Mechanics of Basic Hybridizing

By Albert J. Muller

Hybridizing is easy, right? Bees do it all the time without even trying. Well, if we are willing to go with whatever seed pods that develop in nature, we need not read any further. However, the challenge of hybridizing bites most of us at some time. Since I well remember questions, problems and failures I had in my first attempts, I was determined to do some research to find out what I should do and why. The result – my version of Hybridizing 101A.
The following material is presented only as a guide for the newcomer to the world of hybridizing; it is structured so as to supply the background of plant physiology and the rudimentary steps in the process of hybridizing. It also hopefully will help you cope with some of the bugaboos of hybridizing: the possible disappointment in the seedlings you produce not meeting expectations; the chances of the duplications of the efforts of other hybridizers; coping with space requirements for growing on a reasonable number of seedlings from which to choose the ones worth saving.

First, a few basic definitions - Hybridizing is the process of creating new varieties from already established plants. It is achieved by combining the attributes of one plant with those of another; this is done by fertilizing one plant with pollen taken from another. The result is called crossing, and the hybrid created is called a cross. The notation identifying a cross is written as: Plant A x Plant B, in which Plant A is the "mother" plant i.e. the receiver of the pollen, also called the seed parent; and plant B is the plant from which the pollen is taken, the "father" or pollen-parent. The resulting hybrid is the product of the seed produced in the seed-bearing (mother) plant and will contain varying degrees of the attributes of both parents, yet is recognizably different from each.
Now, we need to know the basic parts of the flower that are involved in fertilization. Below, is a pictorial representation of a typical rhododendron flower in a truss.

Each flower contains male and female sex organs. However, in most instances the pollen is obtained from a separate plant (the exception is in the rare case where self-crossing is contemplated.) The female reproductive organ is a single slender structure growing out of the center of the flower and is called the pistil.

The pistil consists of three parts, namely, the stigma, ovary and style. The stigma is the knob at the end of the pistil which accepts the pollen; the ovary at the base of the flower is which the seed is produced; and a connecting tube known as the style.
The male reproductive organ is the stamen. The stamen consists of the anther, which has two chambers, each with a hole through which the pollen is dispensed. The chambers are called lobes or pollen sacs and contain masses of pollen grains. Anthers are carried on tubes called filaments. Rhododendrons normally have ten stamens surrounding the pistil. (Notable exceptions are Asiatic species which have eight to twenty, and American azaleas which have five.)

Let’s pause for a moment to consider when will be the optimum time and environment to make a cross. Much study has gone into defining ideal conditions for producing seeds from your crosses.

Experimentation has established that a greenhouse environment, with enhanced temperature and humidity control, is most desirable for greatest chance for success. Paraphrasing Weldon E. Delp, a long time hybridizer and pioneer in this field (see sidebar "Optimum Gestation"), it has been established that fertilization is achieved in a much shorter time and more effectively with increased heat and humidity. Short of being able to create such conditions, one must use his best judgement in trying to come as close as possible to setting up such an environment.

We now have to consider harvesting the pollen. Pollen is not a dust as in many plants, but rather long, irregular (tacky) stringy masses. The pollen ripens before the flower opens. After the flower opens, the pollen may soon be lost. Some varieties have great amounts and virtually "drip" pollen (such as the fortuneii series, ‘Janet Blair’, ‘Susan Everett’, and virtually all deciduous azaleas.) Other varieties such as the species Metternichii are all but impossible to use as seed parents due to the copious pollen contaminating the pistil before the flower opens. Conversely, some are pollen sterile (such as ‘Scintillation’) and some are very difficult to obtain pollen from (such as my Yak.) Therefore, just as the buds start to show color before opening, carefully remove the anthers with a pair of tweezers and place the pollen in a paper envelope. If pollen is not visible protruding from the anther holes, hold an anther by the filament (stem) and gently shake it or flick it with your middle finger snapping gently off your thumb, carefully observing whether pollen moves out of the anther hole. On some difficult varieties, it may be necessary to try different anthers from different buds. If no pollen is present, try the following day. If unsuccessful for over several days, you probably have a sterile pollen plant. Some varieties are very stingy pollen producers, so have patience. If you are using a plant that drips pollen, you have a much better chance of success.

It is important to note that prior to making the cross, the buds chosen to receive the pollen must be emasculated, that is, the petals are all carefully cut off with a pair of cuticle scissors and the anthers all carefully picked off and discarded. Also, all the flower buds around the target truss are to be removed to discourage bees from entering the area.

The next step is to present pollen to the seed parent, the mother plant. When to do this? The text book time is "when the stigma ripens and is sticky." However, the stigma doesn’t always run up a flag when this happens – my experience is that it isn’t always very obvious, or you may not be available when it happens. In any event, check other flowers in the seed parent plant.

