Abstract
 
Background and Objectives
Khuzestan Province is one of the most important areas that provide vegetables throughout winter and spring in Iran. The vegetables are mainly picked up in the afternoon, so it cannot be supplied to the market. In this time of the day, the minimum amount of nitrate is accumulated in leafy vegetables, with a gradual increase throughout the night. On the other hand, its highest level is in the early hours in the morning. The purpose of this study is to evaluate nitrate and nitrite accumulation in vegetables produced in different areas of Ahwaz during winter and spring.
 
Materials and Methods
This experiment was conducted in a randomized complete block design arranged in a split-split-plot design with three replications in the Department of Horticulture, College of Agriculture, Shahid Chamran University of Ahvaz. In the treatments viz. first to third harvest dates (1): February 6, 2016, (2):March 4, 2016 and (3): April 7, 2016 respectively were the main factors and nine vegetable species (including coriander, parsley, mint, fenugreek, chard, spinach, leeks, dill and watercress) kept in sub plot, three vegetable production areas in Ahvaz were arranged in sub plot in sub plots and three fields were selected in each of these areas. Before and after cultivation, all planting, harvesting and harvesting operations were carried out on the same field in the selected fields. The concentration of nitrate was measured by the method introduced by Cataldo et al. (1975) and the nitrite concentation was measured via the method of Abu-Dayeh (2009). The data were analyzed using SAS software and the mean comparison was calculated using Duncan's multiple range tests at a probability level of 5%.
 
Results
Nitrate and nitrite contents of the produced vegetables were different depending on various regions, harvesting time and vegetable species. The highest nitrate content was observed in parsley fromthe first harvest in the southern region of Ahwaz (4.4 mg/kg^-1 dry matter) and the lowest amount of nitrate was recorded in watercress from the third harvest in the northern region of Ahvaz (1.4 mg/kg^-1 dry matter). Also, the highest nitrite content was observed at the first harvest in the westhern region of Ahwaz in mint and the lowest amount of nitrate was recorded in the second harvest in the northern region of Ahvaz in chard. In winter, the environmental conditions in Ahwaz are suitable for the growth and development of leafy vegetables (Boroujerdnia et al., 2007); however, soil conditions provide manganese uptake by plants. Moreover, high light intensity is another factor which can decrease nitrate in leafy vegetables. The number of cloudy days in Ahwaz is very low; therefore, the number of sunny days with high radiation intensity is high in winter. This phenomenon greatly reduces the content of nitrate in leafy vegetables.
 
Discussion
The amounts of nitrate and nitrite content in various species of vegetables produced in three regions of Ahvaz during the cold season were much lower than the standard figures. Therefore, we can conclude that the quality of vegetables produced in Ahvaz is very high in terms of nitrite and nitrate contents.