The Effect of Planting Date on Yield and Nitrate Accumulation of Spinach in Esfahan Weather Conditions

The fresh, steamed and boiled spinach were used because of antioxidant compounds that have great biological value. This plant is usually rich in nitrates that can play an important role in reducing heart attacks. Each year, more than 30% of the area under cultivation leafy vegetables in Iran, was allocated to spinach. Spinach growing in both spring and autumn or summer, has poor quality due to stem production. Higher spinach yield have been reported in autumn cultivation (compared to spring or winter planting). The nitrogen-rich lands, little light condition, excessive use of nitrogenous fertilizers and plants during the growing season short cause some plants like spinach, are faced with the problem of nitrate accumulation. The accumulation of nitrate in vegetables will be different depending on the species and genetic differences.
This research was conducted for two years in Kabootarabad Agricultural Research Station of Isfahan in order to determine the most suitable planting prickly seed spinach. Each year, of experiment was based on randomized complete block design with four replications and the results were reported based on two-year combined data analysis. Five planting dates at intervals of 15 days from the beginning of September had been adjusted. Prickly seed spinach was prepared from Seed and Plant Improvement Institute. In full leaf development stage and before the harvest of each plot, 10 plants randomly were selected and measured attributes including number of leaves, leaf length, leaf width and length of the petiole. In order to assess yield, two central rows of plants per plot (surface equivalent to two square meters) were analyzed after removing two distance of half a meter from each side. Nitrate samples were analyzed using ion analyzer. Statistical analyses of the data, correlated traits were performed using SAS software and comparisons of means using LSD at 5%.
Result and According to the results, year had no significant effect on traits. 5 September sowing date, GDD=571 was diagnosed as the best spinach planting date yielding 43 tons per hectare. Delayed planting date of 5 September to 22 October showed significant yield reduction and the amount of nitrate accumulation in plants also increased linearly. Number of leaves can be changed as one of the main factors influencing the performance of spinach according to cultivar and environmental conditions and in some studies they were ranged from 12 to 23. Amounts of nitrate accumulation in spinach were 2675, 2898, 3189 and 3571 ppm of fresh weight in different planting date of 5 September (571 GDD), 22 September (354 GDD), 6 October (193 GDD) and 22 October (84 GDD), respectively. The different sowing date, petiole nitrate accumulation varied from 3513 to 4680 ppm. Nitrate accumulation was ranged from 2135 to 3125 ppm wet weight in leaf blade. Different planting dates can influence on the length of day and intensity of light and thus can affect the amount of nitrate in plant. After planting date of August 22, temperature and radiation also reduced, and therefore it is natural that reduce the activity of the enzyme nitrate reductase and be more prepared conditions for nitrate accumulation. According to the World Health Organization, the uptake of nitrate and nitrite per kg of body weight daily allowance, equal to 0.06 and 0- 7.3 mg has been reported. The quality of light received by the plant also had an effect on nitrate accumulation, and red light compared to blue light has a greater role in stimulating the enzyme nitrate reductase activity and thus is more effective in reducing nitrate accumulation.
Climatic conditions affect the quantity and quality of vegetative organs. The results of this study showed that unsuitable spinach planting date will reduce not only yield, but also increases the concentration of nitrate (over 2500 ppm of fresh weight) and it may causes risks for consumers.