Salinity is a major factor limiting plant growth and development of plants in many areas of the world. As population and potable water demand increase, water shortage is a major problem in many parts of the world (Huang et al., 2014; Marcum and Pessarakli, 2010; Sun et al., 2015). Turfgrasses increasingly experience salt stress because of the accelerated salinization of agricultural lands and increasing demand on the use of reclaimed or other secondary saline water for irrigation of turfgrass landscapes (Huang et al., 2014; Jiang et al., 2013; Sun et al., 2015). Salt stress may reduce turfgrass growth and quality by osmotic stress-induced injury. In many areas with limited fresh water resources, reclaimed water has been applied on golf courses and other turf surfaces to save water.
The complex regulatory processes of plant salt adaptation involve control of water flux, cellular osmotic adjustment, and hormonal regulation (Golldack et al., 2014; Ryu and Cho, 2015). The decline of cell ψS under salt stress may induce stomatal closure (Hu et al., 2013). However, excess accumulation of ions such as Na+ under salt stress may cause toxicity to cells. In addition, salt stress may damage plant physiological processes by over accumulation of reactive oxygen species, impairment of antioxidant defense systems and photosynthetic function, and imbalance of hormones (Hu et al., 2012, 2015; Kim et al., 2016).
Plant hormones function as central integrators that link and reprogram the complex developmental and stress adaptive signaling cascade (Golldack et al., 2014; Llanes et al., 2016; Ryu and Cho, 2015; Strivastava, 2002). Elevated ABA may help plants to acclimate to low water availability by closing stomata (Man et al., 2011; Zhang et al., 2015). Cytokinins (such as ZR and iPA) essentially regulate various plant developmental processes, including cell division and enlargement, chloroplast biogenesis, nutrient mobilization, leaf senescence, vascular differentiation, and apical dominance (Ryu and Cho, 2015). Cytokinins facilitate the responses to delay both stomatal closure and leaf senescence under abiotic stresses (Ryu and Cho, 2015; Zhang et al., 2015). Auxins such as IAA can promote root initiation and also delay plant senescence (Zhang et al., 2009). Among the 136 known GAs, only GA1, GA3, GA4, GA5, GA6, and GA7 have intrinsic biological activity (Davies, 2010). Bioactive GAs such as GA4 are involved in plant growth and development such as leaf expansion, stem elongation, and flowering (Ryu and Cho, 2015). GA4 is two orders of magnitude more active in delaying leaf senescence of Alstroemeria hybrida than GA1 (Davies, 2010). It may be possible that GA4 may delay plant senescence under salt stress. There is cross-talking between GA action and other hormones signaling during abiotic stress to control plant growth and development (Ryu and Cho, 2015). It has been reported that salt stress induced an increase in ABA of maize (Zea mays L.) plants (Jia et al., 2002) and a reduction in IAA of maize (Fahad and Bano, 2012) and CKs in barley (Hordeum vulgare L.; Kuiper et al., 1990). However, limited research has been reported on the relationship of hormonal responses to salt stress with photosynthetic function and visual quality in cool-season turfgrass species.
Kentucky bluegrass (Poa pratensis L.) is one of the most important cool-season turfgrass species in temperate climates and widely used for home lawns, golf courses, urban landscapes, and other sports fields. However, the KBG quality declines because of salt stress in many areas (Huang et al., 2014). The objective of this study was to investigate the responses of hormones (IAA, ABA, ZR, iPA, and GA4) to salt stress and examine if salt stress-induced damage was associated with alteration of hormonal metabolism in KBG.
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