Germinating Paphiopedilums a laboratory overview with attention to rapidly germinating Paphiopedilums - a new phenomena in Complex Hybrid breeding.

Quick Page Contents: Paphiopedilum seed, harvesting, genetics, rapidly germinating Paphs

 

 

 

 

 

 

Of all the major groups of commercial orchids many consider the germination of Paphiopedilums to be the most difficult and challenging. The production of uniform plants in large numbers has always posed problems for the commercial labs and nurseries. According to Paul Phillips of the world famous nursery Ratcliffe Orchids Ltd., 'germinating Paphs has always been difficult and problematical'. Ratcliffe Orchids have been in the business for over 50 years.

This is reflected to some extent in the sale price of the end product. Low volume of seed production is one reason, this low volume can result in very few seedlings germinating and growing into plants. Germination can sometimes be extended over several years, and it is not unusual to find plants geminating 3-7 years down the line. Sowing to germination times can be rapid, slow or continuous and complicated if seeds exhibit dormancy before germination.

It is well known that light plays an important part in the germination response of some Paphiopedilum seed. Exposure to light inhibits germination and can lead to an induced dormancy response. However what is surprising is that not all seed require a period of dark to germinate, and some indeed will germinate quicker if exposed to light.

Reasons for such a range of germination responses can be varied and to some extent reflects the origins of the seed. In this respect the genetic influences of the pod and pollen parent are critical. However the growing conditions of the mother plant, harvesting and correct treatment for in-vitro sowing all have an influence on production results.

Almost all of the work done on germination of Paphiopedilums is based on aseptic seed sowing rather than infield studies. Where field studies have been done they usually record the physical climate and characteristics of the habitat of different Paphiopedilum species and relate them to meteorological records of the nearest weather stations. Indeed it is not even known the 'fate' or germination response of Paphiopedilum orchid seed in the wild over a number of seasons. It is impossible to predict with any certainly how long a Paphiopedilum seed will remain viable when liberated into the natural habitat. Could indeed some seed be lying dormant for many years? Consequently our discussion and results have had to be based on laboratory work.

The genetic influences in Paphiopedilum breeding reflect the huge number of species used by breeders. Depending upon sources some taxonomists consider there are in the region of 125 species within this genus. Of these probably 50 or so are used frequently in the production of seed pods.

There are some basic rules that apply to breeding Paphiopedilums. Species when self pollinated or crossed with similar plants produce either abundant or small amounts of seed following pollination. Some species are easy to germinate and produce a large number of seedlings. Others produce abundant seed but germination can be slow or difficult resulting in few plants as an end product. A third possibility is poor seed formation and poor germination... this usually leads to plants being rare in cultivation or even impossible to raise. Occasionally only isolated seeds will germinate among a whole batch of plentifully produced seed.

Generally we have noticed that those plants that produce abundant amounts of seed will often germinate well though there certainly is no guarantee to that.

The crossing of two orchid species produces what is known as a 'Primary Hybrid'. Primary hybrids are widely produced by breeders of the genus Paphiopedilum. There are a number of reasons for this approach. It is often desirable to cross different or new Paphiopedilum species with other species as an attempt to introduce new breeding genes and characteristics into the nursery breeding lines. The resultant seedlings are often vigorous and produced in plentiful numbers, which on flowering reflect the obvious form of the two parents used. This 'hybrid vigor' is known as heterosis and is the result of a combination of genetic material from distantly related individuals and gives some clue to the shared genetic ancestry of the group in evolutionary terms. Such hybrid seedlings are often more vigorous and early maturing than either of the parents and can also show a high degree of uniformity in flower form. Another reason for making such primary crosses is because with some of the newer Paphiopedilum species, the germination and raising of the parental species through self or cross pollination results in seed that is difficult to germinate and trying to raise seedling plants proves to be a challenge, Therefore is is prudent to produce 'unique' primary hybrids as an attempt to quickly capitalize on the flowering of such plants.

