PROXIMATE CONTENT AND ANTIOXIDANT PROFILE OF PUMPKIN (Cucurbita pepo L.) LEAFY VEGETABLE AS INFLUENCED BY NPK FERTILIZER

OLOYEDE F.M.¹*, OBUOTOR E. M.² and OBISESAN I.O.³

¹ Department of Agronomy ,Osun-State University, Osogbo, Nigeria.

²Department of Biochemistry, Obafemi Awolowo UniversityIle-Ife, Nigeria.

³Department of Crop Production and Protection, Obafemi Awolowo UniversityIle-Ife, Nigeria.

Dr. Oloyede F.M, PROXIMATE CONTENT AND ANTIOXIDANT PROFILE OF PUMPKIN (Cucurbita pepo L.) LEAFY VEGETABLE AS INFLUENCED BY NPK FERTILIZER(2016)SDRP Journal of Food Science & Technology 1(4)

This study evaluated the influence of NPK fertilizer on the proximate content and antioxidant profile of pumpkin leafy vegetable. The vegetable was raised at the Teaching and Research Farm, Obafemi Awolowo University, Nigeria in 2010 for two seasons and the following NPK fertilizer rates were applied: 0, 50, 100, 150, 200 and 250 kg/ha and replicated six times. Six composite samples from the six replications were analyzed in duplicates for proximate content and antioxidant profile. For the antioxidant assays, crude extract was obtained from cold extraction of each of the samples with 80% methanol and evaporated to dryness. The hydrogen donating or radical scavenging of the extract was determined using the stable radical DPPH (2, 2-diphenyl-2-picrylhydrazyl hydrate). Total phenol content was determined by the method of Singleton and Rossi using the Folin – Ciocalteau reagent in alkaline medium while total flavonoid content was determined using AlCl3 method as described by Lamaison and Carnet. The proanthocyanidin content was determined using a modified method of Porter et al, using the AlCl / Butan – 1-0l assay method while the total anthocyanin content of the test samples was determined using the pH differential method of Fuleki and Francis as described by Guisti and Wrolstad. Crude protein, Carbohydrate, Ash, Crude fibre, Ether extract (fat) and Moisture contents were determined using the routine chemical analytical methods of Association of Official Agricultural Chemists (AOAC). The chemical compositions analyzed were reduced significantly when fertilizer rate exceeded 100 kg NPK/ha. The proximate values in pumpkin leaves are 23.8, 0.18, 1.65, and 2.16 g/100g for protein, fat, ash and crude fibre respectively. Fertilizer rates from 150 – 250 kg/ha reduced these values by 10 – 40%. With the increase in fertilizer rates from 100 kg – 250 kg/ha, antioxidant activities was reduced by 19%, phenol by 57% and flavonoid by 65%. For the food value and bioactive compounds of pumpkin to be retained, NPK fertilizer application should not exceed 100 kg/ha if it must be applied.

Leafy vegetables are the least expensive sources of protective nutrients such as minerals, vitamins, dietary fibre, protein and antioxidants. They could be readily available throughout the year. Consumption of locally available green leafy vegetables is an antidote to overcome nutritional deficiencies [1]. Consumption of vegetables and fruits has been associated with the prevention of chronic diseases such as cancer and cardiovascular diseases [2, 3]. This protective action is attributed to the presence of antioxidants such as ascorbic acid, α-tocopherol, β-carotene and polyphenolic components such as flavonoids, anthochanin and proanthocyanidin in plants [4, 5].

Pumpkin young shoot locally called “Gboro” is a palatable and highly nutritious leafy vegetable utilized in the south-western Nigeria. It contains about 44% protein [6]. The vegetable does not require pre-processing and hot-water blanching unlike many other leafy vegetables consumed in Nigeria.

