الفهرس 
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Abstract
Two field experiments were conducted in the Experimental Farm of Tag Elez Station; Dakahlya Governorate, during the two successive winter seasons of 2014-2015 and 2015-2016 to find out mainly: The effect of different N doses including higher and lower rates than the recommended one by the Ministry of Agriculture and Soil Reclamation (MASR) on the nitrate accumulation and oxalate formation of spinach crop. The best N dose which produce vegetables with oxalate, nitrate and nitrite at the permissible limits. The effect of applying Fe and Mo in foliar way on decreasing oxalate, nitrate and nitrite content under the high dose of nitrogen due to its importance in the activity of nitrate reductase enzymes in plants.
Twenty four treatments were arranged in split plot design with 3 replicates, to evaluate the combination effect of: Six levels of N-fertilizer {0, 50, 75, 100, 125 and 150 %} from the recommended dose by the Ministry of Agric. and Soil Recl. (MASR) in the form of NH4 NO3 (33.5% N) as soil application (main plots). Micronutrients in foliar way were arranged in (sub plot) in four treatments including, control treatment, Fe (300 ppm), Mo (100 ppm) and the mixture of (Fe + Mo) at the same rates under study.
1. Vegetative growth parameters: The average values of all the investigated growth parameters for spinach plant were significantly increased as the rate of N-fertilization was increased tell the rate of 100% N. The best plant growth parameters were obtained by spinach plant treated with Fe+Mo followed in descending order by that supplied with Fe and lately Mo. The lowest levels for the previously mentioned traits were recorded for the untreated plants.
2. (N, P and K %): The highest mean value of N % in the leaves of spinach plant was obtained from the plant treated with N-fertilization at the rate of 150% RD, while such effect for P and K % were realized at the rate of 100%N in single form. For all the investigated characteristics (N, P and K %); a superiority effect on the average values of (N, P and K %) was happened due to the application of Fe+Mo in foliar way which recorded the highest mean values compared with the untreated plants.
3. Plant pigments: A stimulation effect was happened on the mean values of chlorophyll (a, b & total mg g-1) and carotene mg 100g-1 FW due to an application of nitrogen fertilization as ammonium nitrate at 100% (RD) leading to record the highest mean values for both of them. The average values of chlorophyll and carotene contents for the plants treated with (Fe+Mo) were more than that obtained for the treatments of Fe, Mo as well as the untreated plants and this trend was significant during both seasons of the experiment.
4. (Fe and Mo ppm): Using the investigated levels of N- fertilizer significantly increased the mean values of Fe and Mo tells the rate of 100% RD. The highest values of Fe were connected with the plants treated with Fe as foliar addition, while such effect for Mo was realized for the treatment of Mo as foliar application.
5. Nitrate, nitrite and nitrate reductase enzyme activity: The highest values of nitrate and nitrite accumulation were associated with the plants treated with N-fertilizer at the rate of 150% RD, while such effect has been recorded the lowest level of nitrate reductase activity. While the lowest values were connected with the plants treated with Fe+Mo. Furthermore, the highest N.R activity was also, influenced by the application of Fe+Mo by foliar way.
6. Soluble, insoluble and total oxalate mg 100g-1 FW: There were significant increases in soluble and total oxalate mg 100g-1 F.W recorded with ammonium nitrate at 100% of recommended dose, while the highest values of insoluble oxalate mg 100g-1 F.W was recorded of 50% of nitrogen fertilizer in both seasons. The foliar application of Fe+Mo on spinach plants significantly decreased the average values of insoluble oxalate mg 100g-1,.
7. Total phenol, VC mg 100g-1 and total soluble solids (TSS %): a higher amount of VC mg 100g-1 FW and TSS% of spinach plant were accumulated due to an addition of ammonium nitrate at the rate of 100% to soil, while total phenol significantly decreased as the level of N-fertilizer was increased compared with the untreated plants. Treating spinach plant with Fe+Mo as foliar way significantly increased the average values of the previously mentioned traits than those obtained from the control treatment.
CONCLUSION:
Under the same conditions of this experiment it can be concluded that, applying N fertilization in doses higher than the recommended one (100%) for spinach plant gave increases in the fresh yield and nutrients content but at the same time increased oxalate, nitrate and nitrite accumulation over the safe limit for human health. This means that, the soil supplying power for Fe and Mo was not enough to supply those elements to the studied vegetables under high doses of nitrogen.
The foliar application of Fe and Mo to the studied spinach crop decreased oxalate and nitrate accumulation under the high N fertilization rates which may be attributed to the increase in the formation and/or the activity of nitrate reductase enzymes which reduced NO3-N to NH4-N leading to the formation of amino acids and protein.