Na+ and K+ homeostasis in different organs of contrasting Zoysia japonica accessions under salt stress

Zoysia japonica Steud., a typical halophyte, can secrete salt through salt glands, which is an important salt stress tolerance mechanism. However, the Na+ and K+ homeostasis in different organs under salt stress has not been reported. In this study, two salt-tolerant (Z011 and Z015) and two salt-sensitive (Z004 and Z103) Zoysia japonica accessions were investigated. We analyzed their Na+ and K+ absorption, transport, distribution and secretion in different organs under salt stress. Under 200 mM NaCl treatment, the Na+ content in the roots of the salt-tolerant accessions was significantly lower but K+ levels and K+/Na+ ratio were both prominently higher than those in the salt-sensitive accessions. Na+ and K+ flux assays in different root zones suggested that, under salt stress, Na+ efflux in the root maturation zone of the salt-tolerant accessions increased significantly compared with the control. The K+ efflux in the root maturation zone of the salt-sensitive accessions showed a prominent increase, but no significant differences were seen in the root meristematic or elongation zones, implying that the root maturation zone is the crucial salt tolerance regulation area of Zoysia japonica for Na+ and K+ absorption and efflux. Studies on cross-sectional tissues of different organs indicated that the roots of the salt-tolerant accessions had a stronger compartmentalization ability aiming to inhibit the entry of Na+ into the stele and prevent the loss of K+ than those of the salt-sensitive accessions. Furthermore, the salt-tolerant accessions could secrete more Na+ and K+ through the salt glands of leaves, while Na+ secretion was much more than K+ under salt stress. Therefore, the efflux and compartmentalization of ions by roots, selective transport of ions by stolons and ion secretion by leaves coordinately regulate the Na+ and K+ homeostasis of Zoysia japonica under salt stress.