Growth response, carbohydrate and ion accumulation of diverse perennial ryegrass accessions to increasing salinity
Highlights
► We examined the growth and physiological responses to increasing salinity. ► Decreased plant height and K+ and increased fructan and Na+ occurred at ≥50 mM NaCl. ► Decreased growth, LWC, Fv/Fm and increased WSC occurred at ≥150 mM NaCl. ► The tolerant accessions had less leaf senescence than that of sensitive accessions. ► DW, LWC, Fv/Fm and Na+ could be associated with variability in salinity tolerance.
Introduction
Salinity is a major abiotic stress limiting plant growth and productivity. Up to 20% of the world's irrigated land, which produces one third of the world's food, is salt affected (FAO, 2008). Salinity affects plant growth and development generally through osmotic stress limiting water uptake and the excessive uptake of ions, particularly Na+ and Cl− that ultimately interfere with various metabolic processes (Munns and Tester, 2008). Salinized plants may suffer from metabolic toxicity, nutrient deficiencies and imbalances, membrane dysfunction, and antioxidative stress, which damage tissue and induce early senescence (Essah et al., 2003). A number of mechanisms enable plants to survive high salinity environments, including but not limited to growth regulation, metabolic and osmotic adjustment, and cellular signaling (Zhu, 2002, Bartels and Sunkar, 2005). Exclusion of Na+ and tolerance of high cellular Na+ accumulation play an important role in minimizing Na+ toxicity above and beyond osmotic tolerance (Munns and Tester, 2008).
Effects of salinity on plant growth vary highly among plant species and/or within a species. Salinity stress reduced the growth of both tolerant tall wheatgrass (Thinopyrum ponticum Podp.) and sensitive wheat (Triticum aestivum L.); however, tall wheatgrasses showed higher relative shoot dry weight (RSDW) than wheat with NaCl ≥ 150 mM (Colmer et al., 2005). Higher RSDW has also been observed in salt-tolerant Thellungiella halophila (Arabidopsis thaliana relative) compared to sensitive Arabidopsis thaliana at 50 mM NaCl (Ghars et al., 2008). In perennial turf or forage grass species, seashore paspalum (Paspalum vaginatum Sw.) had superior shoot dry weight under salinity stress compared to several other warm-season turfgrass species including Japanese lawn grass (Zoysia japonica Steud.), manila grass (Zoysia matrella L.), hybridbermuda grassb (Cynodon dactylon x. Cynodon transvaalensis.) and serangoon grass (Digitaria didactyla Willd.), although inhibition of growth occurred in all species exposed to salinity (Uddin et al., 2012). The results suggest that growth related trait may be important for examining phenotypic variation of salinity tolerance of perennial grass species.
Salinity negatively affects photochemical efficiency of photosystem II, indicated by chlorophyll florescence (Corney et al., 2003, Sixto et al., 2006), thus potentially influencing photosynthesis and carbohydrate metabolism. Plants often adjust metabolism to cope with salinity stress. Accumulation of soluble sugars may enable plants to avoid tissue death and to continue growth and development under saline conditions, contributing to salinity tolerance (Messedi et al., 2006). Salinity tolerant genotypes of rice (Oryza sativa L.) and sunflower (Helianthus annuus L.) maintained higher soluble sugar concentrations than sensitive genotypes (Ashraf and Tufail, 1995, Zhang et al., 2012). However, accumulation of sugars was not related to salinity tolerance in cowpea (Vigna unguiculata L.) (Praxedes et al., 2011) or in safflower (Carthamus tinctorius L.) (Ashraf and Fatima, 1995). Water-soluble sugars of two basil (Ocimum basilicum L.) genotypes also remained unchanged under salinity stress (Heidari, 2012). In addition, avoiding Na+ accumulation in saline environments is an important mechanism contributing to ionic tolerance. The cytosolic K+/Na+ may also be critical for salinity tolerance of plants (Thalji and Shalaldeh, 2007, Azadi et al., 2011). Tolerant wheat genotypes exhibited low Na+, high K+ and high K+/Na+ in the leaf blade (Munns et al., 2000). In contrast, shoot concentrations of Na+ and K+ were not associated with the degree of salinity tolerance in Lolium (Marcar, 1987) and four Triticum turgidum subspecies (Munns and James, 2003). These inconsistent results indicate the complexity of metabolic adjustments that plants undergo to survive saline environments.
