Effects of Phosphorous Application on Growth Performance , Yield and Nutritional Value of Cockscomb ( Celosia argentea L )

Effects of phosphorous (P) level on performance, yield and nutritional value of Celosia argentea L. were evaluated with the aim of determining the P level that supports the best growth, yield and nutritional qualities of the crop. The levels of P were: normal (1.348 mg·kg of P/pot, NP), medium (6.743 mg·kg of P/pot, MP), high (13.48 mg·kg of P/pot, HP) and no P application (control, CP). The crop growth rate (CGR) increased with increase in phosphorus gradients (NP = 0.05 g/m·d, MP = 0.09 g/m·d and HP = 0.12 g/m·d). High and medium P rates increased the relative growth rate (RGR), HP had the highest RGR (0.05 g·g/day) followed by MP (0.04 g·g/day). Addition of P increased leaf area (LA) of C. argentea. Crude protein of the control was higher (5.56%) compared to the other P treatments (NP = 5.14, MP = 5.11 and HP = 5.03%). Phosphorus applications were beneficial for growth and enhancement of nutritional quality of C. argentea. NP and MP are recommended for growing this vegetable crop. DOI: https://dx.doi.org/10.4314/jasem.v24i6.18 Copyright: Copyright © 2020 Jimoh et al. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 22 April 2020; Revised: 27 May 2020; Accepted: 19 June 2020

Leafy vegetables are cheap sources of important nutrition in most countries of tropical Africa (Ayeni, 2016). In Nigeria, a rapid increase in population and urbanization which had led to the use of arable lands for other purposes apart from agriculture had led to a decline in the production of vegetables. Intensive farming activities with little or no fertilization are a key factor contributing to the reduction in productivity of plants (Senjobi et al., 2010). Other factors such as soil type, soil nutrient status and fertilizer management are equally significant determinants of the growth, yield and nutritional quality of plant species (Masanobu et al., 2016). Many vital processes in plants are affected by phosphorous (P) levels in the soil. Some of these processes include nucleic acids syntheses, photosynthesis, respiration, nitrogen fixation and enzyme regulation (Wahid et al., 2015). Application of phosphorus to the soil improves soil P availability, enhances the absorption of some other nutrients, and improves plant tolerance to stress (Cortina et al., 2013). However, energy metabolisms as well as biochemical synthesis-related functions are inhibited by a shortage of P in the soil (Burman et al., 2009). Growth and yield of plants are reduced by abnormality in root structure caused by phosphorous deficiency (Suriyagoda et al., 2014).
Increases in shoot-derived carbohydrates have been observed in many plant species in response to low P availability (Lundmark et al., 2010). Despite several reported studies on effects of P on growth, yield and metabolism of plants, little attention has been paid to the growth, yield formation and nutritional value of cockscomb (Celosia argentea L.). C. argentea is reported to be of great medicinal value (Zheng et al., 2009). The leaves were reported to be rich in folic acid, β-carotene, medium levels of vitamin E, ascorbic acid, calcium, iron and contain 4.7% protein (Oroka, 2015). The plant provides an affordable source of important nutrition at little cost when it accompanies starchy staple foods that are commonplace in Nigerian diets. However, there are few published studies on the biology of C. argentea. It is therefore essential to evaluate effects of level of P on performance of C. argentea to ascertain its level that may be either detrimental or beneficial, for adequate developments of the plant. For good seed production in Celosia and indeed most crops, at least the knowledge of the right amount of nutrients required by the crop for optimum growth and seed reproduction is essential. Therefore, the research was undertaken to investigate the effects of phosphorous application on the growth performance, yield and nutritional value of Cockscomb (Celosia argentea L).

MATERIALS AND METHODS
Preparation of Soil: Soil that was used in raising plant seedlings was collected from the botanical garden of the Obafemi Awolowo University Ile-Ife, Nigeria. The soil was air-dried and sieved to remove particles other than soil. It was later washed with 1N HCl to remove contaminants.
Determination of Soil Physico-chemical Properties of the Soil: The following physico-chemical properties of the soil were determined from the soil sample: particle size distribution (PSD: sand, silt and clay) and textural class, pH, organic carbon, organic matter, total nitrogen, available phosphorus, exchangeable cations (calcium, magnesium, potassium, sodium), exchangeable acidity and effective cation exchange capacity (ECEC).

