Flower Forms and Ploidy Levels Impact Fertility in Althea

Abstract

Althea (Hibiscus syriacus) is a popular shrub known for its vibrant summer blooms and winterhardiness; however, althea produces capsules with numerous seeds that germinate and cause a nuisance in production and the home landscape. Breeding for sterile forms has long been a goal of Hibiscus breeders, yet many popular “sterile” cultivars have been reported as weedy. The purpose of this study was to evaluate female and male fertility of tetraploid and hexaploid cultivars, and to evaluate the female fertility of pentaploid progeny resulting from 4x × 6x and 6x × 4x crosses. More than 600 self-pollinations were performed on 21 cultivars, yet only 24% of self-pollinations resulted in filled capsules, for an overall rate of four seeds per pollination. Significant differences were observed among taxa for seeds per capsule and seeds per pollination. Highest capsule set was observed on self-pollinated White Chiffon® and Pink Chiffon®. Anecdotally, we observed reduced vigor in the S1 generation of most taxa. However, ‘Woodbridge’ produced vigorous seedlings through the S2 generation. More than 2000 cross-pollinations were also performed, resulting in more than 15,000 seeds. To evaluate female fertility, 28 taxa were pollinated with a variety of male parents. Fertility was measured as seeds per capsule and seeds per pollination. Significant differences were found among taxa within and among flower forms (single, semidouble, and double) for seeds per capsule and seeds per pollination. Double-flowered forms had reduced female fertility. Taxa previously reported to be sterile were found to be fertile, including ‘Aphrodite’, ‘Diana’, ‘Helene’, and ‘Minerva’. Two hexaploids, ‘Pink Giant’ and Raspberry Smoothie™, had reduced female fertility compared with tetraploids. Male fertility was estimated for 20 cultivars by pollinating between one and 23 cultivars. For male fertility, significant differences were found among taxa for seeds per capsule and seeds per pollination; however, no significant differences in male fertility were observed among flower forms. Four taxa had relatively high fertility with more than 10 seeds per capsule and seeds per pollination, including Blue Satin®, Lil’ Kim™, Bali™, and Tahiti™. In addition to the significant differences among female and male fertility of each taxon, capsule set varied widely among individual cross combinations. Significant differences of female fertility were found in pairwise comparisons between almost all pentaploid taxa and the mean of tetraploid control cultivars. No difference in percent seed germination was observed between 4x × 6x and 6x × 4x crosses (45% and 45%, respectively) but both were significantly lower than seeds from open-pollinated tetraploids (89%). The reduced fertility of pentaploids will likely lead to new reduced fertility or sterile cultivars for the nursery industry, especially if combined with double flowers.

Near-Hexaploid and Near-Tetraploid Aneuploid Progenies Derived from Backcrossing Tetraploid Parents Hibiscus syriacus × (H. syriacus × H. paramutabilis)

“…Thus, we recommend ‘Resi’ as a pollen parent to breed for larger flowers, not only through transgressive segregation but also ploidy manipulation. Breeders wishing to increase flower size using ploidy manipulation can use treatment of somatic material in vitro or in vivo to create octoploids and backcross to yield hexaploids [ 18 ]. However, using ‘Resi’ as a male parent and selecting progeny with large flowers may be a more efficient method.…”

“…In addition, various ploidy levels of H. syriacus cultivars or breeding materials of tetraploid, pentaploid, and octoploid have been developed and showed altered morphologies [ 7 , 13 , 16 ]. For example, the hexaploid (2 n = 120) cultivars have a larger flower [ 7 ] and decreased fertility [ 17 ] compared to tetraploid cultivars [ 18 ].…”

Hibiscus syriacus ‘ORSTHIB5x1’ Petite Pink Flamingo™ Althea

“…More than a decade of testing available cultivars, including ploidy analysis and testing seed production (fecundity) through controlled crossing and open pollination in fields, has shown that most cultivars are highly fertile tetraploids (4x), but that hexaploids (6x) such as Pink Giant ('Flogi') had reduced fertility (Lattier and Contreras 2022). The availability of multiple cytotypes presented the opportunity to make interploidy crosses to develop 5x plants with reduced fertility (Lattier and Contreras 2022). Pentaploid (5x) plants yielded 0.8 seed per pollination, whereas 4x plants yielded 10 seeds per pollination (Lattier and Contreras 2022).…”

“…The availability of multiple cytotypes presented the opportunity to make interploidy crosses to develop 5x plants with reduced fertility (Lattier and Contreras 2022). Pentaploid (5x) plants yielded 0.8 seed per pollination, whereas 4x plants yielded 10 seeds per pollination (Lattier and Contreras 2022). To date, we are unaware of any available cultivars that are 5x.…”