JARS 42n4 - The Nature Of Evidence For Hybridity In Rhododendron yakushimanum 'Mist Maiden', 'Pink Parasol' and 'Ken Janeck'

The Nature Of Evidence For Hybridity In Rhododendron yakushimanum 'Mist Maiden', 'Pink Parasol' and 'Ken Janeck'
David G. Leach, North Madison, Ohio

In the course of a friendly discussion on R. yakushimanum at my home, biologist Joe Harvey of Nova Scotia proposed that he write an article about the hybridity of the large leaved forms such as 'Mist Maiden', 'Ken Janeck' and 'Pink Parasol' and that I produce a piece in rebuttal, since I did not agree with him.
Joe has made some statements on this matter which do not altogether succeed in being correct. It is not possible, as he suggests, to divide the species into two groups: those of "robust growth, larger leaves, deeper...pink blush to the flowers..." etc. and, by contrast, those of "...dwarfer growth, with smaller flowers...(and)...fatter capsules." The latter would be represented by the Exbury form and by 'Koichiro Wada', F.C.C.

'Pink Parasol' showing effect 
of growing conditions
Figure 1. R. yakushimanum 'Pink Parasol' showing effect of
growing conditions on leaf size and growth habit.
Photo by David G. Leach

The fact is that the stature, leaf size and other vegetative characteristics of R. yakushimanum are determined by the environment, and have nothing to do with hybridity or the lack of it. As proof, the reader is referred to Figure 1. which is a photograph of a plant with small, convex leaves and compressed growth habit typical of the Exbury form and 'Koichiro Wada', F.C.C. However, the plant photographed happens to be 'Pink Parasol'. It was grown in an impoverished soil, such as that on the mountain summits of Yaku Shima, with full exposure to drying sun and wind; it was never irrigated. This harsh treatment transformed the vigorous, large leaved 'Pink Parasol' into a close-coupled 'Koichiro Wada' with small, convex leaves. In early 1988 the plant was moved to a more favored site and its new growth is once again vigorous, with large, flat leaves. So it is apparent that gross morphology is no guide to hybridity in R. yakushimanum . The further truth is that no type specimen of R. yakushimanum exists from its original discovery, so there is no standard of specific validity for taxonomists. The University of Tokyo proposed using a specimen collected by Nakai on Mount Hanano-Ego, Yakushima, in 1923 as the type, which, ironically for this discussion, was the large leaved, vigorous sort similar to 'Mist Maiden' and 'Pink Parasol'. (1) The most that can be said is that the lower elevation forms of the species, which generally grow under more sheltered and benign conditions, tend toward more vigorous growth and produce larger leaves because a higher rate of transpiration can be supported. But of course that does not make them hybrids.

R. yakushimanum 'Pink Parasol'
Figure 2. R. yakushimanum 'Pink Parasol' in full bloom.
Photo by David G. Leach

My friend from Canada then turns to speculate on the other indumented species which might be the second parent of 'Mist Maiden', 'Pink Parasol' et. al. He refers to a Gable mating of R. smirnowii with R. yakushimanum , a cross I and many others have made, but he does not elaborate on it as a plausible combination. In fact, it is not. Plants of this parental derivation have a very different growth habit and foliage density, and they tend to bloom partially in the autumn. But most importantly, they are appreciably less cold hardy than 'Mist Maiden' or 'Pink Parasol'. Since this is the case and there exists no other indumented species which could conceivably produce progeny with the cold hardiness of R. yakushimanum , the reasonable conclusion is that the large leaved, more vigorous 'Mist Maiden' and 'Pink Parasol' are the unadulterated aristocratic descendents of the original settlers.
Finally, my friend from the North whoops it up for pollen abnormalities as an index of hybridity. This does not exactly succeed in being correct either, because of the multitude of ther causes for "empty or malformed" pollen grains which produce meiotic irregularities. For example:
1.  Plants that are forced in a greenhouse, or even grown in a greenhouse, exhibit the same pollen deficiencies that are offered as evidence of hybridity.
2.  Progeny of crosses between two isolated populations of the same species produce much the same pollen abnormalities as true hybrids between different species. On Yakushima, there are possibilities for natural crosses between the low and the high elevation populations of one species, as there are in the United States between two species, R. maximum and R. catawbiense , where the difference in the altitude of some colonies in the southern mountains nullifies the usual discrepancy between their blooming seasons, and produces true natural hybrids such as 'Russell Harmon'.

