Effect of irrigation water salinity and nitrogen rates on growth, chemical composition and salinity stress indices of Valencia orange cultivar

Since the effect of nitrogen rates, higher than optimum levels on salinity adjustment in citrus has not been studied, the interaction effect of irrigation water salinity and nitrogen on growth, salinity tolerance and mineral composition of Valencia orange cultivar were studied in a 4Χ5 factorial experiment arranged in completely randomized design with three replicates. Treatments consisted of four irrigation water salinity (control, 2, 4 and 6 dS m-1) and five nitrogen levels (50, 80, 120, 160 and 200 mg N kg-1 soil as ammonium sulfate). Results showed that as the irrigation water salinity increased, sodium and chloride ions concentration in plant leaf and root were increased to toxic levels. Subsequent result of this event was the significant reduction in chlorophyll content and plant fresh and dry weight of leaves. Although nitrogen had no significant effect on sodium concentration of sodium in plant shoot and root, increments in nitrogen application made a considerable decrease in leaf chloride concentration. In each level of applied nitrogen, increase in irrigation water salinity significantly decreased the concentration of plant nitrogen. In most salinity treatments, nitrogen concentration reached to amounts which were below optimum levels. This reduction can probably be attributed to the restricting influence of chloride on nitrate nitrogen uptake and metabolism which in turn indicates that under salinity condition, decrease in nitrogen uptake is on of effective factors in plant growth reduction. In addition to nitrogen, salinity decreased phosphorus, potassium, iron, manganese, zinc and copper concentration in plant leaf. In some cases these concentrations reached to sub optimal levels which are considered inadequate for optimum plant growth and development. Nitrogen usually improved the decreasing effect of salinity on concentration of plant essential nutrients by increasing their concentration in plant tissues. However, results of this experiment shows that under salinity stress, the plant nitrogen nutrition must be received more attention and also it can be concluded that nitrogen rates, higher than optimum levels, can be utilized to alleviate salinity stress.