Study of nitrate concentration in Varamin plain leafy vegetables and evaluation of its risk for human

Introduction:
Among the plant nutrients required, nitrogen is the most limiting plant growth factor and the key nutrient for achieving optimum yield in most crops, especially in arid regions (Seilsespour and Momayezi, 2005). Therefore, the most widely used fertilizer among the fertilizers is nitrogen-containing fertilizers. Nitrogen is present in the structure of amino acids, nucleic acids, purine bases, alkaloids and chlorophylls. The amount of nitrogen needed by plant depends on the species of plant and soil. It is absorbed by the plant in the form of ammonium and nitrate. Plants mainly prefer nitrate ions for uptake (Dordas and Sioulas, 2008). Nitrate in the human body is converted to nitrite and, in combination with amines, is converted to nitrosamine, which is one of the main causes of cancer. In the neonatal stomach, nitrate is resuscitated to nitrite and, in the bloodstream, oxidizes hemoglobin iron and converts divalent iron to trivalent iron. Thus, hemoglobin pigments disrupt oxygen delivery to the infant's body and eventually lead to choking and death of the infant (Fewtrell, 2004). Nitrate accumulation in vegetables is often dependent on the amount and type of nutrients in the soil and is closely related to the amount and timing of fertilizer use, so that the amount of chemical fertilizer applied is one of the factors affecting it. Vegetable and vegetable products, including leafy vegetables, are of great importance in many countries because of their high nutritional value (Menard, 2008). Vegetables have been suggested by the World Health Organization to prevent no communicable diseases such as cardiovascular disease, cancer, obesity, and type 2 diabetes (FAO / WHO, 2003). However, despite the importance and nutritional value of vegetables, research has shown that about 80% of the nitrates that enter the human body are from vegetables and fruits (Kiani and Gheytasi, 2016). Due to the adverse effects of nitrate on human health, much attention has now been paid to the accumulation of this ion in vegetables and has been considered as a qualitative biological indicator in vegetables (Chen et al., 2004). Due to the lack of information on the nitrate concentration of leafy vegetables produced in the major vegetable production areas of Varamin plain, this study was conducted to determine of nitrate concentration in leafy vegetable and  evaluation of  nitrate risk assessment for vegetables consumers.
Materials and methods:
To investigate the nitrate concentration in leafy vegetables, this study was conducted in 2014 in the agricultural lands of Varamin district (Varamin, Pishva, Pakdasht), which are the hub of vegetable production in Tehran province. The leafy vegetables studied were savory, parsley, coriander, fenugreek, leek, cress, dill, mint, cabbage, celery, spinach and lettuce. Sampling was done for each crop from 30 different farms.. Di azo method (Emami, 1996) was used to determine nitrate concentration. The mean nitrate concentration in each leafy vegetable (at 30 field level) was compared with the maximum permissible nitrate level (National Standard No. 16596) by means of T-test. According to the World Health Organization (Human Health Fact Sheet, 2005) guidelines, which set the daily nitrate uptake for humans to be 3.6 mg.kg^-1 body weight, the maximum daily allowable nitrate uptake through each leafy vegetable for a person with 80 kg weight was calculated.
Results and discussion:
Among the leafy vegetables, lettuce with average 3909 mg­/­kg fresh weight had the highest accumulation of nitrate and fenugreek with an average of 641 mg/kg fresh weight had the lowest nitrate accumulation per kg. The mean nitrate concentration of each vegetable was compared by T-test with maximum nitrate concentration for each vegetable. In all samples of lettuce, spinach and celery studied, nitrate concentration was significantly higher than the maximum, while in all fenugreek and coriander samples, nitrate concentration was significantly lower than Maximum allowed. Considering the maximum allowable daily intake of nitrate for humans (3.86 mg­/­kg body weight) and the average nitrate content in the studied vegetables, the maximum daily intake of each vegetable species were calculated. The results showed that daily consuming of  76 grams of  lettuce or 94 grams of spinach by a person with 80 kg weight, would maximize the amount of nitrate in the body and increase the risk to health.
Conclusion:
The results of this study showed that some of the leafy vegetables such as lettuce and spinach contained high levels of nitrate base on the national standard of Iran. The average nitrate concentration in the leafy vegetables was 1458 mg­/­kg fresh weight. Maximum daily uptake of nitrate for adults is about 190 mg and more uptake, can cause harmful on the body. It is worth noting that this issue is more important for children. Therefore, it seems that the use of nitrogen fertilizers should be revised in the leafy vegetables fields. In this regard, nitrogen fertilizer application based on soil test and soil nitrate is recommended.