Rhododendron microspore mother cells
Figure 3. Rhododendron microspore mother cells,
prior to reduction division; the precursor to pollen.
Photo by Stowe/Fink/Leach
Haploid Rhododendron Research Project
Natural rhododendron chromosome pairing
Figure 4. Result of natural rhododendron chromosome
pairing. The chromosomes in each pair have
separated normally, and appear as dark spots inside
each cell. Viable pollen will be produced.
Photo by Stowe/Fink/Leach
Haploid Rhododendron Research Project
Rhododendron pollen cells
Figure 5. Each rhododendron cell with four segments has 13
single chromosomes, and except for the development of a
heavy exine wall, will remain as pictured from September to
April, when the final cell division will produce functional
pollen tetrads.
Photo by Stowe/Fink/Leach
Haploid Rhododendron Research Project

The evidence suggests that R. yakushimanum was probably an ancient immigrant from the R. metternichii complex to the north at a geologic epoch when the climate was much colder. The species is much hardier than would be expected of a rhododendron growing, as it does, at the latitude of New Orleans or Gainesville, Florida.
Dr. Tor C. Nitzelius of the Botanical Garden at Gteborg, Sweden, concluded after many months of study in Japan, that R. metternichii , R. degronianum , R. makinoi and R. yakushimanum are all one species. (1) I reached the same conclusion after seeing a very large population of "R. degronianum" growing from seeds collected in the mountain wilds near Akiucho, Natorigun, Miyakiken. They included plants with the characteristics of every species recognized in the West, including specimens duplicating R. makinoi and R. yakushimanum , and also varieties recognized only by Japanese botanists. (4) With this wildly heterozygous genetic constitution in an immigrant species on Yakushima, the presence of pollen aberrations is a tribute to its endurance, not an indication of hybridity, in my opinion.
Professor Bengt Kihlman of the Department of Genetics at Uppsala University in Sweden has found (2) that "Pollen sterility can be caused by many factors, genetic as well as environmental (my emphasis), and even by virus infection." Professor John Rouse of the University of Melbourne has found (3) that a plant of R. laetum routinely produces pollen with only two of the four grains developed. He concluded that a plant with only 5% to 10% of the pollen forming normal tetrads still functions fully as a parent. He has further observed that any single plant varies within a season, and from season to season in the quantity and normality of the pollen. (3)
4.  The seed lot of R. yakushimanum which produced about 400 plants at my hybridizing and trial grounds came from Francis Hanger, then head gardener for Exbury Estate, from a controlled cross between the two surviving plants imported from K. Wada by Lionel de Rothschild in 1934. Since the cross was made at the time of the first flowering of the species in England, or close to it, the seeds produced plants which were probably the first R. yakushimanum in America. The seedlings were remarkably uniform, suggesting their authenticity. There were no aberrant plants whatever. The selections, 'Mist Maiden' and 'Pink Parasol' were slow to be made because of the uniformity among the plants, and finally came about when these, with five others, appeared to be somewhat hardier after a severe winter. My Canadian friend has not remarked upon the significance of these heavily indumented seedlings, as nearly identical one with the others as any progeny from species seeds that I have ever seen. Any contamination of the cross by bees or other agents in a southern English garden would almost certainly have produced some tender plants in this seedling population.
Readers can judge for themselves the persuasiveness of the evidence presented by both partisans in this amicable discussion.

(1)  Acta Horti Cotoburgensis XXIV, Nitzelius
(2)  Personal Communication
(3)  Letter of May 1, 1988
(4)  A.R.S. Bulletin, 1978, Leach

David Leach, author of Rhododendrons Of The World , specializes in hybridizing and introducing superior cold hardy rhododendrons.