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Literature Review on Germination of Orchid Seeds.

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Introduction

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). ...read more.

Middle

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. ...read more.

Conclusion

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: 1. 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. ...read more.

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