Abstract

Abiotic stress is one of the major factors limiting production and utilization of cowpea in developing nation of the world. Salicylic acid serve to promote and regulate plant growth. The work was design to examine the impacts of Salicylic acid on phytochemicals and nutritional composition of cowpea under arsenic stress. Three varieties of clean seeds of cowpea each were soaked in 0,75 and 150mg/L of salicylic acid for five hours. They were air-dried and sown in pots of soil containing 0, 250 and 500mg/L of sodium arsenate in a completely randomized design and were replicated thrice. The seeds were harvested at maturity and analyzed for phytochemical composition (total phenolic and flavonoid), Proximate composition (Crude protein, fat, ash, moisture, crude fibre and carbohydrate) and elemental composition (As, Ca, K, P, Mo, Mg and Zn). Data were analysed using ANOVA at α0.05. By comparing treatments with the control, crude protein, fat, phosphorus, calcium, magnesium and molybdenum were by increased by 2 folds among the three cultivars, total phenolic, flavonoid were increased by 4 and 2 folds in ART98-12, 5 and 3 folds in Ife brown and ITOK-568-18 in 250mg/L sodium arsenate-treated soil with SA (150mg/L). The result shows that 150mg/L of salicylic acid could be appropriate concentration to alleviate arsenic stress in cowpea .

Keywords: Salicylic acid, Arsenic, Cowpea, Phytochemical, Nutrition, Varieties, Stress.

Received: 10 February 2022 / Revised: 22 March 2022 / Accepted: 5 April 2022/ Published: 20 April 2022

Contribution/ Originality

This study contributes to mitigation of arsenic stress with the use new method of plant biotechnology. It originates from use of growth hormones to improve plant growth. The paper primary contribution is that 150mg/L salicylic acid could be appropriate concentration to improve cowpea resistance and ameliorate arsenic stress in cowpea.

1. INTRODUCTION

Cowpea (Vigna unguiculata) is one of Africa's oldest crops, and it's also a major crop in other nations, owing to its comparatively low cost of protein, which makes it a good substitute for animal protein, which is expensive in developing countries. Nigeria and Niger, in the African continent, produce over 66 percent of the world's cowpea. The seed contains a higher concentration of proteins, energy, minerals, and vitamins. Cowpea seeds have a low fat content and are high in protein (approximately 25%) [1]. When compared to starch from cereals, cowpea starch is continuously digested, which has been shown to be advantageous to human health. An vital nutrient is found in the grain of cowpea seed that helps to inhibit neural tube defects in unborn babies [2].

Arsenic is a metalloid that can be found in our environment in organic and inorganic forms, with the latter being the more dangerous. It occurs naturally in a variety of minerals, mainly including sulphur and other metals, as well as volcanoes in the atmosphere. Arsenic is found in water, air, and soil as a result of human activities such as smelting, minning, and residues of agricultural pesticides, all of which serve as sources of pollution in our environment [3]. Based on the frequency of occurrence and toxicity, it is considered the most poisonous substance. Arsenic as an element is not water soluble, however its salts (arsenite and arsenate) have a wide spectrum of solubilities. In humans, arsenic toxicity has been linked to lung cancer, bladder cancer, and skin cancer [4].

Arsenic contamination of crops causes oxidative stress, which is caused by the generation of reactive oxygen species, which is mediated by a reduction in antioxidant enzymes that protect crops from harm. Crop consequences include nutritional imbalances, soluble sugar accumulation, withering, curling, necrosis of leaf blades, lower growth and productivity, and a decrease in number and area [5]. Because arsenic is chemically similar to phosphorus, a critical plant nutrient for the growth and production of cowpea, it can be used to replace phosphorus in plant nutrition. Furthermore, the agronomic requirements of cowpea, which include the use of pesticides and insecticides, are increasingly serious. As a result, environmental contamination of soil, water, and air would continue to rise. As a result, bioregulators such as salicylic acid, which plays an important role in promoting growth, are used. Alleviating role of kinetin in maize seedlings under arsenic stress through evaluation of physiological, biochemical and nutritional properties had been reported by Haijuan, et al. [6]. Moreover, heavy metals component of our locally consumed cowpea in major markets in Ibadan had been evaluated and arsenic content were found to be above tolerable standard stipulated by Food Agricultural Organisation [7]. Hence the present study; Phytochemical and Nutritional composition of three varieties of cowpea (Vigna unguiculata) pre-treated with salicylic acid under arsenic stress.