Unfortunately the breeding of primary hybrids is also often associated with infertility in the resultant progeny. Although not universal, it has been found that many primary hybrids incorporating some of the Parvisepalum group of orchids will not produce fertile offspring. Such an end to a breeding line is indeed frustrating though under certain in-vitro conditions this can be possibly remedied by the application of chromosome doubling chemicals such as colchicine. This has to be applied 'in anticipation of this problem' at the stage of the original in-vitro seed raising of the original cross. This can result in chromosome converted tetraploid (4n) seedlings that go on to flower as fertile offspring from an otherwise infertile primary hybrid breeding program. At present it is not possible to multiply adult Paphiopedilum shoots in-vitro by micropropagation or tissue culture with any large degree of success, thereby closing this door to tetraploid conversion of flowering plants. Our work on Paphiopedilum chromosome conversion of seedlings has shown that a range of plants can be produced which are characteristically much slower to grow both in-vitro and in the nursery.

The additional breeding on of Paphiopedilums from the species or the primary hybrid route leads to a diversified classification of what is loosely know as 'Hybrid Paphiopedilums'. Although this can be subdivided into hybrid breeding, complex breeding and novelty breeding. Hybrids are the most diversified group and are the result of a whole range of different parental input. 'Complex Hybrid' breeding reflects the range or age of plants that have been bred for many generations in orchid nurseries. Such complex hybrids may trace their lineage back 15-20 generations and incorporate a huge ancestral gene pool, possibly reaching back through history to nursery raised plants of the late 1880's. In 'Novelty' breeding programs the orchid hybridizer pairs species with primary hybrids, other hybrids or complex hybrids in various combinations. Such novelty breeding is highly speculative but can often result in unexpectedly desirable results and is a short-cut route to achieving a diversity of flowering types and forms. The fertility of novelty Paphiopedilums can be good or poor according the interplay of genes.

Paphiopedilum breeding is dogged by infertility in the seed, dormancy and falling seed yield with the passing generations and complexity of the parental breeding lines. Sometimes it is necessary to reintroduce a Paphiopedilum species into such breeding programs to rejuvenate the production of larger numbers of viable seeds that will go onto produce worthwhile numbers of seedlings and plants.

Botanical definitions state that orchid seed pods are known as fruiting capsules or seed capsules. In orchid nurseries and for many orchid growers the term pod is far more widely used. We prefer to refer to green pod and green pod culture.

Harvesting the seed at the unsplit green pod stage is one method of collecting Paphiopedilum seed. This should be done when the maturation approaches full term. It is difficult to give accurate times for this as it will depend upon the type of plant, growth rates and cross. Generally plants growing in warm climates mature pods quicker. This can be as quick as 6 months for some multifloral types (section Mastigopetalum) or some species such as P. tigrinum take in excess of 13 months to mature. It is best to closely watch the maturing seed capsule and when you detect a change in colour, from perhaps green to yellow, or a browning or slight splitting is observed then harvest time is close. Some green pods become softer when gently squeezed. On occasions a colour change will progress from the pod end tip, or from the stalk carrying the seed pod. As these changes progress it is time to cut the pods and remove them to the laboratory for sowing as green pods. It is very important for the successful raising of Paphiopedilums that the mother plants are grown in optimal conditions and development of seed pods is allowed to progress in a controlled environment and the plants are not subject to cultural stresses. Moving or transporting plants, even to shows is not recommended as this can sometimes cause pods to fail on return to the greenhouse environment. Good plant culture often leads to good seed development and maturation followed by good results in the laboratory. The exact time for seed harvest can only be learnt with experience and requires close observation of seed pods on a day to day basis and attention to pollination dates and observation of the plants. The general aim is to allow for good maturation of seed embryos before removal of the seed pod.

paphiopedilum green pods

When required seed pods can be allowed to split, and then collected as dry seed. On occasions green pods that have started to split, unintentionally, can be allowed to dry off and the seed harvested as dry seed.

paphiopedilum dry seed

Whatever method of seed sowing is carried out, there are advantages and disadvantages. Sowing green pods principally allows the plants to recover quicker from carrying s seed pod for a period of 6-9 months. Removal of the partially mature green seed pod will help the plant make up new growths quicker and recover to flower again sooner. Sowing of the green pods has to take place on harvesting and generally the procedure is quicker in the laboratory. However confidence in the level of aseptic sowing techniques has to be good, as with green pod sowing there is usually only 'one chance' to get it right and if there is any risk of loss due to contamination this technique should not be adopted. The drying of seed and subsequent sowing by the use of bleach to decontaminate the seed should be adopted. Repeated sowings of small volumes of seed is the safest method to avoid loss and is an ideal approach for those new to orchid seed sowing. The principle disadvantages of green pod sowing is the uncertainly about the time of pod harvest, and not knowing if any seed are present in the seed pod until it is opened in the laboratory under aseptic conditions. Some pods are empty of seed on harvest, even though the pods appear to have matured as per normal. In addition there are always risks with green pods that the aseptic integrity of the seed in a maturing pod is not maintained. Sucking insects, premature and unnoticed pod splitting or overhead watering while maturing on the plant, all pose a risk to the seed pod. Dry seed collected correctly and filtered of debris can provide a safe sowing method for Paphiopedilums If correctly stored such seed can be handled at a later date when laboratory sowing allows.

