Flower biology of Hibiscus clayi (Koki‘o ‘ula): Determining the effects of inbreeding on seed set, germination, and growth on ex-situ plants

Abstract

Breeding systems describe the evolved reproductive strategies used by plants. They are the foundation for understanding the reproductive ecology of plant populations and their dependence on pollinator vectors. However, they are not always available to inform conservation procedures, owing to the lack of information about rare plant species. Hibiscus clayi (Malvaceae) is restricted to the island of Kauaʻi and is one of the four endemic species of Hibiscus found in Hawaiʻi. It is listed as endangered by the US Fish and Wildlife Service, assessed as critically endangered on the IUCN Red List of Threatened Species, and is at risk of extinction in the wild. To better inform methods for maintaining ex-situ collections and to enhance outplanting efforts, the breeding system of H. clayi needs to be explored. The objectives for this study were to experimentally investigate the breeding system of H. clayi by determining the degree of self-compatibility, and determine the effects of selfing, outcrossing within a population, and outcrossing between populations on fruit set, seed production, germination, seedling growth, and survivorship. To determine this, ex-situ collections of individuals originating from two different natural populations were used in experiments. Results showed that H. clayi is weakly self-compatible, producing roughly 29% as much fruit when selfed as when outcrossed. The outcrossing treatments had a significantly positive effect on fruit set and seeds per pollinated flower, but made no significant difference to seed count per fruit and mean seed mass. Fruit set when self-pollinated or left open to natural pollinators was low, so hand-pollination from a different individual is necessary to get the highest fruit set. Germination was high, >70% for all treatments, and no significant difference in performance was detected among treatments for seedling growth or survivorship. Outcrossing between populations slightly improved growth; therefore, long-term efforts to increase progeny should prioritize making controlled crosses when possible, to preserve allelic diversity and vigor of plants used in restoration.