Salinity is a build-up of soluble salts (Levy and Syvertsen, 2004). This build-up causes adverse morphological, physiological, and biochemical effects in different organs of citrus plants through an increased concentration of sodium and chloride (Camara-Zapata et al., 2004; Rachmilevitch et al., 2004; Raveh and Levy, 2005; Romero-Aranda et al., 1998; Zekri, 2004). Excess amounts of these salts enhance the osmotic potential (ψS) of the soil matrix, restricting the plant's water intake (Garcia-Sanchez et al., 2002a, 2002b). Plants have developed many adaptive strategies in response to abiotic stresses such as salinity that ultimately influence plant growth and yield (McCue and Hanson, 1990).
Sodium and chloride are major ions and can cause various disorders in citrus plants (Romero-Aranda et al., 1998). Sodium chloride is reported to be a major source of ions in salt solutions compared with Na2SO4 because NaCl liberates ≈60% more ions than Na2SO4 does (Rachmilevitch et al., 2004). Beyond the osmotic effects of salt in the root zone, salt stress causes oxidative stress in plant cells through the generation of reactive oxygen species (ROS), including hydroxyl and superoxide radicals, through various metabolic processes such as photorespiration (Noreen and Ashraf, 2009). Salinity reduces stomatal function and favors the denaturation of chlorophyll (Hernandez et al., 1999), which ultimately leads to a reduction in gS, photosynthetic activity, and the generation of free oxygen radicals, thus inducing oxidative stress. ROS can cause toxic reactions such as lipid peroxidation, protein degradation, and DNA mutation (McCord, 2000). In response, a plant may synthesize more antioxidant enzymes (Sairam et al., 2005), including superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) (Noreen and Ashraf, 2009).
The growth and productivity of citrus plants are inhibited by saline as a result of the ion toxicity of Na+ and Cl– as well as the ion antagonisms that occur with Na+ and Cl– that limit nutrient availability (Munns and James, 2003). In citrus production, rootstocks are used to control a plant's size and form and play a key role in establishing excellent fruit quality and yield. Rootstocks also may impart a tolerance to various biotic and abiotic factors, thus contributing to higher production (Waqar et al., 2007). In citrus plants, excess salts in the root zone may negatively affect plant morphological features, mineral nutrition, and various physiobiochemical mechanisms; i.e., photosynthesis, gS and transpiration (Garcia-Sanchez and Syvertsen, 2006; Garcia-Sanchez et al., 2006). Citrus rootstocks may play an important role with regard to the emerging threat of soil salinity. Therefore, the present study was conducted to assess the effect of salt stress on the various physiological and biochemical aspects of salt-tolerant and salt-sensitive citrus rootstocks. The findings of this investigation also clearly demonstrate the differences that occur with respect to various morphological, biochemical, enzymatic, and ionic attributes of salt-tolerant and salt-sensitive citrus rootstocks. The results may indicate the degree to which rootstocks enhance citrus performance under saline conditions and lead to early screening methods to detect tolerant rootstocks.
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