Literature Review on Germination of Orchid Seeds.
Literature Review on Germination of Orchid Seeds
Liew Kaiyang Kevin, Tan Yong Zi, and Calvin Wong Yun Sheng
The Chinese High School
ORCHIDS
Orchid is the common name for a family comprising one of the largest groups of flowering plants. The family is worldwide in distribution, being absent only from Antarctica and some of the most arid desert zones of Eurasia. The greatest diversity of genera and species occurs in tropical regions that remain poorly explored. For this reason, and because of the complexity of the family, estimates of the number of orchid species vary from 15,000 to 25,000, and the number of genera from 400 to 800.
Orchid seeds are small, with only an undifferentiated embryo. As many as 2 million seeds may be produced from a single orchid seedpod. Unlike most other flowering plants, orchids have no food-storage tissue.
Orchid flowers are pollinated by a great variety of flying animals, and their great diversity in floral structure has resulted from adaptations to various pollinators. About half the orchid species are pollinated by bees; moths, butterflies, flies, birds, and other agents pollinate the rest. Many orchid flowers are adapted for pollination by a single species of insect.
Orchids do not vary as much vegetatively as they do in floral structure, but a great variety of forms exists, reflecting the wide range of habitats they occupy. About half are epiphytic, growing on other plants for support only, but some are parasitic and others saprophytic (living on decaying vegetation). A few Australian species complete their life cycles entirely underground.
Apart from their phenomenal popularity as ornamental flowers, orchids have little economic importance.
SEED GERMINATION
Most orchid seeds cannot germinate naturally in the absence of mycorrhiza. Noel
Bernard discovered the role of mycorrhiza1 in seed germination in the late 1800s and Hans
Buergeff worked on orchid mycorrhiza3 in the early part of last century. Their work showed that orchid mycorrhizal interactions were fairly specific and orchid seeds would not germinate without a fungal symbiont. It is now known that though some species do have specific interactions with certain species of fungi, others have a general relationship with many species of fungi. It is also known that orchids can be germinated without their fungal symbiont as Knudson (1922) found that orchids could be germinated asymbiotically on special media3. Most orchids are now grown in this manner and many clones are cultured from meristems. The Knudson medium has undergone some changes to provide for different species and different methods of culturing orchids.
Asymbiotic seed germination has become the favored method for orchid production. Most tropical epiphytes are produced in this way. However, attempts to germinate terrestrial orchids asymbiotically have not been as successful and only a few species have been germinated asymbiotically. Symbiotic seed germination of temperate terrestrials is more effective as these orchids show a strong dependence on mycorrhizal fungi. However, symbiotic seed germination can be difficult to control and is fairly complicated. Some reviews of the state of symbiotic seed germination suggest that this technique was important for those taxa which do not respond to asymbiotic methods.1
Symbiotic germination of terrestrial orchids has been used as an alternative to asymbiotic methods with some success. Clements found in 1986 that species of Orchis and Dactylorhiza germinated better in the presence of fungi1 though not all seeds germinated nor did seedlings develop. Smreciu and Currah (1989)1 found the same for North American and European species. Several species in their study, including two Corallorhiza, failed to germinate; Cypripedium calceolus L. and Calypso bulbosa (L.) Oakes had low germination percentages in asymbiotic medium only. Zettler (1997) germinated several species with good results, though soil establishment was poor and some ...
This is a preview of the whole essay
Symbiotic germination of terrestrial orchids has been used as an alternative to asymbiotic methods with some success. Clements found in 1986 that species of Orchis and Dactylorhiza germinated better in the presence of fungi1 though not all seeds germinated nor did seedlings develop. Smreciu and Currah (1989)1 found the same for North American and European species. Several species in their study, including two Corallorhiza, failed to germinate; Cypripedium calceolus L. and Calypso bulbosa (L.) Oakes had low germination percentages in asymbiotic medium only. Zettler (1997) germinated several species with good results, though soil establishment was poor and some species did not germinate, e.g Isotria medeoloides and Corallorhiza odontorhiza.2 Work with Spiranthes magnicamporum and Platanthera clavellata showed that while asymbiotic media effected germination, growth and transplant survival were best if seed was inoculated with a symbiont. Knudson, in the course of proving that orchids could be germinated asymbiotically, found that contaminating fungi and a species of bacteria both improved the growth of protocorms.1 It is important to note that though initial stages of germination (water uptake and rupturing of the testa) can occur without a symbiont, the completed process and subsequent seedling growth require infection and thus this technique is referred to as symbiotic germination.
