Measurement of inbreeding depression in Rhododendron kaempferi: seed production, germination, juvenile survival, and growth

To understand the reproductive ecology of a plant species with both selfing and outcrossing reproductive systems, it is important to know how selfed individuals contribute to the maintenance of a population. Further, examination of the magnitude of inbreeding depression after pollination is essential for accurate measurement of reproductive success. In this study, we investigated the characteristics of viable seeds and seedlings of Rhododendron kaempferi in a region where the bumblebee pollinator Bombus ardens are abundant. In a hand-pollination experiment, the fruit set from self-pollination was 32%, significantly lower than that from outcross-pollination (81%). The number of seeds per fruit from self-pollination was one sixth the number from outcross-pollination. However, the number of seeds per fruit was negatively correlated with the weight per seed. In addition, the germination rate was low when the seed weight was light. Although selfed seeds could germinate, 89% of the seedlings died within a year. The inbreeding depression coefficient 1 year after sowing was very high (0.89). These results indicate that R. kaempferi shows extremely strong selection for outcrossing but maintains a slight selfing potential in a natural population in central Japan.