The ornamental and economic value of N. tazetta L. var. chinensis Roem. is high. This variety is widely cultivated in the coastal area of Zhangzhou City, Fujian Province. In this area, there is a large amount of precipitation and uneven distribution that varies greatly with seasonal change. This pattern of precipitation aggravates soil salinization. Strong wind and waves in coastal areas lead to the deposition of numerous water droplets with dissolved sodium chloride content. These droplets settle on plant leaves and permeate the soil. These minerals can be spread to areas that are relatively far from the coast via the forces of wind and gravity. This salinization could pose a severe threat to the plants, as it can lead to withering and even death (Ogura and Yura, 2008). Soil salinization and salt fog have adverse effects on the growth and ornamental value of N. tazetta L. Salt is therefore an important nonbiological limiting factor for the promotion of growth (Tester, 2003). Thus, it is important to understand the salt injury mechanism of salt stress in N. tazetta L.
Most plants are sensitive to salt stress, and their growth is inhibited in saline environments (Deinlein et al., 2014; Maser et al., 2002). Generally, plants are most sensitive to salt stress during the period of seed germination and seedling growth (Kohli et al., 2013). The pathogenic effects of salt on plants are mainly manifested via ion toxicity, osmotic stress, and nutritional imbalance (Flowers and Colmer, 2008; Ghader, 2017; Hand et al., 2011; Julkowska and Testerink, 2015; Munns, 2005). There is some controversy as to whether salt injury is mainly an Na+ effect or a Cl− effect. Increases in Na+ will antagonize the absorption of other ions (e.g., K+, Mg2+, and Ca2+), causing nutritional deficiency and destroying the osmotic balance in the plant (Maathuis and Amtmann, 1999). The reduction of K+/Na+ ratio is a typical indicator of ion imbalance in plant cells under salt stress (Wei et al., 2015). To date, most studies have focused on plant responses to substrate salinity (Aghaleh et al., 2011; Ashraf and Akram, 2009; Julkowska and Testerink, 2015; Munns and Tester, 2008; Tavakkoli et al., 2011; Wang et al., 2017). Studies have rarely investigated the influence of salt spray (Megan, 2006; Ogura and Yura, 2008; Spano and Bottega, 2016), especially salt injury on the different ages and the different parts of plant leaves (Wang and Lin, 1999). There have been many studies on intracellular ion separation, but there have been few studies on the type of extracellular ion separation that occurs in leaves. Therefore, it is difficult to explain the mechanism of leaf apex damage that is primarily caused by salt stress (Tester, 2003), and it is worth studying these effects under salt spray and substrate salinity treatments. The results will be useful for the cultivation and management of N. tazetta L. and other ornamental plants in coastal areas.
In our experiments, N. tazetta L. seedlings were subjected to substrate salinity and salt spray treatments in a greenhouse environment. The growth rate, the allocation of five mineral elements (Ca2+, Mg2+, Na+, K+, and Cl−), and the mass fraction of ash at different ages and in various parts of the leaf were measured and compared. The results may guide salt injury diagnosis and prevention measures and provide a basis for screening salt tolerant species.
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