At present, over one-third of cultivated lands are threatened by soil salinization (Rengasamy, 2010). Salinity affects plant growth by imposing both ionic and osmotic stresses. High Na+ concentration disturbs plant ionic homeostasis and the plasmalemma system, decreases plant growth, and even causes plant death (Munns and Tester, 2008). In addition, Na+ competes with K+ for uptake across the root cell, which caused a decline in the K+/Na+ ratio and nutrient deficiency or ion imbalances (Chen et al., 2015). Salinity-tolerant plants had the capacity to restrict the uptake of Na+, sequester Na+ into the vacuoles, and accumulate compatible molecules or compounds, such as proline, glycine betaine, and sugars, to maintain osmotic equilibrium (Thalji and Shalaldeh, 2007).
Nitrogen and P are the important nutrient elements for plant growth and development (Allen and Bryson, 2007; Sanchez, 2007). Salinity reduced organic matter and nitrogen accumulation in plants (Chen et al., 2015). High salinity prevented plant N uptake, which limited plant growth (Hu and Schmidhalter, 1997; Munns and Tester, 2008). Application of N could positively affect plant growth under salinity stress (Rengasamy, 2010). Phosphate availability is reduced in saline soils due to ionic strength effects that reduced the activity of phosphate, and thus salinity decreased the P concentration in plant tissue (Bloomfield et al., 2014). However, increased P accumulation was also found in the leaf sheath and roots of rice (Oryza sativa) exposed to salinity (Nemati et al., 2011). In addition, P added to saline soils did not necessarily increase salinity tolerance of some crop species, such as maize (Zea mays), carrot (Daucus carota), sugar beet (Beta vulgaris), and tomato (Solanum lycopersicum) (Champagnol, 1979). The results indicate a complex relationship between salinity and P nutrition of plants.
It is well-known that salinity stress increases Na+ accumulation and decreases K+ in plant tissues, including perennial grass species (Hu et al., 2012; Tang et al., 2013a, 2013b). However, shoot concentrations of Na+ and K+ were not consistently associated with the degree of salinity tolerance (Tang et al., 2013a). High Na+ concentration in the soil decreased Ca2+ uptake and transport and inhibited plant growth (Hu and Schmidhalter, 1997). High Ca2+ concentration in plant tissue improved salinity tolerance by excluding Na+ (Hawighorst, 2007; Hu and Schmidhalter, 2005). Ashraf and Fatima (1995) found that CaSO4 treatment enhanced the germination of wheat (Triticum aestivum) under salinity stress and maintained K+ concentration, suggesting a role of Ca2+ in salinity tolerance. In addition, Ca2+ is strongly competitive with Mg2+ at the binding sites of root plasma membranes, which can interfere with both Mg2+ and Ca2+ uptake under salinity stress (Grattan and Grieve, 1998). The reduced leaf Ca2+ and Mg2+ concentrations were more pronounced with increasing NaCl concentrations in sour orange (Citrus aurantium) and Carrizo citrange (Citrus sinensis × Poncirus trifoliata) but such changes in Ca2+ and Mg2+ levels were not consistently shown in Cleopatra mandarin (Citrus reshni) and alemow (Citrus macrophylla) (Ruiz et al., 1997). The results suggest that effects of salinity stress on Ca2+ and Mg2+ concentrations may depend on plant species or cultivars.
Salinity also affects micronutrient uptake in plants. For example, the concentration of Mn2+ increased under salinity stress in barley [Hordeum vulgare (Hassan et al., 1970a)] and rice (Verma and Neue, 1984), but decreased in maize (Hassan et al., 1970b), whereas Mn2+, B, and Fe concentrations were unaffected by salinity in young leaves of wheat (Hu and Schmidhalter, 2001). No clear patterns of changing Mn2+ and Fe concentrations were observed in four Citrus stocks exposed to increasing NaCl as mentioned previously (Ruiz et al., 1997). These inconsistent results indicate that nutrient uptake and metabolism are complex and diverse in different plant species under saline environments.
Perennial ryegrass is a popular cool-season perennial grass species (Cornish et al., 1979). It was widely cultivated in temperate climates. Because of wide geographical distribution, significant natural variations of salinity tolerance existed in this species (Tang et al., 2013a, 2013b). However, alterations of nutrient elements to salinity stress are not fully understood in perennial ryegrass. Therefore, the experiment was designed to identify the genotypic variations in growth traits and nutrient elements in relation to salinity tolerance in perennial ryegrass. The results would provide insights into variability of plant growth and nutritional elements of perennial grass species under salinity stress.
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