Unfortunately there are a number of myths about advantages and disadvantages of green pod or dry seed sowing. Our observations are based on sowing thousands of Paphiopedilum crosses, over the years, either as green pods or dry seed. The principle belief is that green pod sowing avoids dormancy in Paphiopedilum germination. This is simply not the case, for we have experimental evidence to show, that commonly green pod sown seed shows dormancy. However where seeds from green pods germinate well, it is interesting to find that seed from the same pods which germinate just as well when dried and sown as dry seed at the same time. It is also believed that sowing green seed pods will avoid contact with bleaching chemicals which might affect germination. We have found that contact with bleach is not detrimental to the germination of Paphiopedilum seed. We have sown seeds that show dormancy as both green pods and dry seed and observed no difference in the rate of germination after a dormancy period in-vitro. On occasions germination of bleach treated seed is quicker, this may be due to the effect of the bleach on germination inhibitors or due to different rates of wetting of the seed compared to the dry seed resultant from a green pod sowing.

Germination can be observed to start within a few weeks of sowing for those crosses that do not exhibit any dormancy. It is more usual for Paphiopedilums to take in the region of 6 week to 18 months to germinate. It is quite noticeable that some seed will not germinate until a clear 9 -15 months has passed. Ensuring the aseptic integrity of the sown seed during this long time frame is vitally important. Contamination noticed after a number of months is a sure indication that there is a problem with the flasking containers or venting system. The use of loosely capped jars or cotton wool venting systems should be avoided as both approaches can lead to this problem.

In the last fifteen years there has been an explosion of interest in raising Paphiopedilums for the indoor houseplant market, though the idea is not a new one. Back in 1977 the company 'The House of Orchids' was established by Paul and Mary Phillips to sell a range of orchids including Paphiopedilums as houseplants. The main impetus for such work has now centered on the Dutch orchid potplant industry. Demand for such plants has been good in Europe and also there has been increasing interest in the USA and Japan. Orchids generally have proved to be good value plants and perceived by the buying public as exotic, interesting and long lasting. The Paphiopedilum plants grown for these customers have often been found to be remarkably robust and well suited to growing indoors or windowsill culture. A high percentage of such plants are based on the repeat or sequential flowering Cochlopetalum group of species. (P.chamberlainianum and P.glaucophyllum in var., )The resultant progeny are often quick growing and compact plants which readily produce flowering stems. The shelf life of plants in bloom is long due to the succession of flowers produced. Another important factor is many of these primary hybrids produce abundant seed which germinates quickly to produce large number of seedlings.

Selective breeding by the nursery can influence the survival chances of any resultant hybrid or species over time. Selection of vigorous robust plants is one obvious route. For it is well known that poor growing plants often produce poor growing seedlings in-vitro. This is not confined to Paphiopedilums for we have found that a difficult to grow but awarded Phragmipedium besseae cultivar produced seedlings that were notoriously difficult to cultivate in flask, compared to the standard Phrag. besseae. An example of species selection over time in a nursery is illustrated by Paphiopedilum delenatii. Most of the world's nursery stock has been raised from a single plant, cultivated in France and selfed repeatedly since the early 1920's. For many years cultivation of this plant was considered difficult in flask and on the growing bench, but now after many nursery selected generations the germination of this species is good and production of large numbers of seedlings relatively simple. This contrasts strongly, we are led to believe with results achieved by others, for germination of this species when it was rediscovered in the late 1980's.