            Traditionally, pumpkins are planted in organic-rich dump sites. However with the change from an agrarian society with a subsistence type of agriculture to a modern mechanized society there is a marked increase in the use of fertilizer to enhance crop productivity [7]. Fertilizers are sources of plant nutrients that are added to soil to supply or enhance the soil fertility. They are intended to supply plant needs directly through modification of such properties as soil pH and structure [8]. There is usually a dramatic improvement in both quantity and quality of plant growth when appropriate fertilizers are added.

            Application of fertilizers to soil does not only improve yield but also has significant effect on the quality and bioactive compounds of food crops. The objective of this study therefore was to evaluate the effect of NPK compound fertilizer on the protein, ash, crude fibre, fat, carbohydrate, antioxidant activities and phenolic antioxidant components of pumpkin leafy vegetable.

Field study

Field study was conducted at the Teaching and Research Farm, Obafemi Awolowo Univerity, Ile-Ife, Nigeria for two seasons, (May - August and August – November) in year 2010. Physical and chemical compositions of the soil of the experimental site were determined. The experiment was a randomized complete block design consisted of 6 rates of NPK 15:15:15 fertilizer at 0, 50, 100, 150, 200, 250 kg/ha. The experiment was replicated six times, there were 6 plots per replication and each plot size was10 m X 12 m and consisted 7 rows. Young shoots of pumpkin biomass was harvested at 6 weeks after planting. The dry matter was obtained by drying the samples to constant weight in the oven at 70⁰C for 24 hours. The dry matter at 6 weeks was then milled and stored in the refrigerator. Six composite samples from the six replications were analyzed in duplicates for proximate and antioxidant compositions.

Laboratory analyses

From the composite samples, 5 g were weighed into different vials and 15 ml each of 80% methanol was added and vortexed. The cold extraction, that is extraction not involving heat, took place for 24 hours. The crude extract was obtained by evaporation of the methanol soluble extract to dryness. The hydrogen donating or radical scavenging of the extract was determined using the stable radical DPPH (2, 2-diphenyl-2-picrylhydrazyl hydrate) according to the method described by Brand-Williams [9] DPPH reacts with an antioxidant compound which can donate hydrogen, it is reduced. The change in colour from deep violet to light yellow was measured spectrophotometrically at 517 nm. Total phenol content was determined by the method of Singleton and Rossi [10], using the Folin – Ciocalteau reagent in alkaline medium. Total lavonoid content was determined using AlCl₃ method as described by Lamaison and Carnet [11]. The proanthocyanidin content was determined using a modified method of Porter et al., [12], using the AlCl / Butan – 1-0l assay method. The total anthocyanin content of the test samples was determined using the pH differential method of Fuleki and Francis [13], as described by Guisti and Wrolstad [14]. Crude protein, Carbohydrate, Ash, Crude fibre, Ether extract (fat) and Moisture contents were determined using the routine chemical analytical methods of Association of Official Agricultural Chemists (AOAC) [15]. All data were subjected to combined analysis of variance SAS [16]. Means squares, where significantly different, were separated using Duncan Multiple Range Test (DMRT) at 5% level of probability.

Fertilizer influence showed that proximate values of protein, fat, ash and crude fibre in pumpkin young shoot were similar at 0, 50 and 100 kg NPK/ha. The nutrient values reduced significantly when compared to the control from the application of 150 kg/ha of NPK and this continued to the highest fertilizer rate (250 kg NPK/ha). The carbohydrate result was contrary to others. The values increased with increased fertilizer rates. Carbohydrate content in pumpkin young shoot was highest with the application of 200 and 250 kg/ha (Table 1). Seasonal influence showed higher nutrient values during the early season than in the late season except for carbohydrate (Table 2). Variation between the nutrient values range from 3 to 17%. Protein was 8.5% higher in early season while carbohydrate in late season was higher by 3% than in the early season. The influence of season x fertilizer interaction on protein showed that the nutrient concentration was higher across fertilizer rates in the early season (Figures 1). The crude fibre contents of pumpkin young shoot was higher in early season than the late season. The values were significantly lower than in the control when fertilizer application was above 100 kg/ha both in the early and late seasons (Figures 2). The carbohydrate was higher in early season than in the late season. The values were constant with fertilizer rates below 150 kg/ha but this increased sharply at higher rates (Figure 3).

