African eggplant (Solanum macrocarpon) and African nightshades (Solanum scabrum) are very important underutilized vegetables of southwest Nigeria and other countries of Africa  . The vegetables are good sources of vitamin C, riboflavin, folic acid, carotenes, protein, iron, vitamin A, iodine, calcium, potassium and zinc  . Good consumption of the vegetables could contribute to healthy diets in the face of the current COVID-19 pandemic. Nitrogen is the most essential element for plant nutrition and the most limiting nutrient for crop growth and yield . It is important for chlorophyll formation, protein synthesis and coenzyme. Nitrogen content of savanna soil of Nigeria is low because of wide C/N ratio, which greatly affects N availability compared with forest soil. Limited soil water and nitrogen are major factors militating against crop production in the region. Climatic factors, temperature and moisture also have profound effect on N availability through their effects on N mineralization, transformation and movement. Approximately 50% of total N fertilizer applied was being taken up by the crop. Vegetable crops require high amount of nitrogen; however, research attention on indigenous vegetables has over the years focused on diversity, production and marketing, role in poverty alleviation, nutritional quality, processing techniques, livelihood diversification studies and fertilizer requirement.
Little information is available on nitrogen uptake and nitrogen use efficiency of indigenous underutilized vegetables of southwest Nigeria . Genotypical differences in nutrient efficiency are related to differences in efficiency in acquisition by the roots, or in utilization by the plants or both. Efficiency in acquisition is frequently defined in terms of total uptake per plant or specific uptake rate per unit root length, and efficiency in utilization (nutrient use efficiency) as dry matter production per unit nutrient in the dry matter. For a given genotype, nutrient efficiency is reflected by the ability to produce a high yield in a soil that is limiting in one or more mineral nutrients for a standard genotype. According to , N use efficiency is the result of two main components: N uptake efficiency, which is the ability of crops to take up N from the soil and use efficiency with which crops use the absorbed N to grow and give yield. These efficiencies may differ within the same crop because they depend on different organs and mechanisms and on different environmental factors as well. At application of large amount of nutrients for plants with lower nutrient use efficiency, a luxury consumption may occur .
Since leafy vegetables have high demand for nitrogen, intensive production of vegetable in order to provide balanced nutrition for rapidly increasing population in developing countries and reduce environmental pollution, demand for high fertilizer used efficiency. The nitrogen use efficiencies of Solanum macrocarpon and Solanum scabrum in soils of savanna have not been documented. Understanding plant N uptake dynamics is critical for increase fertilizer N uptake efficiency and minimize the risk of N loss. The aim of the study was to examine the effects of nitrogen fertilizer application on dry matter yields, N uptake, nutrient use efficiencies and N recovery of Solanum macrocarpon and Solanum scabrum with a view to improving food and nutrition security in Nigeria.
2. Materials and Methods
2.1. Description of the Study Area
The study was carried out in Iwaro Oka, Ondo State, southwest Nigeria in 2013. The study location lies between Latitude 07˚25'25.99"N and Longitude 05˚46'29.14"E with elevation ranging between 350 and 353 m above sea level. The area climatic region was characterized by alternating wet and dry season and underlain by the Precambirian Basement complex .
2.2. Soil Sampling, Preparation and Analysis
Bulk surface soil samples (0 - 15 cm) were collected from unfertilized plots from Obafemi Awolowo University, Teaching and Research Farm, Ile-Ife. Soils at the sampling site have been classified as Iwo series (the local name)  and Typic Paleustult (the international name) . Soil samples were air dried, crushed in agate mortar, passed through a 2-mm sieve, and analyzed for following properties. Particle size distribution was determined by the modified hydrometer method  using 0.2 M NaOH solution as the dispersing agent. Soil pH was determined using a glass electrode pH meter in both distilled water and 0.01 M CaCl2 solution, using mass ratio for 1:2 soil:CaCl2 solution as described by . Soil organic carbon was determined by the chromic acid digestion method of . The total N concentration was determined by macro-Kjeldahl method according to , and the available P was extracted by Bray-1 method as described by  and determined using Spectrometer (Model 721 Visible Spectrophotometer, Axiom Mediral LMD. UK). Exchangeable K, Ca, Na and Mg were extracted with neutral (pH 7) solution of 1 N NH4OAc. K, Ca and Na were determined using the flame photometer (flame photometer model 2655-00 Digital Flame Analyzer, Cole-Parmer Instrument Company, Chicago, Illinois 60061) and Mg by the atomic absorption spectrophotometer (PG-900 Atomic Absorption Spectrophotometer Model, PG-instrument Ltd. United Kingdom).
