Getting The Max Out of R. maximum or "Apomixis Anyone?"
George W. Ring, Fairfax, Va. - Raymond H. Goodrich, Vienna, Va.
In a few instances notable hybrid rhododendrons have been achieved using R. maximum as one of the parents. Some have been reported by Leach1, Tichnor2, and Wister3. On the other hand Leach has documented serious questions concerning the value of R. maximum as a parent4. It is the intent in this report to offer a dissent to one of the major criticisms reported. Perhaps with a little time, R. maximum may be started back on the road to respectability. However, for the moment the subject is "apomixis."
One of the major drawbacks, according to Leach is the "extraordinary large number of seedlings lacking evidence of hybridity." This absence of hybridity has been attributed to apomixis*, or more exactly to a special form of apomixis called apogamy.
(* - apomixis: "reproduction involving the specialized generative tissues but not dependent on fertilization." apogamy: "development of the sporophyte from the gametophyte without fertilization; - where the embryo arises from a cell or cells other than the egg and without fertilization." - Webster.)
Since most rhododendrons bloom earlier than R. maximum, it is most logical to collect their pollen and apply it to the later blooming species as the seed parent. It is this sequence of crossing that has probably been the source of the lack of hybridity in maximum seedlings. As our beloved master rhododendron hybridizer, the late Joseph Gable, once wryly observed, "when apomixis occurs with a particular seed parent, try making the reverse Cross." If this had been truly practical, we might now have a larger assortment of R. maximum hybrids. However, the problems of storing the pollen over the winter and distributing it to twenty or more seed parents are obvious, particularly if some of the seed parents will survive only in greenhouses in the eastern U.S.
Unhampered by too much expertise, equipped with some good specimens cf R. maximum and stimulated by the desire to blend the hardiness, foliage, and late bloom of R. maximum with some of the more beautiful flowers not available here, the authors have made nearly fifty assorted crosses on R. maximum. At this time most of the seedlings of the crosses exhibit clear evidence of hybridity coupled with the vigor to survive one or two severe winters. Typical of these are crosses with 'Disca', 'Autumn Gold', 'DavidCrest', and chrysanthum-degronianum. Some, such as 'Sir Charles Lemon' and R. mallotum, yielded very few seeds with only one or two surviving seedlings, but clearly the survivors are hybrids. A few others, typified by R. recurvoides, yielded clearly hybrid seedlings, but many were unable to survive the hot summer. There are some others, particularly large leaved types, such as R. fortunei, R. calophytum, and others having leaf size and stature comparable to R. maximum, which produced many seedlings whose hybridity might be questioned. But, usually, there have been a few seedlings in the group which leave no doubt of their hybrid parentage. Incidentally, the authors have found that it is often the smallest seed pods which yielded the most obvious hybrids. All of the seedlings in these experiments were started in the fall or winter under indoor lighting and grown on to several inches height before setting out in the spring. In most cases, the hybridity was evident even before planting out-doors. As a summary, we have obtained clear hybrids in the majority of R. maximum crosses, and have yet to make a cross in which hybridity is clearly absent.
We now believe that what has occurred for others in R. maximum crosses is erroneously attributed to apogamy, and is actually self-pollination. With this suspicion in mind, we observed very carefully during the past blooming season and found reason to strengthen our belief. Florets approximately three days from blooming had the corolla removed, and it was noted that the stigma was not yet receptive, and the stamens were sill short but curved toward the center of the floret and actually touched the style. Pollen could be seen and extracted from the anthers at this time. When the same emasculation was performed on a floret about one day before opening, the situation was about the same with one major exception. In the growth process, the stamens had extended and were about the same length as the stigma, and in some cases were in contact with it. In at least one case. actual pollen was observed (by magnifying glass) on the still non-receptive stigma. It was normally a day or two after the floret opened when the stigma became receptive; the style curved upward and the stigma became red-to-pink, shiny, and sticky. It is, of course, our hypothesis that pollen may be deposited on the stigma even before the floret opens and may remain viable until the stigma becomes receptive, i.e. the physical characteristics are such that self pollination is so easy as to be very frequent, if not unavoidably so.
If this hypothesis is correct, and we will plan an experiment to verify it, the procedure for making crosses on R. maximum without apogamy, becomes clear. Just be sure to remove the corolla and stamens about three days before the floret is large enough to open, and before the stamens are of significant length to deposit pollen on the stigma. In practice, this is done by emasculating the floret as soon as it is large enough to be conveniently snipped with small scissors.
Using this reliable method for obtaining true hybrids, we believe that there is a much higher chance of obtaining outstanding plants having the good qualities of R. maximum combined with other rhododendron genes. We plan to continue trying to get the max out of R. maximum by making crosses with new species and notable hybrids, especially in combination with yellow-flowered clones. We will also continue our observations of growth habits as our seedlings mature and plan to report on their adaptability and flowers in another article.
- Leach, David G., The Maximum Effect, Qu. Bull. A.R.S., Vol. 23, No. 2.
- Ticknor, R. L., Some Notes on Rhododendron Maximum as a Parent, Qu. Bull. A.R.S., Vol. 19, No. 1.
- Wister, John C., Ph.D., For the East Coast a Few Early Flowering and Late Flowering Rhododendrons, Qu. Bull. A.R.S., Vol. 19, No. 3.
- Leach, David G., The Rosebay Rhododendron: Its Value as a Parent, Qu. Bull. A.R.S., Vol. 19, No. 2
R. MAXIMUM CROSSES
Hybridity Not Certain
1969 'Disca' 'Autumn Gold' 1970 recurvoides grande beanianum bureavii bureavii, Exbury aberconwayi 'Sir Charles Lemon' 'F.C. Puddle' 'Brinny' orbiculare tsariense 'Elizabeth' 'M. Dunn, Talisman' wasonii 'Fabia', James form 'Odee Wright' 'Leo' 'Moon Mist' crinigerum 'China' 'Noyo Chief' 'Cheyenne' mallotum 1971 'David' x 'Crest' 'Inca Gold' x yakushimanum chrysanthum x degronianum
as of date
chlorops, Lackamas Cream fortunei houlstonii fargesii Rock # 30 decorum, Dr. Hu 'King of Shrubs' wightii Dido x williamsianum calophytum, pink coriaceum bainbridgeanum 'Carolyn Grace' yakushimanum x sutchuenense 'Leo' (repeat cross) fictolacteum unnamed white hybrid yellow hybrid 1972 wardii falconeri 'Mary Garrison'
(The authors are deeply indebted to all those who so kindly sent pollen.)