4.3.3. Effect of Salinity on *S.* × *hainanensis* Seed Germination

The optimum salinity for *S.* × *hainanensis* seed was between 0‰ and 7.5‰. Within this range, the germination potential, germination rate, radicle length, and radicle perfectness ratio, which reflect the sprouting ability, are better than those under other salinities. Further increases in salinity caused a drop in germination potential, germination rate, radicle length, and radicle perfectness ratio, which demonstrates that low salinity is conducive to *S.* × *hainanensis* seed germination, whereas high salinity inhibits germination. This result is consistent with findings that most halophytic plants have higher germination rates at low salinities [49–51].

Generally, seed germination at low salinity only slightly differs from that at 0‰ salinity. Increasing salinity gradually inhibits germination, but the seeds of most plants regain their vigor after being transferred into freshwater, which increases their accumulated germination rate [52]. For example, placing seeds that cannot germinate under high salinity into fresh water partially restores their germination rate, which shows that high salinity triggers dormancy in *S.* × *hainanensis* seeds. Exposing the seeds to optimal conditions allows them to germinate. This may be an important adaptation mechanism for *S.* × *hainanensis* in saline environments. Studies in China on the seed germination and seedling growth of other mangrove trees such as *S. apetala* under saline stress revealed that seed germination is better under low salinity than under high salinity [53]. This observation is partially consistent with the findings in this experiment. The *S.* × *hainanensis* seeds exhibited the highest germination index under 0‰ salinity and the low salinity value (2.5‰). However, radicle growth was more robust and the radicle perfectness ratio was higher under low salinity. This observation implies that *S.* × *hainanensis* seed germination requires stimulation under a certain salinity to allow the radicle to grow more robustly, more adaptable to the saline environment, and to improve the survival rate of seedlings. In addition, the seawater salinity control demonstrates that salinity limits the seed germination of *S.* × *hainanensis*.

#### **5. Conclusions**

Ecological analysis of the reproductive system, seeds, and seedlings indicated that the following factors limit the regeneration of *S.* × *hainanensis*: (1) Pollen limitation and inbreeding recession caused by the extremely small population of *S.* × *hainanensis*. (2) Seeds near parent trees are susceptible not only to high fruit drop rate, but to high predation beneath parent trees canopy as well. (3) Seed germination has weak adaptability to light, temperature, and salinity.

To protect *S.* × *hainanensis*, we recommend implementing artificial xenogamy during the flowering stage to improve pollination efficiency, the fruit setting rate, and the fruiting, as well as building a small simulation greenhouse and collecting seeds in time. The populations can be expanded through indoor seedling breeding and domestication to factors such as salinity.

**Author Contributions:** Conceptualization, M.Z. and X.Y.; data curation, M.Z. and X.L.; funding acquisition, D.L.; investigation, M.Z. and X.L.; methodology, M.Z.; project administration, X.Y.; resources, W.L.; writing—original draft, M.Z.; writing – review and editing, M.Z. and X.Y.

**Funding:** This work was financially supported by the China National key R & D Program during the 13th Five-Year Plan Period (2016YFC0503100).

**Acknowledgments:** We thank LetPub (https://www.letpub.com.cn/) for editing this manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