2.3. Experimental Design
The experiment was arranged into a Completely Randomized Design with N at 0, 40, 80 and 160 kg/ha−1 applied as urea fertilizer and replicated four times, as previously described by .
A plot was 2 m × 3 m with 1 m between the plots. Planting method was by drilling and control of weeds and pests were as previously described by . Fertilizer was applied at two weeks after planting and immediately after the first and second harvest.
2.4. Data Collection
2.4.1. Shoot Fresh and Dry Matter Weight
Solanum scabrum shoot was harvested at six weeks after planting (WAP) whereas Solanum macrocarpon shoot was harvested at eight WAP. The shoots of all vegetable plants on each plot were harvested by cutting the stem at 4 cm above the soil surface and the weight recorded. Shoot subsample was collected in polyethylene bag and immediately transported to the Soil Testing Unit of the Department of Soil Science and Land Resources Management, Obafemi Awolowo University, Ile-Ife. Leaves and succulent stem of the sub-sample were collected and dried in oven at 65˚C until a constant weight was obtained. The dry weight was determined.
The shoot dry matter weight was estimated using the following expressions
Oven dry weight of shoot per plot (kg):
2.4.2. Determination of Nitrogen Composition
Plant tissue N content was determined using micro-Kjeldahl method.
2.4.3. Determination of Shoot Dry Matter Yield, N Uptake, N Use Efficiency and N-Recovery
N uptake, NUS and N-recovery were estimated using the following expressions
where Nr is the amount of N applied (kg∙N∙ha−1), YN is the dry yield with applied N, and Y0 is the dry yield without N applied. To further explore the response to applied N, the NR was calculated using the following equation
where Nr is the amount of N applied (kg∙N∙ha−1), UN is the plant N uptake (ton N∙ha−1) with applied N and U0 is the plant N uptake (ton N∙ha−1) without applied N .
2.4.4. Data Analysis
Data collected were subjected to analysis of variance (ANOVA) to assess treatment effects and when it was significant means were separated using Fisher’s Least Significant Difference at 5% level of probability using SAS 9.1 .
3. Results and Discussion
3.1. Growth of the Vegetables
Plate 1(a) & Plate 1(b) show the photographs of Solanum macrocarpon and Solanum scabrum. Solanum macrocarpon has broad, thicker leaves and more drought tolerant compared with Solanum scabrum. The vegetables were planted under rainfed.
3.2. Characteristics of the Soil Used for the Study
The native soil used for growing the vegetables were loamy sand, slightly acidic, medium to low in organic matter and high available P but low exchangeable cations and cation exchange capacity CEC (Table 1). Total N was also low for vegetable production .
3.3. Effects of N Fertilizer Application on Dry Matter Yields and N Uptake of the Vegetables
The result showed a significant (p < 0.05) increase in dry matter yield only at 160
Plate 1. Photo of (a) Solanum macrocarpon and (b) Solanum scabrum at the experimental station.
Table 1. Properties of the soil (air dried) used for the experiment at 0 - 15 cm depth.
kg N/ha compared with control, which was similar with what was observed at 40 and 80 kg N/ha for S. macrocarpon whereas application at 40 kg N/ha and higher rates were markedly (p < 0.05) increased the yield compared with the control for S. scabrum (Figure 1(a) & Figure 1(b)). The average dry matter weight at 40 kg N/ha was 93% and 9.2% higher than the values for the control for S. scabrum and S. macrocarpon, respectively. The values obtained for nitrogen content at applied N were lower that the control due to dilution effect (Figure 1(c) & Figure 1(d)) . The average nitrogen uptake was 10 kg/ha and 240 kg/ha for S. macrocarpon and S. scabrum, respectively. The result showed that S. scabrum had ability for nutrient acquisition .
3.4. Effects of N Fertilizer Application on N Use Efficiency and N-Recovery
Negative values for nitrogen use efficiencies were obtained for N-recovery for S. macrocarpon while 40 kg N/ha gave the highest value (87.33 kg/ha), which decreased consistently at 80 kg∙N∙ha−1 and 160 kg N/ha for S. scabrum (Table 2).
