Effects of NaCl stress on growth and ion homeostasis in pomegranate tissues

Pomegranate is a 'super fruit' that is always adaptable to a saline environment. To elucidate the mechanism underlying the response to salinity, we investigated the effects of NaCl stress on growth and ion balance in six-month-old self-rooted pomegranate cuttings. The plants were fertigated with half-strength Hoagland's solution containing 0 (control), 100, 200, or 300 mM NaCl every six days. After 18 days, the growth parameter was determined, and dry samples of root, stem and leaf were analyzed for ion content. We found that the growth of pomegranate was not inhibited under 100 mM NaCl stress. With the increased salinity level, excessive accumulations of Na+ and Cl- occurred in the pomegranate tissues and the net accumulations were ranked as leaf > root > stem. The uptake of K+, Ca2+ and Mg2+ was enhanced at low salinity (100 mM) and inhibited at high salinity (300 mM). With increasing salinity, the K+ contents in the roots and leaves initially increased and then decreased, with peaks at 200 mM NaCl. The Ca2+ and Mg2+ contents increased in the leaves and decreased in the roots. The absorptions of NO3- and SO42- were inhibited by salt stress, but the H2PO4- uptake had no obvious change. The ion ratio and transport coefficient of pomegranate tissues were significantly lower under stress than in controls, but the K+/Na+ and Ca2+/Na+ ratios of the roots and stems were higher than 1 at 100 mM. This study revealed that salinity affected the ion balance in pomegranate tissues through impeding the ion uptake, accumulation and transport. Furthermore, this pomegranate cultivar 'Taishanhong' can adapt to 100 mM NaCl stress by maintaining a stable or higher uptake of K+, Ca2+ and Mg2+ to mitigate the detrimental effects of Na+ and Cl- in the cells.