Dry and fresh spinach (Spinacia oleracea L.) is considered to be the one of the extremely nutritious vegetables, rich both in phytochemicals and core nutrients. Nowadays, phytochemicals in plants are raising interest in consumers for their roles in the maintenance of human health. Variation in content of bioactive compounds and core nutrients is the main concern in vegetable production. Factors such as cultural practices specially fertilization, may affect the nutritional and medicinal properties of the plants

Therefore, three parallel trials for NPK to investigate the response of dry and fresh spinach leaves to nitrogen (N), phosphorus (P) and potassium (K) on chemical composition were conducted, with treatments arranged as follows: 0, 45, 75, 105, 120 kg·ha-1 N and P and 0, 60, 85, 106, 127, 148 kg·ha-1 K in a randomized complete block design (RCBD) with four replications. The results demonstrated that, application of nitrogenous, phosphorus, potassium fertilizers significantly increased the total phenolic content, total antioxidant activity, total flavonoid content and vitamin C while magnesium, iron, zinc and selenium did not exhibit significant response to all treatments applied. The increase in concentrations on total phenolic content, total antioxidant activity, total flavonoid content and vitamin C was observed, reaching maximum at 45 kg·ha-1 N, 75 kg·ha-1 P and 85 kg·ha-1 K.  The optimum rates of 45 kg·ha-1 N, 75 kg·ha-1 P, 85 kg·ha−1 K were then used to formulate the NPK treatment combinations as follows: 0, 30: 30: 40, 45:45:60, 60:60:70, 75:75:90 kg·ha-1,arranged in a RCBD with three replicates. The results showed that total phenolic content, total antioxidant activity , total flavonoid content and vitamin C reached maximum in dry and fresh spinach leaves at N45:P45:K60 kg·ha-1.


Keywords: dry and fresh spinach, bioactive compounds, minerals, NPK nutrition, concentrations, variations, phytochemicals, fertilizer, response, quality. 



1.1       BACKGROUND

Spinach is a leafy and extremely nutritious vegetable, rich both in core nutrients and phytochemicals. It is a vegetable that is provided fresh, frozen or canned to the consumer. Spinach harvested after a shorter growth period than normal is called dry and fresh spinach, and is marketed fresh to the consumer. This is a fairly new product that has turned out to be popular in recent years because of its nutritional value (Hedges and Lister, 2007).  Its nutrients comprise a range of vitamins and minerals, as well as phytochemicals. The major micronutrients in spinach are vitamins A (from β-carotene), C, K and folate, and the minerals, calcium, iron and potassium. The phytochemicals of most importance are the carotenoids, flavonoids, β-carotene, lutein and zeaxanthin and phenolic compounds (Bergquist, 2006). A number of studies have shown spinach to have strong antioxidant activity and high levels of antioxidant compounds such as phenolics and carotenoids (Hedges and Lister, 2007).


Agronomic practices such as chemical fertilizers have made major contributions to improve crop yields and food nutrition (Fageria, 2009; Wang et al., 2008). There are many contributing factors in terms of fertilizer application that can influence the effectiveness of fertilizer such as the application method, application timing and the rate of application. All these factors must be investigated before fertilizer application recommendations to ensure effectiveness and to prevent problems associated with fertilizers. Problems such as under application and over application of fertilizers are seen to be challenging. On the other hand, excessive fertilizer application can have detrimental consequences. It can have adverse environmental effects on water quality, leaching, and runoff (Heckman, 2007; Heckman et al., 2003; Manotti et al., 1994; Sims, 1998; Sims et al., 1998). Consequently, it is imperative to investigate and determine fertilizer application rates that are beneficial and maximize the yield while minimizing environmental contamination (Fontes et al., 1997; Heckman et al., 2003). The efficient use of fertilizers and optimal fertilizer management of nitrogen, phosphorus and potassium are necessary to minimize production cost and to improve yield and quality (Fageria, 2009).


