photosynthetic capacity of shoots is also one of the consequences of aluminium toxicity. This is associated with cellular and ultrastructural modifications in leaves, reduced stomata opening and CO 2 assimilation, reduced chlorophyll concentration, chlorosis and leaf necrosis (Vitorello et al., 2005;Chen, 2006;Miyasaka et al., 2007;Chen et al., 2010). Accessions IT97K-568-18, TVU-9256 and IT98K-555-1 had 100% emergence under the control with respect to other treatments in contrast to TVU-4886 (93.3%) and IT96-610 (80%) as shown in Table 1. Plant biomas was not inhibited by aluminium treatment, with control having the lowest number of biomass in most accession (Table 2). According to Blamey et al. (1998) either Al 3+ or Al(OH) + are predominantly responsible for decreases in soybean growth. Similarly, Pavan and Bingham (1982) suggested that shoot growth of coffee in nutrient solution was more closely associated with the calculated activity of Al 3+ than with the activity of other monomers in the shoot environment. Each value is a mean of 5 replicates. For each parameter, means with the same letter(s) in superscript on the same column are not significantly different at P > 0.05.
The effect of aluminium on leaf length varied significantly among the accessions, and was not affected by the treatment with IT98K-551-1 having the lowest terminal leaf length and breadth respectively (Tables 3 and 4). Leaf area represent an integral component of plant growth, hence could be affected by different stresses. A significant decrease in leaf area of sugar cane (Beta vulgaris L) in response to salt stress of NaCl has been reported (Jamil et al., 2007). The notable decrease in the number of nodes of IT96-610 and IT97K-568-18 under the control regime (Table 5)  of Aluminium chloride could be explained by the negative effect of salt on photosynthesis that leads to reduction of plant growth, leaf growth and chlorophyll content (Netondo et al., 2002).  (Table 5). According to Alva et al. (2005), among the Individual aluminium monomers, relative root length of soybean was most highly correlated with calculated activity of Al(0H) 2+ followed by AlSO 4 , A1(OH) + , and Al 3+ .
They also found through reinterpretation of data from other studies with soybean, subterranean clover, alfalfa, and sunflower that root growth was most highly correlated with activities of Al(0H) 2+ or Al(OH) + . In the majority of cases, the relationship between root growth and activity of Al 3+ was relatively poor. This situation is further complicated by the fact that Ca and other cations, as well as pH, influence the expression of aluminium treatment (Cameron et al., 1986). Each value is a mean of 5 replicates. For each parameter, means with the same letter(s) in superscript on the same column are not significantly different at P > 0.05. Each value is a mean of 5 replicates. For each parameter, means with the same letter(s) in superscript on the same column are not significantly different at P > 0.05. Each value is a mean of 5 replicates. For each parameter, means with the same letter(s) in superscript on the same column are not significantly different at P > 0.05.
Shoot elongation when treated with low concentration of salt may induce osmotic adjustment activity in the plants which may improve growth. On the contrary, the observed decrease in plant height in IT98K-551-1 (Table 6) could be due to debilitating effect of salt on photosynthesis, changes in enzymatic activities and decrease in the level of growth hormones, both of which can lead to inhibition of growth (Mazher et al., 2007). Reports of this finding from the number of leaves was corroborated with the results of Welfare et al. (2002) and López-Aguilar et al. (2003) with their study on Phaseolus vulgaris L. and V. unguiculata. Their findings revealed that treatment with sodium chloride salt reduced the number of leaves compared with control plants. The decrease in leaf number in IT97K-568-18 and TVU-9256 at 50 µm and 100 µm may be due to accumulation of aluminium chloride in the cellwalls and cytoplasm of the older leaves (Table 7). In addition, their vacuole sap cannot accumulate more salt and thereby decrease the concentration of the intracellular ions (Jamil et al., 2007;Kapour et al., 2010).
Result dry weight of the shoot agreed with the findings of Andriolo et al. (2005), while working on lettuce, they reported that increased concentration of NaCl increased fresh and dry weight of the seedlings (Table 8). The inhibition of shoot elongation is one of the most important and visible effects of toxic concentrations of aluminium in plants (Kochian et al., 2004). The mechanisms of Al-induced inhibition of shoot elongation are a complex process involving physical, anatomical and morphological modifications as well as cell division (Silva, 2012). Complexity in the soil environment increased with aluminium supplement to a greater extent that performance of crop became unpredictable and increasingly variable among the accessions as tolerant to aluminium became more crucial. Each value is a mean of 5 replicates. For each parameter, means with the same letter(s) in superscript on the same column are not significantly different at P > 0.05. Each value is a mean of 5 replicates. For each parameter, means with the same letter(s) in superscript on the same column are not significantly different at P>0.05 Reduction in root dry weight was significantly affected with increase in aluminium toxicity. Root dry weight of TVU-4886 (0.70) at 200µm relative to the control in contrast to IT98K-551-1 (0.49) which was reported to be the lowest (Table 9). The present result confirmed that root biomass was significantly affected compared to other physiological parameters. Decrease in shoot biomass among the various accessions possibly indicates an inverse relationship between aluminium toxicity and biomass production. Findings from this study is in agreement with Gururaja Rao et al. (2005), that root growth was significantly affected by salinity levels than shoot growth. The biomass accumulation in TVU-4886 could therefore indicate optimal acquisition and uptake of nutrient for efficient metabolic activities (Ologundudu et al., 2012). Each value is a mean of 5 replicates. For each parameter, means with the same letter(s) in superscript on the same column are not significantly different at P>0.05 Each value is a mean of 5 replicates. For each parameter, means with the same letter(s) in superscript on the same column are not significantly different at P > 0.05.

Conclusion
The biomass accumulation in TVU-4886 could therefore indicate optimal acquisition and uptake of nutrients for effective metabolic activities. TVU-4886 appears to dictate the growth pattern of other accessions based on its performance under aluminium treatment. Such physiological and biochemical changes exhibited are important strategies to adapt to Al-toxic environment.
For further study, biochemical mechanism of aluminium tolerance in acid soils can be explored.