Influence of Urea on Some Growth Responses and Nutrients of Spinach (Spinacia oleracea L.) under Different Levels of Soil Salinity

Salinity is an environmental problem in the world, especially in arid and semi-arid regions. High amounts of salts like sodium chloride (NaCl) in the soils and water have destructive effects on yield of plants. The harmful effects of salinity on plant growth are related to the low osmotic potential of the soil solution (water stress), the nutritional imbalance, the specific ion effect (salt stress), or the combination of these factors. The relationship between salinity and plant mineral nutrition is complicated. Under salinity stress, occurs the sodium and chlorine accumulation, resulting in ionic imbalance and the deficiency symptoms of nutrients in plants. The sodium (Na+) competes with the uptake of potassium (K+), calcium (Ca2+) and magnesium (Mg2+) by plant, and the chlorine (Cl-) with the uptake of nitrates (NO3-), phosphates (PO43-) and sulfates (SO42-).
In order to evaluate the tolerance of spinach cv. “virofly” to salinity levels in application with different nitrogen rates, a greenhouse experiment was conducted as completely randomized design based on factorial arrangement with three replications at Shiraz University Agricultural Faculty. Treatments include four levels of salinity (without salinity, 1, 2 and 3 gr of sodium chloride per kg of soil, equivalents to 0.7, 4.5, 8 and 11.5 dS/m in saturated solution extract of soil, respectively), and five levels of nitrogen (unfertilized, 75, 150, 225 and 300 mg N/kg of soil) as urea source. Nitrogen treatments were applied in two installments in water soluble (half before planting and another half, 20 days after planting). In order to prevent sudden stress, saline treatments were applied gradually after complete plant establishment with irrigation water. The irrigation of the pots was carried out with distilled water and at field capacity. After 56 days of sowing, in every pot the spinach shoots were discarded near the surface of the soil and the required parameters were measured.
The application of 4.5 and 8 dS/m salinity had no significant effect (≤0.05) on the relative yield and spinach leaf area, but 11.5 dS/m salinity significantly (≤0.05) decreased relative yield and spinach leaf area compared to without salinity level, 4.5 and 8 dS/m. Nitrogen application (75 and 150 mg/kg of soil) alleviated negative effect of salinity on yield and leaf area. Application of 225 and 300 mg N/kg of soil with 11.5 dS/m salinity significantly decreased the relative yield of spinach. The highest and lowest shoot water content changes in salinity conditions were observed in no-nitrogen application and 150 mg N/kg application, respectively, which shows that the application of nitrogen in the medium level controls the water changes in the spinach plants. In this study, increasing the amount of nitrogen at all levels of salinity, elevated the shoot water content. The tolerant plant species in the face of environmental stresses maintain the water content of their cells in the higher levels. Therefore, it can be said that the maintenance of high leaf water content is an important mechanism for tolerance to salinity, and the cultivars that can hold more water in their leaves under stress conditions, will have more tolerance to salinity stress. Linear regression (R2 = 0.8198) showed that in the salinity levels of 4.5 to 11.5 dS/m, there is a negative relationship between the yield and the chlorine to nitrogen ratio (Cl/N) of spinach shoots, so that with increasing Cl/N, the spinach shoot yield decreased by gradient of -3.077. Application of nitrogen up to 225 mg/kg of soil gradually reduced the ratio of K/Na, Ca/Na and Mg/Na, however, the application of 300 mg N/kg of soil had no significant effect on these ratios. The application of different levels of salinity gradually reduced the K/Na, Ca/Na and Mg/Na ratio.
The threshold of salinity of spinach cv. “virofly” was about 8 dS/m in our study, this was above the threshold mentioned (2 dS/m) for spinach in most sources. The application of nitrogen in medium level as urea can improve the negative effects of salinity in spinach but intensive nitrogen fertilization may increase the negative effects of salinity on plant yields.