About three days after the flowers open the stigmas should be receptive, and should stay receptive for about three to five days, so you should cover the entire stigma with pollen at this time. (I have found that fresh pollen adheres to the stigma anyway, and have been successful in my limited experience in having this work whether or not I was sure if the pistil was completely receptive. I theorize that pollen easily remains viable for the several days it may take for the stigma to fully ripen.

Therefore, when the stigma ripens, the pollen is already there, provided it has been covered to prevent rain from washing it away. At least six to eight flowers should be pollinated to give the best chance of getting some seed pods. If you make just a couple that don’t "take," you have to wait until next year.
After a flower is pollinated you break of all the other buds on that truss and cover the remaining hybridized truss with a plastic bag tied loosely at the base to protect from the wind and rain, but allowing for some air movement, Bees should not be a problem on the emasculated buds, since no color is present to attract them.

Another method of covering used by Jack Rosenthal, a well know NY Chapter hybridizer, is to make short aluminum foil tubes using a pencil as a mandrel and bend on end over. Slide the open end over each pistil and tie groups of these together for support as convenient. Carefully label each cross, seed parent (mother) first x pollen parent and date.

Now, let’s understand the fertilization process, as outlined by Dr. Clement Bowers. Once the stigma ripens, a critical process must take place whereby the stigma excretes a thin syrup and furnishes a nourishment to the pollen, which then forms a sprout-like process called a pollen tube. The style (stem) portion of the pistil has a channel running down its center from the stigma to the ovary configured with loosely formed cells through which materials easily pass. The pollen tube grows heading down this canal. The pollen tube develops into a long slender thread-like structure as it grows down the style canal toward the ovary. At its lower end, the pollen tube contains the male cells and vegetative nucleus. In the ovary, the female (egg) cells are present. After a minimum of 24 hours at approximately 68° F., the pollen tube enters the ovule; the male nucleus is discharged and fuses with the egg nucleus to form a new seed. A separate ovule and a separate pollen cell are required to produce each seed. Dr. Bowers further writes that, "up to several hundred pollen tubes may pass down the style at any on time, and these may not be of the same variety or species." Therefore, it is entirely possible in open pollinated flowers for one rhododendron flower to be successfully pollinated by several different sources of pollen at once. The implication is, therefore, that based on all the above, on open pollinated seed pod could produce seed with several different crosses. (This information on open pollinated crosses is added here as food for thought.)
If you have been at all successful, you should observe a swelling or elongation of the ovary well within a month. This in no guarantee of success, however. Observe the pods periodically through the developing cycle – June through September – and start watching for ripening, i.e., turning brown late September and certainly after the first frost. The seed pods may be harvested now. Wait too long and the pods can break open and lose the seeds.
The remaining husbandry is obvious – label and store your seed carefully, and donate generously to your chapter’s seed exchange!

1. Proceedings of Breeders Round Table, 1973, American Rhododendron Society.
2. Rhododendrons and Azaleas, Clement Gray Bowers, The Macmillan Company, New York, 1960.
Note: Al Muller is a member and former president of the New York Chapter of the American Rhododendron Society. Email Al Muller at:aljmuller@juno.com

Optimum Gestation

Weldon E. Delp, Harrisville, PA wrote and lectured about his studies and experiences on the ideal hybridizing environment. He states, "Improved seed production with high heat and humidity has been used to produce viable seed from hitherto "impossible" crosses. I have found (Delp continues) that the temperature range which plants will withstand is 110° to 114° F for a period of about three hours, with the humidity as close to 100% as possible. The plants remain in good active condition with no detrimental effects. Van’t Hoff’s rule states that for every 10 degrees rise in temperature, the speed of chemical reaction (in the pollinated plant) is doubled.1 With this in mind, I decided that if I could hold this temperature for around two hours, without any ill effects on the plant, it would be the same as having the pollen on the plant for a period of 32 hours. As a matter of fact, I am sure the pollen reacted in a much shorter time and was more effective with the increased heat and humidity.." AJM

Editor’s note: The above was taken from an article "How to Succeed in Producing Rhododendron Seed" by Weldon E. Delp. His article is reproduced in its entirety in the paperback book "American Rhododendron Hybrids", published by the American Rhododendron Society, 1980, Meldon Kraxberger, Editor.

Mechanics of Basic Hybridizing
Previously published in:
1. Rhodora. New York Chapter Official Publication. Winter, 1995 Edition
2. ARS Journal, Winter 1996
3. Pukeiti. New Zealand (Rhododendron Trust). June 1996 Edition