Perennial grasses used as turf and forage are increasingly subjected to salinity stresses in many areas due to the accelerated salinization of agricultural pasture lands and increasing demand on effluent water use for irrigating turfgrass landscapes (Carrow and Duncan, 1998). Perennial ryegrass (Lolium perenne L.) is a popular cool-season grass species cultivated in temperate climates. Originating in Europe, temperate Asia, and North Africa, it is commonly used as a turf and forage grass around the world. Perennial ryegrass has been ranked as moderate in salinity tolerance for commercial cultivars, tolerating soil ECe (saturated paste extract) ranging from 4 to 8 dS m−1 (Harivandi et al., 1992). Due to wide geographical distributions of perennial ryegrass, significant natural variation in growth and whole-plant physiological responses to salinity stress are expected in diverse ecotypes within this species. However, growth and physiological responses of diverse perennial ryegrasses to increasing levels of salinity stress as well as traits associated with genetic variability in salinity tolerance are not yet fully understood in perennial ryegrass accessions varying in origins. Therefore, the objectives of this study were to investigate growth response, carbohydrate and ion accumulation of diverse perennial ryegrass accessions exposed to increasing salinity and to determine phenotypic traits associated with variability of salinity tolerance. The results will aid in determining natural variations in salinity tolerance of perennial ryegrasses and in providing a mechanistic formation for creating perennial grass materials that are more tolerant to salt-affected soils and water.
Section snippets
Plant materials and growth conditions
Ten accessions of perennial ryegrass from various origins were used in the experiment including 2 wild, 3 cultivars, 2 cultivated, and 3 uncertain materials, according to USDA National Plant Germplasm System (USDA-NAGS) classification (Table 1). Nine materials including two cultivars (Aberystwyth S. 101 and Ellett) were obtained from USDA-NAGS at the Western Regional Plant Introduction Station in Pullman, WA, USA, and 1 turf-type commercial cultivar (BrightStar SLT) was obtained from the seed
Genetic relationship among perennial ryegrass accessions
Perennial ryegrass accessions varied in collection status and showed diverse origins (Table 1). Eight out of ten accessions were core collections according to USDA classification. Phylogenetic analysis based on 32 SSR markers across perennial ryegrass 7 chromosomes revealed genetic relationship among accessions (Fig. 1). PI231595, PI231597 and PI231587 all with uncertain status were genetically close to each other and were somewhat distinct from PI462339, PI275660, PI303011 and BrightStar SLT.
Discussion
Diverse perennial ryegrasses from different origins demonstrated substantial variations in leaf senescence, growth and physiological responses under higher salinity stress. The results suggest that diverse accessions are suitable for identifying a range of salinity effects on the growth and physiology of perennial ryegrass. The sensitivity of these accessions to increasing salinity was generally consistent with their genetic relationship. For example, accession PI231595, PI231597, PI231587 and
Conclusions
The decreased HT, K+ concentration and K+/Na+ and increased concentrations of fructan and Na+ were observed at ≥ 50 mM NaCl, while decreased FW, DW, LWC, Fv/Fm and increased WSC occurred at ≥150 mM NaCl in diverse perennial ryegrasses. Accessions varied largely in tolerance to high salinity at 200 mM to 300 mM NaCl. Traits of DW, LWC, Fv/Fm, WSC and Na+ accounted for larger variations across accessions and could be more associated with variability in high salinity tolerance, severing appropriate
Acknowledgements
The authors would like to thank Chun Zhao and Steve Sassman for assisting in extraction and analysis of Na+ and K+. This research was supported by the O.J. Noer Research Foundation and the Midwest Regional Turfgrass Foundation at Purdue University.
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