Work with symbiotic germination implies that the fungal interaction is not highly
specific, though some books suggest that it may be in some species. Zettler and
McInnis (1993)2 found that the mycorrhizae isolated from a species of Platanthera promoted germination of several North American orchids. However, Isotria medeoloides failed to germinate in vitro with fungal isolates and in the field with naturally occurring fungi. Smreciu and Currah (1989) also found that though 15 species of North American and European orchids responded to different fungi, several fungal isolates induced germination in more than one species of orchid. They also showed that Ceratobasidium cereale Murray and Burpee could become pathogenic on North American species.2
Though germination was stimulated in vitro, field transplant survival was low suggesting that the fungus used to germinate seeds in vitro may not be suitable for growth in situ (also known as in vito). Masuhare and Katsuga (1994) showed that seeds germinated in a turf grassland were infected by a single species of Rhizoctonia1 even though field isolates of other Rhizoctonia species were able to induce germination in vitro. Further, Knudson (1922)1 found that species of Penicillium, which are not considered orchid endophytes , were pathogenic, but could also stimulate growth. Symbiotic seed germination has proven useful in the propagation of terrestrial orchids from Australia, Europe and North America. However, it is not always efficacious and has not been successful in all species tested. Many books and reviews state that "even with the best methods, germination [of temperate terrestrials] is relatively poor and not easily reproducible."1
Factors such as light and temperature can also affect orchid germination. Zettler and McInnis3 (1994) found that light increased the symbiotic seed germination of Platanthera integrilabia. The same was found for the European species Dactylorhiza majalis. However, other reviews stated that "light may inhibit the germination of some temperate terrestrials,"3 as Stoutamire found with seeds of Cypripedium species. Temperature effects on germination are unclear as different species show different responses to cold and warm temperatures. Zettler and McInnis (1993) found that Spiranthes cernua responded favorably to chilling storage while Goodyera pubescens did not.2 Rasmussen showed that a warm incubation followed by cold storage promoted germination of Epipactis palustris.5
Abscisic acid (ABA) (induces dormancy/inactivity in organisms) has been found in orchid seeds though hormonal applications that break ABA induced dormancy do not affect orchid seed germination.4 Of interest, however, are the results of a study by Wilkinson et al. (1994)4 that showed that IAA enhanced symbiotic germination of an Australian terrestrial orchid. They believed that bacteria associated with orchids were the source of IAA in situ.
MYCORRHIZAL INTERACTIONS
The relationship of orchids with fungi is relatively unique in the plant kingdom. Of the greater than 80 percent of plant species that form mycorrhizal relationships, orchids along with members of the order Ericales do not form endomycorrhizal relationships with genera of the Zygomycota. The main group of fungi inhabiting orchid roots is Basidiomycetes, though Ascomycetes have been found.6
Some of the Basidiomycetes with which orchids form a relationship are pathogenic on
other crops, e.g., Rhizoctonia solani Kahn (Hadley, 1982)4. Even within the Orchidaceae, symbiotic fungi of one orchid species may be pathogenic on another. However, most orchids are able to control the infection and growth of endomycorrhizal fungi. Orchid mycorrhizal fungi are found intracellularly in cells of the cortex and they are confined to roots (Hadley, 1982)5. Infection is limited to suspensor cells of the embryo and epidermal hairs and is highly restricted compared to fungal infection of non-orchid species. This suggests that orchids control the infection process, and that fungal symbionts are adapted to this control (Hadley, 1982)5. Within the cells, the mycorrhizae form dense coils of mycelium called pelotons which are thought to be adaptations to the host cell. Within the cell, the pelotons are surrounded by a membrane and interfacial matrix material. The membrane lacks adenylate cyclase activity but is otherwise similar to the plasma membrane. The orientation of microtubules and cell wall microfibrils is altered during infection and may be necessary to alteration in the cytoplasm and synthesis of the membrane surrounding the pelotons.
