Hybridization

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Hybrids result from the crossing of two species and occur regularly in nature. The pollen from one species is transferred to the pistil of another species by insects, wind, birds, etc. The resulting hybrids may or may not survive in nature. If not, they will not be perpetuated.

Hybridization is used in culture to produce plants which differ from the species in respect to vigor, disease resistance, size, growth habits and form.

When we cross-pollinate species artificially the potential for survival increases. We carefully collect, plant and grow the hybrid seed under optimum conditions for maximum survival.

Hybridization is both fascinating and challenging. To date, the hybrids produced are those within a genus, that is Drosera crossed with Drosera or Sarracenia crossed with Sarracenia and not, for example, Drosera crossed with Dionaea.

Successful hybridization is enhanced by crossing species having the same number of chromosomes. While all the Sarracenia species have equal numbers of chromosomes, not all of the Drosera do. To ascertain the chromosome number of the species, the literature must be consulted. An ideal reference is the Kew Index to Taxonomic Literature. Also Katsuhiko Kondo has done much work on chromosome studies, and published his results in The Journal of Japanese Botany.

Hybridization by man involves not only the transfer of pollen from anther to stigma,

Fig. 9-3 Propagation of Drosera by decapication. The plant is cut from the roots near soil level. The removed portion is firmly replanted. Both the removed portion and the roots will regenerate their missing part.

but also protection of the ovary from unwanted pollen and careful recording ol cross as well as marking the ovary for identification of the seed when mature.

To increase the probability of subsequent fertilization and seed development, it t* necessary to use mature pollen. Mature pollen is visible to the naked eye or with the aid of a 10 X magnifying lens. It usually appears as dense masses of pale to deep yello spherically-shaped bodies when the anther splits open.

Mature pollen can be transferred using flat toothpicks, small camel's hair brushes <i the stamen itself. Toothpicks have the advantage of being inexpensive and disposal*I. We usually use small camel's hair brushes for our Sarracenia hybridizing. To keep tr.u V of which brush was used to transfer what pollen, we insert the brush, handle first, intht soil next to the plant supplying the pollen. After each transfer we shake out as much pollen from the brush as possible. Before the brush is utilized to transfer the pollen ol another species, it is washed thoroughly in warm, soapy water and allowed to dry. If water or nectar drops are mixed with the pollen, and adhere to the brush, as sometimes happens with Sarracenia, the brush should be washed before being used again.

If the stamen is used to transfer the pollen, remove it gently from the donor flower with tweezers. Rub the anther with its pollen on the stigma of the other flower. This operation should be done quickly as the stamen dries rapidly. The anther cannot be used as a pollen source another day unless it is wrapped in wax paper or plastic and refrigerated.

Once you have made the pollen transfer, you should check to make sure the job has been done. There should be pollen on the stigma. If it is not visible to the naked eye, use a magnifying glass to make sure. Whenever possible, transfer pollen each day for several days or until the flower closes.

To insure that the cross you have made is maintained and not contaminated with undesirable pollen, the flower that is expected to produce the seed should be protected from insects that may transfer pollen during their quest for nectar, by covering the flower. Plastic bags, gauze, fine cheese cloth, muslin or paper bags are used to cover the flowers or the whole plant to prevent contamination. If plastic bags are utilized, tiny holes should be punched in them and the plant kept out of direct sunlight while covered.

Unwanted pollen also comes from the anthers of the flower that will produce the hybrid seed. The easiest way to prevent pollen in the same flower from pollinating the stigma is to remove the anthers from the flower before they mature. We usually remove the anthers from Sarracenia flowers that are to be crossed unless the pollen is needed to effect another cross. In some Drosera species, the flowers open during the day, usually before noon, and close the same day in the afternoon. Upon closure, the petals press the pollen to touch the flower's stigma, thereby causing self-pollination. In order to prevent self-pollination, the anthers must be removed. If possible, it is safest to remove the anthers from the flowers before the pollen is mature to reduce the incidence of accidental self-pollination.

