Journal ARS Article
Vol. 50: No. 1: Year 1996
Tips for Beginners: Mechanics of Basic Hybridizing
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 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. This action is called a cross. The notation identifying a cross is written as Plant A x Plant B, in which Plant A is the "mother" of the new 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. Each flower contains male and female sex organs. However, in most instances the pollen is obtained from a separate plant (the exception is in rare cases where self-pollenization 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 that accepts the pollen; the ovary at the base of the flower is where the seed is produced; and the connecting tube is known as the style.
The male reproductive organ is known as 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 with five.)
Let's pause for a moment to consider when will be the optimum time and environment to make the 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 the greatest chance for success. Paraphrasing Weldon E. Delp, a long-time hybridizer and pioneer in this field, 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 judgment 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 Fortunea subsection, 'Janet Blair', 'Susan Everitt' and virtually all deciduous azaleas), and some varieties such as the species R. metternichii are almost impossible to use as a seed parent because of the copious pollen contaminating the pistil prior to the flower opening. 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 the 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 cross 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 the pollen to the seed parent, the mother plant. When to do this? The textbook 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 on the seed parent plant. About three days after the flowers open, the stigmas should be receptive and should stay receptive for 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. There, when the stigma ripens the pollen is already there, provided it has been covered to prevent it washing away in the rain.) 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 off all the other flowers 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 is to make a short aluminum foil tube using a pencil as a mandrel, bend one end and slide the open end over each pistil. Then tie groups of these together for support as convenient, and carefully label the cross, seed parent (mother) first pollen parent (father), and date.
Now let's understand the fertilization process, as outlined by Dr. Clement Bowers.1 Once the stigma ripens, a critical process must take place whereby the stigma excretes a thin syrup and furnishes 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 discharge 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 one 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, one open pollinated seed pod could produce seed with several different crosses. (This information on open pollinated flowers is added here as food for thought.)
If you have been at all successful, you should observe a swelling or elongation of the ovary starting well within a month. This is no guarantee of complete success, however. Observe the pods periodically through the developing cycle - June through September - and start watching for ripening, i.e., turning brown in or by 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 seeds.
The remaining husbandry is obvious - label and store your seed careful, and donate generously to the ARS Seed Exchange or your chapter's seed exchange.
1Rhododendrons and Azaleas, Clement Gray Bowers, New York: The Macmillan Company, 1960.