Growth, Nutrient Status, and Biochemical Changes of Sour Orange Plants Subjected to Sodium Chloride Stress

Sour orange is a valuable citrus rootstock. It is characterized as salt tolerant, based mainly on the mineral content of the leaves and roots under saline environment and to a lesser extent based on any biochemical indices. Therefore, the aim of the present experiment was to study both nutrient content and biochemical changes involved in this rootstock's tolerance. One-year-old sour orange (Citrus aurantium L.) trees growing in 5-L pots were subjected to four levels of sodium chloride salinity stress (control, 40 mM, 80 mM, and 120 mM sodium chloride) for 60 days. At the end of the stress period, plant growth was evaluated by measuring leaf, shoot, and root dry weights. Carbohydrates, chlorophylls, lipid peroxidation, and electrolyte leakage were also determined in leaves. Plant nutrient status was evaluated in leaf, shoot, and root samples, and the soil was also analyzed for nutrient content. Leaf sclerophylly indexes were determined to assess water stress induced by salinity. Leaf dry weight was not significantly affected, whereas shoot and root dry weights decreased with salt stress. Soil electrical conductivity and sodium and chloride concentrations increased with increasing salt concentration, whereas available potassium concentration decreased. Salt stress induced a significant accumulation of sodium and chloride in leaves, shoots, and roots. Potassium and phosphorus concentrations in leaves were enhanced under salt stress, whereas inconsistent changes were detected concerning magnesium, zinc, manganese, and copper concentrations. Sucrose and glucose concentrations were significantly reduced under salt stress while fructose concentration did not exhibit significant changes. Chlorophyll concentration was also reduced under stress. Based on lipid peroxidation and electrolyte leakage indexes, cell membrane integrity was maintained. According to leaf characteristics indexes, leaves’ water stress was negligible, even after 2 months under salt stress. Based on the present results, sour orange accumulated significant amounts of sodium and chloride ions, inducing a reduction of growth, possibly through a reduction of carbon assimilation rate under low chlorophyll content, resulting in reduced carbohydrate concentration in the leaves.