Recently I’ve been thinking about hibiscus breeding. I have a few varieties of tropical hibiscus and have been unsuccessful at making any crosses. I have a couple species hibiscus, Hibiscus arnottianus which is one of only a few fragrant species which come from Hawaii, as well as H. schizopetalus, unique for its frilly petals. These have been used for making a number of hybrids, but the real challenge would be making a hardy woody Hibiscus.
There have been some recently reported putative successes (http://www.houzz.com/discussions/1940307#18752192), and there is a definite possibility that this can be obtained. Others have reported this possible in the past. A former University of Florida plant breeder named Sam McFadden claimed to have made a hybrid called ‘White Angel’* reported to be H. rosa-sinensis x H. syriacus ‘Diana’. I have never seen this offered for sale, but you can see pictures of it here: http://aggie-horticulture.tamu.edu/archives/parsons/heroes/mcfadden/mcfadden.html
*UPDATE: I did a search for this and it is going to be on the market: http://www.gardendebut.com/plant/WHITE-ANGEL-ALTHEA
Rose-of-Sharon, H. syriacus, is one of three temperate species in section Hibiscus, along with H. sinosyriacus and H. paramutabilis. These species are closely related and also cold hardy shrubs. Some hybrid cultivars are available between H. syriacus with H. paramutabilis.
H. ‘Tosca’ (H. paramutabilis x H. syriacus) http://www.woodlanders.net/index.cfm?fuseaction=plants.plantDetail&plant_id=1726
H. ‘Lohengrin’ (H. paramutabilis x H. syriacus)
Researchers in Belgium made some interspecific crosses with those three hardy species: “Interspecific hybridisation between Hibiscus syriacus, Hibiscus sinosyriacus and Hibiscus paramutabilis” (http://link.springer.com/article/10.1007/s10681-006-9328-8). They found that crosses tended to work unilaterally, using H. syriacus as the female parent. From that article: “Basic chromosome number of Hibiscus syriacus is x = 20 and most cultivars are tetraploid, 2n = 4x = 80 (Skovsted 1941). However, in literature frequently the term diploid is used instead of tetraploid. Breeding work resulted also in hexaploid (so called triploid) and octaploid (so called tetraploid) cultivars, which in general have larger flowers and a longer flowering period (Egolf 1971, 1981; Van Huylenbroeck et al. 2000). Hibiscus sinosyriacus, 2n = 4x = 80 (Skovsted 1941), has broader leaves compared to Hibiscus syriacus. The leaves have short triangular lobes and the involucratal bracts outside the calyx are as long as the calyx or even longer (Bates 1965). Hibiscus paramutabilis has big round-shaped leaves and the flowers are larger compared to H. syriacus (Bates 1965). The chromosome number is 2n = 82 (Niimoto 1966).”
To parse that information out:
- H. syriacus (2n=4x=80)
- H. sinosyriacus (2n=4x=80)
- H. paramutabilis (2n=82)
There has been some confusion in the past regarding the ploidy of H. syriacus, which was reported as diploid but has been discovered is actually tetraploid. Another study from 2013 looked at the ploidy of common cultivars of Rose-of-Sharon, and they discovered that most are tetraploid, even those that were supposed to be ‘triploid’ (which would have actually been hexaploid). It is unknown if they have reverted to a tetraploid state or if they ever were actually hexaploid. (https://www.oregon.gov/ODA/shared/Documents/Publications/NurseryChristmasTree/NurseryResearchPloidy.pdf)
They did discover a single hexaploid (2n=6x=120) cultivar: ‘Pink Giant’. I have not seen this for sale anywhere in the U.S., but as this came from Oregon, I assume it must be available in some places. It did make some seed, so this could be useful for hybridization. As a female parent: 3 seedlings from 39 flowers. As a male parent: 25 seedlings from 50 flowers.
It would be nice to have the larger and wider color variation in flowers of H. rosa-sinensis with the hardiness of H. syriacus. The tropical hibiscus, H. rosa-sinensis, isn’t a true species, but rather an amalgamation of various species with the section Lilibiscus (http://www.malvaceae.info/Genera/Hibiscus/Lilibiscus.php) There are thousands of H. rosa-sinensis cultivars around the world, and their chromosome counts are all over the place, which complicates breeding. From that same link: A wide variety of chromosome counts have been reported from Hibiscus rosa-sinensis (36, 38, 40, 44, 46, 52, 63, 70, 76, 84, 90, 92, 118, 132, 144).
From a Chinese study (CHROMOSOME NUMBERS AND PLOIDY OF SEVERAL PLANTS IN HIBISCUS L. SONG Juan-juan, ZHUANG Dong-hong. http://en.cnki.com.cn/Article_en/CJFDTotal-RYZB200103006.htm):
- H. schizopetalus (Masters) Hook.f. (2n=42)
- H. mutabilis L. (2n=92)
- H. rosa-sinensis L. (2n=84)
- H. rosa-sinensis cv. DoubleRainbow (2n=105)
- H. rosa-sinensis cv. Flavo-plenus (2n=138
- H. rosa-sinensis cv. Carminatus (2n=147)
- It is shown that the ploidy relationship exists among them except cv. Flavo-plenus, the basic chromosome number being x=21.
So now what? Where do we go from here? Try your hand at breeding some Hibiscus. H. syriacus ‘Lucy’ doesn’t make pollen so would be a possible good female parent. It’s fairly easy to emasculate H. rosa-sinensis flowers, because they’re so big. You’ll want to strip the style of the anthers before the pollen sheds, usually before the flower opens. Some self pollinate, others don’t. ‘Dustin Rae’ on the Houzz forum used H. ‘Lutea’ because it is self sterile, and he read somewhere it was triploid (2n=63 I guess) and fertile. Many of the large flowered H. rosa-sinensis will be tetraploids at least, but again the ploidy is all over the place, so it may not matter. It appears that within H. rosa-sinensis that there is some mechanism that allows for fertile aneuploidy.
If the pod aborts quickly it probably won’t work, but I would try the same cross many times before I would definitively say that a cross doesn’t work. (i.e. 1 out of 50 is still success) If after a week the pod aborts, it may be a good candidate for embryo rescue or ovule culture. I haven’t been able to find a lot of information on embryo rescue in Hibiscus, but I was able to find information on cotton, which is related. ‘Dustin Rae’ used Murashige and Skoog (MS) media with success, which I don’t know if he used any plant hormones, but a couple of the articles I read used plant hormones: Stewart and Hsu reported using Beasley and Ting (BT) media with 5 μM indole-3-acetic acid, 0.5 μM gibberellic acid, and 0.05 μM kinetin. (Stewart, James McD. and Hsu, Cecilia Lee. 1977. In-ovulo Embryo Culture and Seedling Development of Cotton (Gossypium hirsutum L.) Planta 137,113-117). I read the Beasley and Ting article from 1973, and I think MS would probably work just as well and be less work. I read another more recent article where the researcher used 2,4 D but I can’t remember where I saw it. I’ll update this if I find it.
Embryo rescue is more complicated but can be done. Here is a link to some instructions by Sandra Reed about how it is accomplished: https://naldc.nal.usda.gov/download/42085/PDF
If you make a cool hybrid, let me know!