The nitrogen content required for optimal growth varies between 2% to 5% of the plant dry matter weight depending on the plant species, development stage and organ  . An increase in the nitrogen supply not only delays senescence and stimulate growth but also changes plant morphology in a typical manner, particularly if the nitrogen availability is high in the rooting medium during the early growth.
The results of our study showed 9% and 93% increase in dry matter weight at
Figure 1. Effects of nitrogen fertilizer application on (a) & (b) shoot dry matter and (c) & (d) N uptake of S. macrocarpon and S. scabrum.
Table 2. Effects of nitrogen fertilizer application on (a) N use efficiencies (NUE) and (b) N-recovery of S. macrocarpon and S. scabrum.
application of 40 kgN/ha, which were not significantly different from the values obtained at 80 kgN/ha and 160 kgN/ha for S. macrocarpon and S. scabrum, respectively, showing a luxury consumption. These observation confirmed the previous finding of  on luxury consumption of nutrient and nutrient use efficiency at high nutrient availability There was a wide difference in the use of nitrogen take up between S. macrocarpon and S. scabrum. The high dry matter weight produced by S. scabrum at 40 kgN/ha could be explained by the report of earlier study by  that photosynthesis of C4 plant species of tropical origin have high photosynthetic rates and produce large amounts of dry matter. The C4 mechanism enables plants to utilize both CO2 and water more efficiently. S. scabrum gave highest % N-recovery (5.85%) at 40 kgN/ha while negative values were obtained for S. macrocarpon.
The study concluded that S. scabrum had higher nitrogen uptake and nitrogen use efficiency in the soil compared with S. macrocarpon and that highest NUE and N recovery were obtained at 40 kgN/ha. There is a need for further studies on N use efficiency of indigenous underutilized vegetables of Africa in order to achieve their potentials for improved nutrition security.
The authors are grateful to the Foreign Affairs, Trade and Development Canada (DFATD) and International Development Research Centre (IDRC) for the project research grant number 106511. This study was a portion of the Nigeria-Canadian Vegetable (NI-CAN VEG) Research.
 Idowu, M.K., Oyedele, D.J., Adekunle, O.K., Akinremi, O.O. and Eilers, B. (2014) Effects of Planting Methods and Seed Density on Vegetable Yield and Nutrient Composition of Solanum macrocarpon and Solanum scabrum in Ile-Ife, Southwest Nigeria. Journal of Food and Nutrition Sciences, 5, 1185-1195.
 Musyimi, D.M., Chemisto, J.K. and Buyela, D.K. (2012) Growth and Physiological Response of African Nightshades (Solanum scabrum Mill) to Sodium Chloride Salinity Stress. Global Advanced Research Journal of Food Science and Technology, 1, 66-73.
 Hommels, C.H., Kuiper, P.J.C. and Tanczos, O.G. (1989) Luxury Consumption and Specific Utilization Rates of Three Macro-Elements in Two Taraxacum microspecies of Contrasting Mineral Ecology. Physiologia Plantarum, 77, 569-578.
 Okusami, T.A. and Oyediran, G.O. (1985) Slope-Soil Relationships on an Aberrant Toposeqence in Ife Area of Southwestern Nigeria: Variabilities in Soil Properties. Ife Journal of Agriculture, 7, 1-15.
 Thomas, G.W. (1996) Soil pH and Soil Acidity. In: Sparks, D.L., Ed., Methods of Soil Analysis, Part 3, Chemical Methods, SSSA and ASA, Madison, 475-490.
 Walkley, A. and Black, I.A. (1934) An Examination of the Degtareff Method for Determining Soil Organic Matter and a Proposed Modification of the Chromic Acid Titration Method. Soil Science, 37, 29-38.
 Baitilwake, M.A., De Bolle, S., Salomez, J., Mrema, J.P. and De Ne, S. (2011) Effects of Manure Nitrogen on Vegetables’ Yield and Nitrogen Efficiency in Tanzania. International Journal of Plant Production, 5, 417-430.
 Mina, R., Mohammad-reza, C., Mohammad-reza, J., Hamid, R.M. and Hamid-reza, S. (2011) Effects of Water Stress and Nitrogen Fertilizer on Multi-Cut Forage Pearl Millet Yield, Nitrogen and Water Use Efficiency. Communications in Soil Science and Plant Analysis, 42, 2427-2440.