Other factors of the orchid-mycorrhizal interaction are also important to our knowledge of symbiotic seed germination. Several studies have shown how terrestrial plants grown with a symbiont have greater growth compared to plants grown asymbiotically. Anderson shows that Spiranthes magnicamporum had greater growth when grown with a fungal symbiont than without.3 There have also been reviews of the process of nutrient exchange between plant and fungus. It has been shown that vitamins, amino acids and sugars are translocated from the fungus to the orchid. However, it is not known whether these compounds are translocated across a living interface or released upon digestion of the fungus. Some reviews state that starch, accumulated during asymbiotic culture, is rapidly broken down upon infection and seeds germinated in the presence of an endophyte do not accumulate starch.5 Some reviews also state that orchids do digest their fungal symbiont and others note that, though digestion is the ultimate end for endomycorrhizal fungi, it is not known whether the enzymes for digestion are produced by the plant or the fungus. A study done in 1976 showed that the activity of peroxidase, ascorbic acid oxidase, polyphenol oxidase and catalase increased upon mycorrhizal infection and that this was in part due to digestion of the fungus.1 It has even been suggested that mycotrophy, the digestion of mycorrhizal fungi within infected seedlings can increase the water content of infected seedlings.
Some studies point out that orchids control the level of infection and that phytoalexins appear to mediate this control.3 The level of this control may relate to the orchid-fungus specificity of some species and the pathogenecity of some fungi in other orchid species. Also, it implies a moderate defense response by the orchid. In 1995, at study found orhcinol in protocorms of Orchis. This was surprising as orchinol is a phytoalexin (the first discovered) and believed to inhibit fungal growth.4 Fungi readily infect protocorms so orchinol would not be expected to be present; however, its presence suggests expression of defense genes albeit at a level that does not interfere with infection. That mycorrhiza are not found in mature pseudobulbs or tubers supports the expression of defense genes at some point during mycorrhizal infection.
A 1976 study hypothesized that an increase in oxidative enzyme activity upon mycorrhizal infection was similar to the oxidative activity of plants resistant to pathogens again indicates a defense response on the part of the orchid.2
FACTORS INVOLVED IN THE RATE OF SEED GERMINATION
Orchid seeds are fine and dust like, and contain no endosperm to nourish the plant during germination. Therefore, the environment would have to be sterile. Home growers can achieve a 50% to 75% germination rate, compared to the 97% to 99% germination rate in laboratories.
Therefore, the factors that help improve the rate of seed germination are:
. Amount of water
2. Amount of Nutrients and Fertilisers
3. Sterility of environment (Orchid seeds are very susceptible to contamination.)
4. Temperature (Most Orchids grow between temperatures of 13?C to 32?C.)
5. Humidity (40% to 70% for most orchid species.)
6. Exposure and amount of light (Orchid plants in general need around 50% of a full sun)
7. pH of germination medium
Therefore, the mediums used can affect the rate of germination. The nutrients, sterility, pH and moisture of the medium all affect the germination of the orchid seeds.
We can measure the rate of germination quantitatively by considering the following factors:
- Number of seeds germinated over a fixed period of time
- Percentage of seeds germinated over a fixed period of time
- Time taken for each seed to germinate
- Average time taken for all the seeds to germinate
REFERENCES
. Arditti J. 1992. Fundamentals of Orchid Biology. John Wiley and Sons. New York, NY.
2. Benzing DH. 1987. Major patterns and processes in orchid evolution: a critical synthesis.
3. Clements MA. 1981. The germination of Australian orchid seed. In Proc. Orchid Symposium 13th International Bot. Congr. Sydney, Australia.
4. Esau K. 1977. Anatomy of Seed Plants. John Wiley and Sons, Inc. New York, New York.
5. The American Orchid Society. All about Orchids. AOS 2002, California. (orchidweb.org)
6. (2001). Orchids. Microsoft(r) Encarta(r) Reference Library 2002.
By: Liew Kaiyang Kevin, Tan Yong Zi, Calvin Wong
