There was some excellent research done in Europe in the 1970s by J. Heurshel and W. Horn where they identified six pairs of genes controlling flower in evergreen azaleas. It was all in German and I just have a few translated notes I got from Augie Kehr many years ago. Their study showed that flavenols which give that ivory color also seem to increase the "purpling" effect when anthocyanin pigments are present. Anthocyanins are water soluble but they control red, purple, and blue colors we see in many flowers. The exact flower color will depend upon the pH of the plant tissue, though. Flowers in rhododendrons are apparently buffered, so we can't change the pH of the soil and expect that will change the pH of the flower and thus the color. We can do that with hydrangeas, though.
Dealing with unexpressed anthocyanin pigments will add to the complexity, especially when trying to breed for yellow. Many white azaleas like 'Rose Greeley', 'Desiree', and kiusianum album have those hidden purple genes. The ability to produce flower color is recessive, so for a plant to have white flowers it must have two of those recessive genes. Unfortunately, purple flower color is a dominant gene and will totally dominate in crosses with salmon or red shades. It will often show up with abandon in future generations depending upon the whites one uses since only one of those purple genes needs to be present to get a purple flower.
The effect of flavenols on purple genes might explain why Joe Klimavicz's cross of 'Leopold Astrid' with 'Girard's Fuchsia' produced a seedling with a definite yellow color. 'Girard's Fuchsia' is probably that intense purple shade because it has a high concentration of flavenols which modified the pigment expression. 'Girard's Fuchsia' was probably not homozygous for purple so when those genes segregated out in 'Sandy Dandy', he still had the flavenols but no purple genes anymore. 'Sandy Dandy' is gorgeous and a very unique color... yellowish-cream with a bit of a pink blush. I picked one up at the 2006 Convention and it has buds for next spring. I haven't used it yet in hybridizing yet but maybe this year.
Good yellow rhododendrons often contain both flavenols and carotenoid pigments. Carotenoid pigments are insoluble in water, but those are the ones that provide the deep yellow color we see in native azaleas. I suspect the orange shades in some deciduous azaleas like calendulaceum might be the result of carotenoid pigments combined with anthocyanins but I am not sure. Every time I see that pink flush on a calendulaceum flower I suspect that both pigments are present. If anyone knows of some research in that area I would appreciate hearing about it. I know there was an article by Bob Griesbach (USDA) in the Winter 1987 ARS Journal on "Rhododendron Flower Color" that briefly discussed chlorophyll, flavenoids, and carotenoids in relation to flower color.
I agree that we need to build on Augie Kehr's work. My approach is to find hybrids between evergreen and deciduous azaleas that may already have genes for those carotenoid pigments... a.k.a. hybrids like 'Pryor Yellow' although that plant was sickly and has already died. I also think we need to get away from the deciduous azalea leaf quality as soon as possible since that will probably dominate those primary crosses. Most of the Azaleodendrons I have seen that are crosses of deciduous azaleas with rhododendrons have the worst looking foliage... very thin and ratty looking. They might look better if that foliage dropped off but it usually just hangs on looking awful.
If I can get plants that might have those carotenoid pigments in the background, I might first cross them with white azaleas that have a high concentration of flavenols and hopefully no genes for purple. Once I get a batch of those, I hope to cross the best "yellows" with one another and raise a huge batch of seedlings. Some might have higher concentrations of both pigments and then I can select for good evergreen foliage. That will likely take many generations... probably not in my lifetime either.
I know there will be sterility issues and selecting tetraploid parents will be important, especially since now know from Tom Ranney's research in the Fall 2007 ARS Journal that in addition to calendulaceum, plants we thought were diploid like austrinum and 'Admiral Semmes' are actually tetraploid. I now have concerns about your cross with 'Snow' since I suspect it is diploid and unfortunately, that means most of your seedlings will probably be triploid and sterile. I wish my 'Puck' was tetraploid, too, but I have no way to tell and I suspect that it is diploid. I expect to have lots of sterile plants with that cross as well. That just means we have to plan better when we make our crosses this spring. :-)
The route to a yellow evergreen azalea is complex, but Augie Kehr was heading in the right direction and we just need to continue his fine work. Wish he were still around to give us some advice, and that we could thank him for all he has done. He was such a great man and I feel honored to have know him!
Don Hyatt McLean, VA
