Soil and water salinity is a global problem in the 21st century that threatens agricultural sustainability. In fact, the extent of salinized land is expanding at the rate of 10% per year and, experts speculate that by the year 2050, more than 50% of the global arable land would be salinized (Shrivastava and Kumar, 2014). Inappropriate farming systems contribute to the large-scale development of saline soil and massive loss of arable lands, especially in the arid and semiarid areas (Cheong and Yun, 2007). Soil salinity may have a profound impact on plant growth. Some studies have reported that salinization of soils inhibits the growth, development, and differentiation of plants and, causes low productivity of crops and grasses (Koca et al., 2007; Munns, 2002; Zhu, 2001). Salinity stress in bottommost soil is extremely frequent in the field; nonetheless, the upper soil may be sufficient for nutrient uptake by the plants if accompanied by regular irrigation and remediation. However, the physiological mechanisms underlying plant adaptation to soil salinity at subsurface depth have not been adequately understood.
The high soil salinity due to neutral salts, such as NaCl, has destructive effects on plant growth, development, and differentiation. Neutral salts mostly occur as a result of a neutralization reaction between a strong base and a strong acid or less often between a weak acid and a weak base. Neutral salts dissolve in water to form a solution of pH 7 (Masterton et al., 2011). High NaCl content in soil affects plants through both the direct action of Na+ and Cl− ions culminating in osmotic stress (Hasegawa et al., 2000; Munns, 2005). Soil salinity can disrupt ionic balance causing severe damage to plant roots (Hasegawa et al., 2000). Salinity also interferes with nutrient acquisition and hence causes micronutrient deficiencies in plants (Turkan and Demiral, 2009). The extent of damage resulting from salinity mainly depends on its influence on the plant’s physiological and biochemical processes as well as the plant’s ability to adapt to or tolerate salinity (Munns and Tester, 2008; Takahashi et al., 2001; Zhu, 2001). Patently, the root system is the first part of a plant to encounter and respond to high salinity environments. Therefore, plant roots have evolved several physiological and metabolic strategies to adapt to/tolerate salinity stress to survive in saline conditions. For instance, plants adjust the content of free amino acids and modify carbohydrate metabolism to cope with stress.
Free amino acids are organic components essential for the synthesis of proteins and other growth substances. Therefore, they play a vital role in plants’ stress tolerance through the modulation of stomatal conductance, detoxification of reactive oxygen species, and regulation of intracellular ion transport (Parida and Das, 2005; Rai, 2002; Szabados and Savoure, 2010). The accumulation of free amino acids enables osmotic adjustment a phenomenon speculated to improve plants’ ability to cope with stress (Serraj and Sinclair, 2002). Furthermore, the buildup of free amino acids provides carbon and nitrogen for future use either under stress or less stressful environments (Greenway and Munns, 1980). About 18 free amino acids have been substantiated to have functions in plants’ physiological and biochemical processes. Glutamic acid (Glu) and glycine (Gly) have been identified as the principal metabolites in the synthesis of chlorophyll (Kannangara et al., 1988). Proline has been considered to have the ability to stabilize the structure of proteins and membranes against adverse effects of drought, salinity, and extreme temperatures (Szabados and Savoure, 2010; Verslues and Sharp, 1999). Gly, arginine (Arg), asparagine, and serine (Ser) accrued in spinach (Spinacia oleracea) and coleus (Coleus blumei) subjected to salt stress (Di Martino et al., 2003; Gilbert et al., 1998). Asparagine and valine increased in bermudagrass (Cynodon dactylon) growing under water stress (Barnett and Naylor, 1966). Kerkeb and Kramer (2003), on the other hand, proposed that asparagine and histidine have the capacity to bind heavy metals hence acting as antidotes.
Carbohydrates synthesis in plants involves a series of complex reactions including photosynthesis. The growth, development, and differentiation of plants mainly depend on carbohydrate metabolism, which includes the synthesis of sucrose, fructose, and glucose (Hasegawa et al., 2000). High content of carbohydrates before and during abiotic stress in plants may, therefore, indicate better tolerance to stress (Kafi et al., 2003). Rolland et al. (2016) also put forward that the biochemical breakdown of carbohydrates can supply energy and metabolites to participate in biosynthetic processes. Soluble sugars, including glucose, fructose, and sucrose, have been considered as typical osmoprotectants and essential components for stabilization of cellular membranes. They also scavenge for radical reactive oxygen species besides serving as carbon storage components in plants (Farooq et al., 2009; Parida and Das, 2005; Rolland et al., 2006). The accumulation of free amino acids and carbohydrates may help in the following vital processes: maintenance of ionic balance in homeostasis, eradication of free radicals, stabilization of organelles and micronucleus, and the maintenance of the cytosol’s acid–base balance pH (Gilbert et al., 1998). As of now, there is little information concerning the metabolic profiles of free amino acids and carbohydrates that influence plants’ ability to cope with extreme soil salinity stress conditions, especially in perennial grasses.
Perennial ryegrass, native to western Europe, north Africa, and southwest Asia, is one of the most widely used cool-season turfgrasses in the northwest of China (Xiong et al., 2007). The rapid establishment of perennial ryegrass makes it suitable as a vanguard in combination with other turfgrasses on lawns, athletic fields, and golf courses (Xing et al., 2007). Furthermore, perennial ryegrass possesses desirable qualities including impressive regeneration, high density of tillers, excellent palatability and digestibility, high yield potential, and high protein content (Wilkins and Humphreys, 2003). As a result, it is a preferred perennial forage grass in the temperate regions of the world. However, there is limited information documenting the effect of extreme salinity stress, present in subsurface soil layers, on the physiological metabolisms in perennial ryegrass.
The first objective of this study was to analyze the effects of salinity present at subsurface soil layers on the root growth and root activity of perennial ryegrass. The second objective was to decipher salinity-induced changes of free amino acids and soluble sugars in the roots of perennial ryegrass exposed to extreme soil salinity. The final objective was to identify the suitable top-soil depth for root elongation and growth of perennial ryegrass exposed to salinity stress.
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