Some studies are available on the identification of bioactive compounds in fruits and vegetables, pre-harvest and postharvest factors.  However, information on the effect of mineral fertilization on nutritional quality of dry and fresh spinach is scarce. As the consumption of fresh-cut spinach increases, it is important to investigate the good management practices, specifically fertilization and its effects on quality of dry and fresh spinach. The consumption of spinach is increasing because people are becoming health cautious and spinach is regarded as one of the healthy vegetables which is rich in nutrients. The purpose of the present study was to investigate and quantify the variations on different bioactive compounds, namely, antioxidants, flavonoids, total phenolic content and carotenoids, as well as the trace elements (magnesium, zinc, iron and selenium), in dry and fresh spinach with reference to mineral nutrition. 



Spinach is an important agricultural crop, not only because of its economic importance, but also for the nutritional values of its leaves, mainly due to the fact that they are an excellent source of nutrients and phytochemicals.  Spinach is increasingly becoming important in health because of its micronutrients and phytochemicals. It has an additional advantage of being low in calories, which is very important in weight management. Therefore, it is becoming a food of choice to many people because of its nutritional importance.  Nowadays, phytochemicals and antioxidants in plants are raising interest in consumers for their roles in maintaining human health. Phenolics and flavonoids are known for their health promoting properties due to protective effects against cardiovascular disease, cancers and other diseases (Kaur and Kapoor, 2001; Sardas, 2003)


The intake of these compounds in foods is an important health protecting factor. They have been recognized as being beneficial for preventing widespread human diseases, including cancer and cardiovascular diseases, when taken daily in adequate amounts (Kaur and Kapoor, 2001; Sardas, 2003). It is generally known that environmental factors and agricultural techniques have an effect on vegetable and fruit quality. In particular, mineral fertilization influences antioxidant composition in some fruits and vegetables (Jeppsson, 2000; Kaur and Kapoor, 2001; Kopsell et al., 2006; Kemal et al., 2007).  Some studies are available on the effect of postharvest storage and processing on the antioxidant constituents (Flavonoids and vitamin C) of Fresh-Cut spinach (Bergquist, 2006). However, information on the effect of mineral fertilization on nutritional quality of spinach is scarce. Variable content of bioactive compounds and minerals is the main problem in the production of vegetables, due to many factors especially fertilization. Consequently, the nutritional and medicinal value of vegetables may be affected. Producers often apply large quantities of fertilizer in an attempt to maximize yield and advance growth, which can result in the accumulation of nitrate in case of excessive nitrogen, reducing the quality of some vegetables. There are no reports about the content of bioactive compounds and minerals in dry and fresh spinach grown on the principles of best management practices in South Africa. 


Currently the demand of dry and fresh spinach exceeds the supply and the applications of 2245Nkg/ha, 22-45Pkg/ha and 63-138Kkg/ha have been reported in California, USA. These are the application rates from the growers since there is no scientific validation of these rates. Limited studies are available on the effect of nutrients on bioactive compounds of dry and fresh spinach and data that provide recommended rates such as N, P and K on quality have not been well established under South African conditions.  


Therefore, it was important to investigate the variation in concentrations of bioactive compounds (antioxidants, flavonoids, carotenoids and total polyphenols) and trace elements (magnesium, iron, zinc and selenium) in dry and fresh spinach with reference to mineral nutrition. The main aim of this study was to investigate the effect of fertilization (N, P and K) dry and fresh spinach on their bioactive compounds. Additionally, any information on the physicochemical properties and bioactive compounds of dry and fresh spinach will provide a knowledge base that may be of some benefit to the production of dry and fresh spinach in South Africa.



Spinach is one of the world's healthiest vegetables. Rich in vitamins and minerals, it is also concentrated in health promoting phytonutrients such as carotenoids (betacarotene, lutein, and zeaxanthin) and flavonoids to provide powerful antioxidant protection (Mehta and Belemkar, 2014).  Leafy green vegetables such as spinach provide more nutrients than any other food, when compared calorie for calorie (Longnecker et al., 1997).  Its nutrients include a range of vitamins and minerals (micronutrients), which can prevent deficiency diseases and are essential for normal physiological function, as well as phytochemicals thought to help prevent chronic health problems such as cancer and heart disease, as well as other health problems  associated with ageing (Hedges and Lister, 2007). 