Experimental Design:
A factorial experiment laid out in completely randomized design (CRD) consisting of 3 levels of P and a no P control was established in a screen house at Obafemi Awolowo University Ile-Ife, Osun State, Nigeria. To produce seedlings, a big plastic pot was filled with loamy soil, and seeds of C. argentea were sown and maintained for 30 days. The big plastic pot was supplied with 500 mL of water in the morning and evening throughout the 30 days. The experiment was carried out under screen house conditions to minimize extraneous factors from interfering with the plants. The mean daily temperature under the screen house was measured using a thermometer and the intensity of light was determined with the aid of a digital Lux meter.
Soil samples for raising test plants were stabilized for soil nutrient using a modified method of the long Ashton formula (Hewitt, 1952) and carefully homogenized. Homogenized soil samples (3.5 kg/pot) were transferred into 40 plastic pots measuring 10.0 cm by 15.0 cm. Levels of P added to pots were: normal (1.348 mg·kg -1 of P, NP), medium (6.743 mg·kg -1 of P, MP), high (13.48 mg·kg -1 of P, HP) and no P application (control, CP). Phosphorus was administered as NaH2PO4·2H2O, after mixing with 100 mL of water. Each treatment was replicated 10 times.
Evaluation of plant Yield: Leaf area measurement (calculated using the formula of Osei-Yeboah et al., 1983) and shoot heights were carried out weekly starting from 30 days after planting. The yield of C. argentea determined from the biomass of the plant components at the end of each week. Plants were harvested 30 days after planting and at the end of the experimental period. They were then separated into leaves, stems, and roots. These samples were ovendried at 80°C and used for quantification of net assimilation rate (NAR), relative growth rate (RGR), crop growth rate (CGR) and leaf area ratio (LAR). Photosynthetic pigments were extracted in the dark from leaves of each plant in each treatment. Eight grams of frozen fresh leaves from each treatment was extracted for chlorophyll and carotenoid contents using 0.1 g of sodium bicarbonate and 16 mL of 80% acetone. The absorbance of each extract was determined using digital spectrophotometer at wavelengths of 470 nm, 646 nm, and 663 nm.
Soluble sugar content was determined at the beginning and the end of the experiment to ascertain the level at P fertilization that may produce stress for C. argentea. A 0.1 g of dry leaf samples were homogenized with deionized water, filtered and the extract treated with 2% (w/v) phenol and 98% sulphuric acid (Dubois et al., 1956). This mixture was incubated for 1 hr at room temperature and the absorbance at 490 nm read using a spectrophotometer.
Proximate content Analysis: The percent moisture, crude fat, and ash content were determined using the AOAC (1975) method. Crude fiber was determined using a Fibretec 2021/2023 system (Foss Tecator, 2002). The dry, defatted sample (0.875 g) was placed in a Kjeldahl digestion flask and analyzed (AOAC, 1980). Standard solutions using nitrate salts of the metals were prepared (AOAC, 1980). The digest was analyzed with a flame photometer for determining K and Na. UV-Visible spectrophotometer was used for determining P at 660 nm wavelength. Atomic absorption spectrophotometer (AAS) was used to determine other metals (Donald & Clyde, 1979). The metal concentration in the sample was determined from a standard curve by extrapolation. The defatted sample (0.292 g) was hydrolyzed in 6 N HCl at 105°C for 22 hr in nitrogen. The hydrolysate was analyzed for amino acids using the sequential multi-sample amino acid analyzer as described by Sparkman et al., (1958). The chromatogram of the sample was compared using norleucine as a standard.

Data analysis:
The data obtained from the study were subjected to analysis of variance (ANOVA) using SPSS (version 18.0, SPSS Inc., Chicago, IL). One-way ANOVA was used to test the effect of different levels of phosphorus application on the growth indices, photosynthetic pigment accumulation and proximate contents of the plant.

RESULTS AND DISCUSSION
Crop growth rate (CGR) was affected by P level (Table 1), with CGR increasing as P level increased. High and medium P level increased RGR, with HP having the highest RGR followed by MP. The NAR was not different among treatments except for MP which had the highest value. The NAR of C. argentea was in an increasing order: MP > HP > NP > CP (Table  1). Compared with the control, P, at all levels, decreased LAR in C. argentea. The increment in CGR is directly proportional to that of phosphorus. The effects of High and medium P level on the plant indicated that P application improves soil fertility and possibly lowers the incidence of biotic stress (Ayeni, 2016). With increase in P fertilization, accumulation of dry matter by C. argentea might increase. The RGR signifies the rate of dry matter production and CGR corresponds to accumulation of dry matter (Adebo & Olaoye, 2010). There were gradual and steady increases in shoot height and leaf area of plants in the treatments from the beginning to the end of the experimental period. Leaf area and shoot height of HP plants were higher than the other treatments towards the end of the experiment. Root and shoot biomasses of plants could be described to have erratic pattern throughout the experiment (Fig. 1). Phosphorus is an important nutrient element and its role in plant growth, development and productivity cannot be overemphasized. It is a component of many cell constituents and plays a major role in several key processes, including photosynthesis, respiration, energy storage and transfer, cell division, and cell enlargement. Adequate phosphorus is vital to early root formation, growth, improvement in crop quality and is necessary for seed formation.  There was no significant difference in chlorophyll a, chlorophyll b, carotenoid and soluble sugar contents except in the high P treatment ( Table 2). The chlorophyll a content in the high P treatment at week 1 was higher than at week 8. The carotenoid and soluble sugar contents at week 1 were lower than week 8. Chlorophyll is a major component of chloroplasts and has a positive relationship with photosynthetic rate. Chlorophyll and carotenoids play roles in the maintenance of plant quality. Reduction in chlorophyll content is a typical symptom of oxidative stress and a consequence of pigment photo-oxidation and chlorophyll degradation (Demmig-Adams and Adams, 1996). The chlorophyll a content, affected by the high P treatment, was higher at initial than later stages of plant development. Carotenoid and soluble sugar contents were lower as plants advanced in age. Soluble sugar is essential in plant metabolism, mainly as substrates in biosynthesis processes, energy production and hydrolytic processes. The increase in soluble sugar content with high P in the later growth stage of C. argentea could indicate inhibition of their utilization and translocation.
There were no significant differences in ash content and crude fat of C. argentea due to P level and the control ( Table 3). The crude protein of the control plants was higher when compared to those for P treatments. High P increased crude fiber content of C. argentea. Our results indicated that using the high rate of P at 13.48 mg/kg produces the highest ash and crude fiber contents.
Conclusion: Phosphorus applications were found to be beneficial for growth and improvement of nutritional quality of C. argentea. However, its application might not be beneficial for crude protein and fat synthesis. None of the P levels used were detrimental for adequate yield and development of this plant.
edition. Association of Official Analytical Chemists (Washington DC: AOAC International).