The discovery of new Chinese Paphiopedilums belonging to the Parvisepalum group stimulated a flourish of interest in the latter part of the 20th century. Generally these were difficult to raise in flask with the species P. armeniacum, P.micranthum and P.emersonii being frustratingly challenging worldwide. However now many nurseries offer these plants as seedling raised stock and some of the problems have been overcome. Where large numbers of self pollinations have been made those plants producing offspring have opened a window of opportunity to select cultivars which produce seed that is not subject to dormancy problems and can produce viable numbers of seedlings. Our belief is that some plants within these species produce seed that is more capable of producing progeny and this is strongly under the control of genetic. Such good breeding plants are likely to be producing the vast majority of plants from flask in the future through nursery selection.

Rapidly germinating Paphiopedilums which exhibit no seed dormancy is an exciting development that we have observed at Tissue Quick Plant Laboratories. Our seed sowing work on Complex Hybrid Paphiopedilums has revealed that on occasions these plants will produce seed that germinates immediately on sowing, this is also often accompanied by a high yield of seedlings. It is well known that the yield of seedlings from complex hybrid breeding programs can sometimes be infuriatingly low. This is often put down to the belief that vigor and weakness occurs when such plants have been inbred for many generations. Rapidly germinating Paphiopedilums have only been observed in complex hybrids, both as green pod harvested plants and full term dry collected seed. Sowings of such seed as green pod, or subsequently dry sown does not make any difference to these results. Germination is rapid, in both dark or light and signs of germination can easily be observed within 15 days, with an 'explosion' of germinating seed over the next few weeks. This germination is different to that observed in primary hybrids, Corypetalum group or some maudiae type crosses

Such observations have been recorded on our standard Paphiopedilum seed sowing media, without charcoal.( Paphiopedilums and Phragmipediums Media A).

It is our speculation that over a number of generations of nursery grown selections, plants have been brought together for their vigor and flower performance and have as a by product been subject to seedling germination selections. During the process of selfing, sib crossing (brother/sister mating) or selection from other breading lines the gene pool of characteristics starts to suffer from inbreeding depression. To some geneticists the production of uniformity in plants is a sign of such genes at work, and can be clearly seen in some inbred flower breeding. This may lead to the accumulation of genes affecting germination or dormancy of seed. In non-orchid plant breeding the repeated self pollination of out-pollinating plants leads to weak inbred lines, but when these lines are brought together the resultant offspring exhibit good uniformity and hybrid vigor or heterosis. We have already seen the effects of heterosis in the production of Paphiopedilum primary hybrids, which often produce high yields of seedlings from well germinating seed. Could indeed such processes be an explanation why some complex hybrid breeding produce such rapidly germinating Paphiopedilums? Such ideas have to remain speculative as it is not possible to investigate this subject from a conventional plant breeder's perspective due to a number of factors including generation time for the slow maturation of this group of orchids. 5-9 year waits does not allow many generations of orchids to be studied. Most orchid breeding programs are concerned with rapid turn around, volume throughput and the production of award winning plants or quality seedlings. The laboratory process is only one element in a long cultivation cycle.

There may also be an argument that such an observation is nothing to do with the above mechanisms and any inbreeding depression does not exist in such a diversified group such as complex hybrids due to the large number of different species and hybrids combining in a heterozygous gene pool. While the observation of rapidly germination Paphiopedilums is a simple knocking out of seed dormancy expression genes, and will be expected in any large and varied complex hybrid breeding program.

At the other extreme, consideration should be given to the breeding of complex white and pink plants These have always been slow to germinate and yield of seedlings is poor. Germination numbers can typically run from 0-100's, this is in contrast to other complex breeding pods where 2000-3000 or more seedlings may be produced. This is a worldwide problem and is seen in the high price commanded for such plants. This low output of plants may be a reflection of the influence of the parental FC Puddle breeding lines or other factors. Although widely fertile this plant will breed with a range of other complex plants but the seed yield can be low of difficult to germinate and the seedlings slow to grow in-vitro and in the nursery. It is hoped that perhaps some breeding programs will be able to unlock these problems with the aim to produce larger numbers within this challenging group.

The future for raising Paphiopedilum seed has to be an optimistic one with increasing knowledge about the laboratory and seed producing processes. With each passing month new seed are germinated and seedlings deflasked to flower in years to come. The seed germination and rooting media are greatly superior nowadays compared to the past, while the use of lightweight containers and a move towards plastic all benefit the economics of producing these orchids. The days of reliance on leaking venting systems have become history by the use of PTFE which can eliminate contamination through air exchange and help speed up the growth rates of seedlings allowing them to be better suited to their life in community pots and on the nursery bench.

   
 
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