*4.1. Reproductive System of S.* × *hainanensis*

The understanding of the plant reproductive system is a prerequisite for understanding the life history of plants, and is also the basic background knowledge required for other related research [30]. Due to the different gender systems of plants, they often exhibit diverse types of reproductive systems [31]. Different gender systems have different effects on their hybridization rate, pollination mechanism, and breeding system [32].

According to the standards by Dafni, further experiments can verify that the sexual reproductive system of *S.* × *hainanensis* is partially self-compatible, outcrossing, and needs pollinators [28]. Based on the P/O ratio by Cruden, its reproductive system is facultative outcrossing. Cross-pollination, especially xenogamy, has a high maturation rate [29]. The fruiting rate of artificial pollination is higher than the other pollination approaches. Thus, *S.* × *hainanensis* flowers have a mixed mating system. Selfing may mainly come from geitonogamous pollination. Moreover, the study by Tomlinson showed that geitonogamous plants have an outcross reproductive system [24]. The differences in these studies may be attributed to the narrowed population of *S.* × *hainanensis*, which led to a higher chance of selfing and inbreeding. During evolution, *S.* × *hainanensis* gradually changed from obligate outcrossing

to facultative outcrossing with self-compatible tendencies. This reproductive system possibly evolved into a stable mixed mating system.

Research shows that small populations weaken the attraction of insects for pollination [33]. As a result, individual plants in small populations only acquire limited pollen with inferior quality compared to large populations, which leads to the pollen restriction on the reproductive success of small populations [33]. According to population genetics, small populations tend to undergo genetic drift, which reduces genetic diversity. This process is often accompanied by the accumulation of harmful mutations and it enhances inbreeding [34]. When populations shrink, the amount of pollen and its quality during pollination are greatly affected, which undermines reproduction [34]. According to research, only eight strains of *S.* × *hainanensis* are naturally distributed, with remarkable distances from each other. Under natural conditions, *S.* × *hainanensis* mainly conducts self-pollination. The artificial pollination experiment shows that artificially improving pollination notably improves the fruit setting rate of *S.* × *hainanensis*. The seed germination rate from the outcrossing population is much higher than from selfing. Therefore, pollen restriction and inbreeding depression can be attributed to the low seed germination rate of small populations, which is consistent with the finding of Li on *Lumnitzera littorea* (Jack) Voigt [35].