            Fertilizer influence showed consistent decreased in values of antioxidant activities and its components as fertilizer rates increased. The control had a significantly higher values than the values from the application of 150 to 250 kg/ha. The addition of 50 and 100 kg gave similar antioxidant values with the zero fertilization at 0.05 probability level (Table 3).

Seasonal variation revealed higher values during the early season in all the determined profiles when compared to the late except for anthocyanin. The values in flavonoid were similar in both seasons (Table 4). The significant (season x fertilizer) interaction on flavonoid concentration showed that early season had higher values than late seasons across fertilizer rates. The concentration at both seasons was similar between the control, 50 kg and 100 kg fertilizer, while significant reduction in values was found at 150 kg/ha fertilizer rate and above (Figure 4).

A lot of attention has been focused on the use of vegetable and fruits in the supply of protein, vitamins and antioxidants that are of benefit to human health. The protein from pumpkin young shoots range between 21-24g/100g. The protein value at control and 100 kg NPK/ha fertilizer rates (24 g/100g) was higher than at higher fertilizer rates. This value is comparable or higher than some vegetable crops like Amaranth on dry weight basis (5.9g/100g), Celosia argentea (5.8g/100g), Cowpea leaves (4.7g/100g), Cabbage (1.6g/100g), Carrot (1.0g/100g), Fluted pumpkin leaves (4.3 g/100g) [17, 18].

The free radical scavenging capacity/antioxidant activities of “Gboro” under control and lower NPK rates was 95%. This high value is similar to those of Basella alba (88.9%), Crassocephalum crepidioides (88.9%), Celosia argentea (90.2%), Talinum triangulare (88.9%), Piper guineese (90.1%) and Amaranthus caudatus (88.9%) [19].

It has been reported that about 29 g of protein is required by adults daily [20], it means that pumpkin leafy vegetable can easily meet the protein required by an adult per day. The plant protein produced by pumpkin leafy vegetable undoubtedly will contribute significantly to the mitigation of protein deficiency in the nutrition of people in developing countries. Though, some essential amino acids are not present in high concentration in plant protein (e.g. Methionine, Lysine, Thiamine), the % crude protein from pumpkin leafy vegetable is still comparable to that of some animal products. For example, animal protein per 100 g in pork is 14.5 g, Chicken is 20.6 g, Beef is 17.9 g, Egg is 12.1 g while fish (sardine) gives 19.6 g [18].

Early season leafy vegetables had higher protein compared to the late season while the antioxidant activities was also higher in the early season. This finding was similar to (Trichosanthes cucumerina L.) as observed by Oloyede and Adebooye [21]. Also, Konova and Rainova [22] reported that soybean that received less water had higher crude protein and lower pH since high moisture causes dilution in the cell plasma and this retarded protein forming enzymes and eventually reduce protein synthesis.

Beyond 100 kg/ha of NPK fertilizer application, food values and health benefits of pumpkin leafy vegetable reduced. Oloyede and Adebooye [21] found out that there was reduction in fruit proximate composition of snake tomato (Trichosanthes cucumerina L.) belonging to the same family of pumpkin- Cucurbitaceae, as the concentration of phosphorous application increased. Increase in biomass due to breeding or improved cultivation methods have decreased nutrients and metabolites in plant. Decreased levels of N have been positively correlated with production of plant defense compounds such as phenolics and flavonoids [23, 24]. The present study confirmed that excessive inorganic fertilizer decreased nutrient composition and antioxidant profile in pumpkin leafy vegetable.

The result of this study shows that pumpkin leafy vegetable does not require fertilizer for optimal proximate and antioxidant profiles. However, if fertilizer has to be applied, the application of 100 kg NPK/ha is sufficient for the antioxidant concentration and the proximate content not to be affected negatively. Exploiting this bioactive and protein rich pumpkin at 6 weeks after planting to alleviate national food and nutrition insecurity is paramount.