About 950 million hectares of land are salt affected, almost 6% of the world’s total land area (Rengasamy 2010). And a considerable proportion of cultivated land is becoming saline because of human acts and climate change (Smajgl, Toan et al. 2015). Soil salinity is becoming an increasing threaten to agricultural production and plant growth. Growing salt-tolerant plants has considered as one of the most effective measures to ameliorate saline soil. Chinese Iris is a widely distributed green vegetation which has a strong tolerance to high soil salinity (Xia, Guo et al. 2018), so the study to the salt tolerance mechanism of Iris would be significant.
Salt tolerance is a complicated process. Salt stresses plants mainly in two ways: hyperosmotic and hyperionic. Hyperosmotic makes it hard for roots to extract water and hyperionic can be toxic to plants (Xiaoli Tang 2014). During the long-term adaptation to salt stress, plants have evolved a variety of adaptive strategies. For now, osmotic adjustment (such as compatible solutes accumulation), hormone (ABA, ethylene) regulation (Achard, Cheng et al. 2006, Ahmed, Yadav et al. 2017, An, Zhang et al. 2018) and ionic balance (especially K+/Na+ homeostasis) are considered as the main mechanisms of salt tolerance in plants (Chen, Zhou et al. 2007, Adem, Roy et al. 2014, Deinlein, Stephan et al. 2014).
Under NaCl stress, excessive Na+ in cytoplast decreased cellular absorption for K+ and Ca2+, caused serious damages to physiological metabolisms (Zhu 2003). Preventing excessive Na+ accumulation and maintaining optimum Ca2+/Na+, K+/Na+ ratios in cytoplasm are important mechanisms for plant to growth in saline condition(Wu, Shabala et al. 2015). The ability to transport, compartmentalize, extrude, and mobilize Na+ are crucial for plants to tolerate high NaCl concentration (Apse and Blumwald 2007, Deinlein, Stephan et al. 2014). Wu (2015) found that salt tolerant genotype of wheat has the ability to sequestrate Na+ in mature root zone vacuole, maintained a low Na+ content in cytoplasm. This ability were also found in tomato, soybean etc. (Wei, Zhang et al. 2017, Zhao 2017). Zhao (2017) used the non-invasive micro-test technique (NMT) measuring the fluxes of Na+ and K+ of roots and leaves of different genotypes of wheat, found that the tolerant genotype exhibited a significant Na+ efflux under NaCl stress to keep ion homeostasis. Na+ extrude mainly through three pathways: SOS pathway mainly for Na+ secretion (Munns and Tester 2008), HKT transporters for K+/Na+ anti transport (Park, Yu et al. 2017) and NHX transporters for exchange of cations through vacuolar H+-ATPase and H+-PPase (Himabindu, Chakradhar et al. 2016, Li, Wang et al. 2017).
Chinese Iris is a high salt tolerant vegetation, our previous study has revealed that Chinese Iris could increase the synthesis of proline to keep a proper osmotic potential and decrease the absorption of Na+, meanwhile increase the transport of K+, Ca2+ from roots to leaves to alleviate Na+ toxicity and maintain ion homeostasis (Bai, Li et al. 2008, Xia, Guo et al. 2017). Nevertheless, despite a lot of studies have been conducted on the physiological responses of Chinese Iris, the ion distribution and ion fluxes of roots and leaves remain poorly understood. In this study, we analyzed growth status, distribution and fluxes of Na+, K+, Ca2+ in roots and leaves of Chinese Iris under NaCl stress, in order to reveal the mechanisms of salt tolerance and offered a theoretical basis for improving the salt tolerance of halophytes.