In order to hybridize two species, either they must flower at the same time or the pollen from the anther of one species must be available when the pistil of the other species is receptive. There are two ways to deal with this problem. One is to delay the flowering of the earlier flowering plant so that it will flower at about the same time as the late flowering species. The other method which is the simplest, once a procedure is developed, is to store the pollen until it is needed.

We will use the genus Sarracenia to illustrate our technique for delaying flowering of the early flowering species. Experience with Sarracenia kept in our greenhouse the year around reveals the following sequence of blooming, which fall in 2 distinct flowering groups, early and late. S. flava flowers first, followed by S. alata, S. oreophila, then S. purpurea and S. leucophylla. A few weeks later S. minor, S. rubra, S. alabamensis and S. psittacina come into bloom.

We have been very successful in producing hybrids between the 2 groups using the following procedure. About 5 dormant plants or rhizomes of each species to be crossed are stored under refrigeration. In the spring, we plant the late flowering plants one at a time, at 2-week intervals. What date in the spring depends upon your location. In Florida it is late February or early March, whereas in New York State, where we grow, it is mid-to late-April in order to avoid the expense of heating the greenhouse when the plants are flowering. When one of the early planted plants produces a visible flower bud, commence planting the early flowering plants or rhizomes one at a time at 1-week intervals. Using this procedure we have made 26 different successful crosses between species.

This timing procedure can be accomplished without refrigeration. The flowering time of the early blooming species is delayed by keeping the plants cooler, by placing them on the ground beneath greenhouse benches, on the side of the greenhouse that is not exposed to direct sunlight or is shaded.

If you are working with species whose blooming sequence you don't know, this sequence can be ascertained as follows. Plant one or two of each species that are involved. The first species to bloom will be classified as the early flowering type. Now follow the procedure described for Sarracenia. While plants used in the above illustration are kept dormant by low temperatures other plants such as the tuberous Drosera are kept dormant by a dry, warm environment.

A much simpler procedure for making crosses is to collect the desired pollen when it is mature and store it until it is needed. Sarracenia pollen can be collected, wrapped in wax paper and stored in a refrigerator (not freezer) for up to 2 months and still retain its viability. Storage of viable pollen for this length of time will enable you to execute any of the crosses in the Sarracenia genus by collecting and storing pollen of the early blooming species. Perhaps pollen storage will work for some other carnivorous plant genera.

Freezing pollen until needed has been done with other groups of plants, but as far as we know it has not been tried with any carnivorous species. The procedure, developed by C. D. Clayberg of the Connecticut Agricultural Experiment Station with Gloxinias, follows so that if any reader wants to try it with carnivorous plants they will have some guidance.

Gather the fresh pollen, place it in a small open bottle or open vial. This container is, in turn, placed inside a slightly larger bottle (the dessicator) in which about tyz teaspoon or 2.5 milliliters of silica gel has been placed. Then the larger bottle is tightly capped. The purpose of the dessicator is to remove moisture from the pollen for better storage.

The pollen should be left in the dessicator for 3-4 hours at room temperature, after which the pollen, in its dessicator, is placed in a yet larger storage jar containing a layer of silica gel. The storage jar is tightly capped. If you are storing pollen from more than 1 plant or species, each should be placed in its own small dessicator but several can be placed in the storage jar. (Fig. 9-4) Place the large storage jar in a freezer (not refrigerator) at temperatures below 32°F (0°C) until needed. When the pollen is needed, remove it from the freezer and allow it to stand at room temperature for about 10-15 minutes before opening the smaller dessicator. If there is more than one kind of pollen in the larger dessicator, remove only the vial you want to use and immediately return the rest to the freezer. Viability of frozen pollen depends upon the genus and species of the plants and can last from a few weeks to over a year.

It is difficult to know when ordinary silica gel has absorbed all the water it can, whereas Tel-Tale, a brand product, is an indicating gel. It has been treated so that when it is dry its color is blue and when it is translucent pink it has absorbed all the moisture it can. To regenerate the gel, spread a thin layer in a shallow pan such as a baking dish and bake at 250°F (121°C) until it changes back to a blue color. It is then ready to be reused. While still warm the silica gel should be placed in sealed containers for storage until needed.