2. MATERIALS AND METHODS

The present work was carried out in a screen at Institute of Agricultural Research and Training Ibadan Oyo State, Nigeria. Pot (5kg) were filled with soil and were arranged in a complete randomised design, replicated thrice with 15 treatments/ varieties of cowpea. Sodium arsenate (250 and 500mg/L) were later prepared. Aqueous sodium arsenate (500ml) prepared above was applied to each 5 kg of soil in the pot. It was left for 14 days for proper acclimatization. Cowpea seed varieties were obtained from Institute of Agricultural Research Training (Ife Brown and ART 98-12) while (ITO7K-568-18) was obtained from International Institute of Agricultural Research & Training.

2.1. Salicylic acid Preparation

Salicylic acid (75 and 150 mg/L) was prepared by using procedure of Coolbear [8] and Heydecker [9].

2.2. Planting of Seeds

Three different cowpea types (Ife brown, ART98-12, and ITO7K-568-18) were soaked in two different salicylic acid concentrations (75 and 150 mg/L, respectively). Other parts of the seeds that constitute the control were soaked in distilled water. They were stored at 37oC for 6 hours in the dark. The seeds were then drained of these solutions, rinsed three times with distilled water, and air-dried for roughly an hour. The seeds were placed in arsenic-stressed soil and left to germinate until they were ready to harvest.

2.3. Preparation of Seeds for Analysis

Seed from each of the varieties were crushed using laboratory blender.

2.4. Determination of Total Phenolic

Total phenolic compounds were estimated according to method reported by Singleton, et al. [10] using Folin- Ciocalteau reagent. Standard curve was plotted using gallic acid and the results were expressed as mg/GAE/g of the dry weight.

2.5. Determination of Total Flavonoid

The content of total flavonoids were determined by colorimetric method reported by Zhishen, et al. [11]. Flavonoid contents were expressed as mg/QUE/g/ dry weight.

2.6. Determination of Proximate Composition

Proximate composition is the routine analysis of a sample. However, salicylic acid are bioregulators which are involve in various processes that boost food production.

2.7. Procedures

Samples were analyzed chemically according to the official methods of analysis described by the Williams and George [12].

2.8. Determination of Mineral Elements and Arsenic Composition

Arsenic toxicity resulted in the loss of essential nutrients required for plant development and production. This is due to the inhibition of micronutrient and macronutrient uptake, most likely due to competitive binding with uptake carriers. However, SA facilitates the uptake of these nutrients through different mechanisms, such as using both the first and second line of defense, resulting in enhanced growth during stress.

2.9. Procedure

Mineral nutrients and arsenic content were determine according to the official methods of analysis described by the Williams and George [12]. The extracts obtained was analysed for magnessium, zinc, arsenic, molybdenum, phosphorus using Atomic Absorption Spectrophotometer while Flame photometer was used to determine potassium and calcium

2.10. Statistical Analysis

The data were analysed using Statistical analytical for science (SAS). Anova was used for separation of means. Duncan multiple range test was used to determine the level of significant (P ≤ 0.05) among the treatments.

3. RESULTS

The results of phytochemical composition of cowpea pre-treated with salicylic acid under arsenic stress is shown below in Figures 1-6. Cowpea exposure to arsenic significantly reduce the total phenolic and flavonoid contents when compared to the control but cowpea initially pre-treated with salicylic acid show increase in the total phenolic and flavonoid contents when exposed to arsenic. Moreover, nutritional composition of cowpea pre-treated with salicylic acid under arsenic stress is shown below in Tables 1-6. Cowpea exposure significantly reduce the nutritional and mineral compositions examined when compared to the control but cowpea initially pre-treated with salicylic acid show increase in the nutritional compositions.

Figure 1. Effect of salicylic acid on flavonoid contents of cowpea (Ife brown) under arsenic stress.