In recent years, considerable attention has been directed towards identifying natural antioxidants, namely those plant-derived that may be used for human consumption regarding health promotion and disease prevention. It is vital to determine a method of how we could grow healthy vegetables vegetable so that human body could have a higher intake of health promoting bioactive compounds. This fact is very important in these days when people are exposed to lot of health related challenges. This study addresses such areas where the effect of mineral fertilization can be related to the concentration of health promoting phytochemicals.


One of the advantages of spinach is that it is readily available, just about all over the world and is easy to prepare. What is more interesting about spinach, it is easy to find in the market or easy to grow and prepare. With its fine taste, dry and fresh spinach is a versatile food that can be easily included into a range of dishes. It can be eaten raw in a salad or it can be cooked (steamed or boiled) and eaten as a dish on its own or added to soups and other dishes. Unlike other vegetables, it is harvested after a short period after planting. Over the years, ready to use fresh-cut spinach has become more popular as one of the important dietary vegetable (McGill et al., 1966; Burgheimer et al., 1967; Izumi et al., 1997).

Dry and fresh spinach was chosen for this study for two main reasons. It is becoming an increasingly popular product in South Africa and elsewhere in the world. Spinach is known to be a healthy product and contains relatively high concentrations of bioactive compounds (USDA, 2005). In addition, dry and fresh spinach has the advantage of a short culture time and shelf life, making it an excellent model crop. The objective of the study was to investigate the response of chemical composition and trace elements in dry and fresh spinach to fertilization with N, P, and K nutrition.  



The aim of this study was to investigate and determine the response of total phenolic content, total antioxidant activity, total carotenoid content, total flavonoid content and vitamin C as well as minerals to fertilization with nitrogen (N), Phosphorus (P) and potassium (K) nutrition. 



1.5.1 To determine the effect of different rates/levels of application of N, P, and K fertilizers on chemical composition and minerals in dry and fresh spinach.


1.5.2 To determine the interaction effects of NPK on the bioactive compounds in dry and fresh spinach.



1.6.1 Rate of applications of fertilizer N, P and K have no influence on the bioactive compounds in dry and fresh spinach.


1.6.2 No interaction effects on bioactive compounds in dry and fresh spinach after NPK application.



Given the research aim and objectives of the study delineated above, the scope of the study entailed three aspects – rate of applications of fertilizer N, P and K, the variation in bioactive compounds in dry and fresh spinach as well the combined effect of N, P and K on the bioactive compounds. 


However, the study had a set of limitations inherent in the topic investigated. Firstly, not all varieties of dry and fresh spinach could be investigated for this research, given the nature of the research methods used and the time-frames selected. Secondly, the study was conducted in a protected environment and the effect of this was not taken into consideration in this study. Lastly, only one method of fertilizer application, side dressing was used in this study. 



No ethical clearance of this study is required (Appendix C).



The thesis is presented in five chapters. Chapter 1 provides the research background on dry and fresh spinach, the importance and research problem of the study. An outline of brief research motivation, aim and objectives, scope and limitations of the study is also discussed. Chapter 2 consists of a review of the relevant literature pertaining to dry and fresh spinach. Chapter 3 provides a brief description of study areas, research design and methods, as well as data collection and analyses. Chapter 4 deals with set of research findings, with particular reference to the mineral nutrition and bioactive compounds in dry and fresh spinach. The chapter details the analyses and discussion of the variation in concentrations of bioactive compounds in dry and fresh spinach. The thesis ends with Chapter 5 which gives a summary of research findings, concluding remarks and recommendations which set goals for further research.

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Item Type: Project Material  |  Size: 88 pages  |  Chapters: 1-5
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