Record-keeping is a very important part of hybridizing plants. Your records should be accurate and thorough. Minimally they should include: date, plant species receiving and plant species supplying the pollen. The usual procedure is to indicate the Iim the female plant first followed by an "X" and then the male plant, the pollen suppliei For example, if pollen from Drosera spathulata is placed on the pistil of D. capensis, the cross would be written: D. capensis X D. spathulata. Failure to record the cross when il I executed is an all too common mistake. Procrastination in record-keeping can be disasterous, especially if several crosses are made at the same time. A cardinal ml. should be to record the cross at the time it is made.

Hybrids resulting from crosses should be identified so that when seed is produced you will know its parentage. Plastic plant markers with the crosses written on them serve this purpose, but care should be taken to make sure they are not removed anil replaced in the wrong pot. Visitors in your growing area can, however, make .1 shambles of markers. So it is wise to tie the plastic tag to the plant. Hybrids growing in beds must be identified in the same way. We use plastic tape dispensed after being printed or numbered by a labeling gun. A hole is punched in one end of the tape, then the tape imprinted with a code is tied to the flower stalk with nylon string or a rubber band.

Do not label hybrids with any label material or marking ink which will fade when in a humid environment, or from water and sunlight. It is very disheartening to have successfully produced seed from a cross and then not know the parentage. Sometimes the hybrid seed yields progeny whose parentage is obvious, but when dealing with complex parentage it is difficult or impossible to determine the parentage by looking at the offspring. So label the crosses that you make.

When dealing with plants which have several flowers on a single flower stalk such as most Drosera, we use short pieces of colored thread to identify the cross. For example, we use green thread to represent pollen from Drosera capensis. Every time D. capensis pollen is used we tie a piece of green thread around the pedicle or base of the flower. Thus, the green thread identifies all the flowers which have been pollinated with D. capensis pollen. When we make more crosses than colors of thread available, 2-3 colors are combined to represent pollen from a particular species.

We have found that the basic colors red, blue, green, black, white and yellow are the safest to use. Under greenhouse conditions the thread color may fade and become lighter in color, therefore, shades of the basic color should not be used with the basic color. For example, red thread may fade during the summer so it could look like or be confused with pink or rose.

If a code is utilized, such as colored thread or numbers on plastic tape, be sure to record the code and its meaning in a permanent record book at the time you make the cross. Record keeping is a tedious but necessary undertaking.

Anyone attempting hybridization of small-flowered plants should consider purchasing a magnifier worn on the head such as the Magni-Focuser Mark II. The advantage of such a device is that both hands are free for making the cross.

When working with very small flowers a fine pair of tweezers is necessary to maneuver the floral parts.

Since a slight undesirable movement can result in damage to the pistil or unwanted pollen dispersal, it is safer to sit while you are working rather than stand.

We have noticed that when anthers were removed from some Drosera species such as D. rotundifolia, the petals closed over the pistil in a relatively short period of time. Whether petal closure was caused by stamen removal or by some unseen injury to the flower due to the procedure, we do not know. There are 2 ways to handle the problem. One is to gently open the de-antherized flower by pulling the petals apart with fine tweezers when the pollen for the cross is ready. The pollen is then deposited on the exposed stigma. The alternative solution is to wait until the desired pollen is ready, remove the anthers from the flower whose stigma will be utilized and immediately

Fig. 9-4 Freezing of pollen necessitates removal of excess moisture. Pollen is placed in a small vial which is placed in a jar containing silica gel which is then tightly capped. After 3-4 hours are room temperature the jar with the vial is placed in a larger jar containing silica gel which is tightly capped. The 3 jar container set-up is placed in a freezer.

Fig. 9-4 Freezing of pollen necessitates removal of excess moisture. Pollen is placed in a small vial which is placed in a jar containing silica gel which is then tightly capped. After 3-4 hours are room temperature the jar with the vial is placed in a larger jar containing silica gel which is tightly capped. The 3 jar container set-up is placed in a freezer.

effect the cross. Extreme care should be taken so that self-pollination does not occm accidentally.

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