Let's Talk Hybridizing, Hybridizing with a
Genetic Mission: Line Breeding
Dr. Mark Konrad
Finding your way along the genetic trail of plants is a challenging and daunting experience. Who among us can begin to imagine the complexity? Fortunately, this is balanced by the ease with which new things can be created.
The purpose of this article is to focus on the development of genetic lines that could be very important to future hybridizers. Because of the staggering amount of material available, I would like to concentrate on one group of plants in subsection Pontica in the genus Rhododendron, which can be used as an example for other groups. Happily, much of the primary work has already been done.
The eastern part of our country is host to most of the rhododendron species native to the United States. They are a very important group of plants for a number of reasons. They should be considered a national treasure. Two of these species, Rhododendron catawbiense and R. maximum, are in subsection Pontica.
RHODODENDRON CATAWBIENSE. Rhododendron catawbiense imparts great cold tolerance as well as heat resistance, two very important characteristics. In addition to these special qualities, they have the ability to impart great beauty. The truss is uniquely formal and can be very appealing. A great contribution would be to cross the better white forms and select the best of a large population. Seedlings from a cross between 'Catalgla' and 'La Bar's White', both white forms of the species, show great promise.
RHODODENDRON MAXIMUM. Rhododendron maximum is also a very desirable plant for heat and cold resistance but possibly slightly less resistant than R. catawbiense. Primary crosses can throw very beautiful things, but it may not always be predictable. It might be better used as a hybrid.
RHODODENDRON DEGRONIANUM SSP. YAKUSHIMANUM. We all know the five star quality of this species. It also has proven very excellent for heat and cold resistance. Many beautiful hybrids have been produced with restrained growth, a big bonus, along with wonderful foliage and indumentum.
RHODODENDRON AUREUM. Rhododendron aureum (formerly classified as R. chrysanthum) is a species that does well for cold but poorly for heat. The yellow color of the flower is dominant, which may be very desirable at times.
WHITE CATAWBIENSE BY SSP. YAKUSHIMANUM. White catawbiense species forms crossed with ssp. yakushimanum have produced some very beautiful hybrids. F2 crosses should be exploited to fix the genetic pattern and capture the very best possible. Many outstanding hybrids from this cross are available in commerce.
'White Genes'* ('La Bar's White' x R. degronianum ssp. yakushimanum)
Photo by Mark Konrad
WHITE MAXIMUM BY SSP. YAKUSHIMANUM. This cross has produced magnificent hybrids. This year I have made F3 crosses in an attempt to further fix the genetic pattern.
'Angel Blush'* (R. maximum x R. degronianum ssp. yakushimanum)
Photo by Mark Konrad
WHITE MAXIMUM BY WHITE CATAWBIENSE ('LA BAR'S WHITE'). My first bloom occurred this year. I feel the cross should have great merit, but for some reason it has been a challenge to produce blooming plants.
'METTERNIANUM'1 BY AUREUM. The cross has produced extremely slow growing plants with pale yellow flowers. The F2 cross has just been made. It appears to have a very restraining influence on the progeny. 'Metternianum' is extremely tolerant of heat and drought conditions. The seeds were obtained from Koichiro Wada.
SSP. YAKUSHIMANUM BY SMIRNOWII. This cross has produced very beautiful hybrids both in bloom and foliage. The leaves are heavily indumented.
Other good primary crosses for line breeding in subsection Pontica are:
Select white form of R. catawbiense by R. brachycarpum and select white form of R. catawbiense by R. smirnowii. Exceptional plants from these crosses have been produced in Finland for cold hardiness.
'La Bar's White' by R. makinoi has produced seedlings with beautiful foliage. It should be mentioned that the plants from hybridizing in subsection Pontica might stand the best chance for survival in the regions that have extreme heat, cold and adverse soil conditions.
Line breeding can be a very important part of hybridizing. By crossing the best of two species very beautiful things can be produced. An F2 cross allows for the further fixing of the genetic pattern in addition to recovering recessive characteristics in the second generation. Line breeding allows for more predictable results in any future hybridizing programs.
In a simple way I have tried to stress the importance of line breeding. The results will be much more predictable. Innumerable F1 crosses have been made so that one-half of the work is already done. Two siblings should be sought to cross for an F2 population. Selfing is often done; however the plants may lack vigor and often hybrid sterility is encountered.
It is interesting to note that the late Bill Fetterhoff of the Great Lakes Chapter produced beautiful sibling plants between R. maximum and R. wardii. This year Bettye Bowen, a member of the Great Lakes Chapter, made the F2 cross between two of the named siblings, 'Susan Kay' and 'Adele's Yellow'. What a great way to honor and perpetuate the work of a hybridizer.
1 'Metternianum' is a form of R. degronianum ssp. heptamerum, selected by Koichiro Wada.
* Name is unregistered.
Mark Konrad, a frequent contributor to the Journal, is a member of the Great Lakes Chapter.