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Orchids are made up of the largest and most diverse groups of flowering plants in the world. There are more than 25, individual species, which can be found in every continent except Antarctica. The high volume of orchid flower seeds is due to the competition with other plants to attract pollinators. While choosing the kind of orchid to plant, keep in mind where it will be kept. Some species, like the epidendrum, do well in an outside garden, while paphiopedilum thrives indoors.
Due to their showy and sweet-smelling fragrance, orchid flower seeds are ideal for fragrant outdoor plants and bloom in all other colors except black and blue. The orchid seeds for sale can be germinated at home through a process known as asymbiotic germination. This process involves germinating the seeds in vitro and agar, which is a jelly-like substance that contains growth hormones and necessary nutrients. This type of germination is also known as flashing.
All orchid seeds must be sterilized without damaging the seed. This work clearly demonstrates the pathway of solute movement and the importance of suspensor in embryo nutrition.
In flowering plants, the suspensor can take on structural and biochemical specializations Yeung and Meinke ; Kawashima and Goldberg Transfer cell morphology is one of the common structural specializations observed in suspensor cells. In Paphiopedilum delenatii , wall ingrowths are found in suspensor cells Lee et al. Wall ingrowths increase the surface area of a cell which can aid in short distance transport. Hence, the formation of wall ingrowths further strengthens the notion that orchid suspensor is specialized in nutrient uptake.
The membrane associated with wall ingrowths is unique, e. Future studies demonstrating the presence of biochemical specialization s will serve as proof of orchid suspensor function.
In Phaseolus species, having large suspensors, high quantities of different phytohormones have been detected see Yeung and Meinke ; Kawashima and Goldberg Although orchid suspensor is small, it may have a similar biosynthetic ability. If this is true, the orchid suspensor will have additional roles to play during embryogenesis.
Not all orchids have a well-developed suspensor see Swamy ; Clements In genera such as Spiranthes , many species are considered to be suspensorless. At present, the nutrient uptake pathway by these embryos is not known. In the absence of a suspensor, it is logical to speculate that the entire surface of the embryo proper must be able to absorb nutrients directly from surrounding maternal tissues, i.
In Cyrtosia javanica , the seed coat is multilayered and the cells maintain their cytoplasm until seed maturation Yang and Lee Furthermore, dense cytoplasmic accessory and antipodal cells are present, locating at the chalazal end of the seed coat, near the developing embryo Yang and Lee In flowering plant, a cuticle is present on the surface of the embryo proper Rodkiewicz et al.
Moreover, in C. These structural features indicate that the C. In order to determine how nutrients enter the embryo in the absence of a suspensor, it is necessary to study the microenvironment surrounding the developing embryo and to perform tracer experiments similar to the study for the nun orchid.
The pattern of embryo development in flowering plants can be divided into several phases Goldberg et al. Once the polar axis is established with the formation of the proembryo, histodifferentiation begins with the formation of a shoot and a root apical meristems, the primary meristems protoderm, ground meristem, and procambium , and the cotyledon s. Upon the completion of the histodifferentiation phase, storage product synthesis and accumulation begins within embryo cells.
Concomitant with storage product accumulation, the maturation phase sets in and the embryo will acquire desiccation tolerance and prepares for developmental arrest and germination events. This pattern of development contributes to the success and survival of flowering plants in the natural habitats. In lower vascular plants such as bryophytes and ferns, once the histodifferentiation phase is completed, the embryo will germinate immediately without a resting phase, as they do not have the ability to undergo storage product biosynthesis and developmental arrest.
Moreover, in flowering plants, given the right conditions, developing embryo can also precociously germinate Raghavan after the completion of the histodifferentiation phase, as the embryo has already established a functional body plan ready for germination. Structural descriptions of mature orchid embryos are detailed by Yam et al.
The number of cells and embryo sizes vary among species of Orchidaceae. Only 8 cells were reported for Epipogium aphyllum and approximately cells for Bletilla striata see Yam et al. The mature B. Does histodifferentiation occur within a tiny orchid embryo proper? Although there is no obvious tissue pattern formed, histodifferentiation can still be identified Vinogradova and Andronova The following discussion argues that histodifferentiation is an integral and a key step during early embryogeny.
In orchid embryos, a gradient of cell size is often recognized with smaller cells located at the apical chalazal end and larger cells located at the basal micropylar end of the embryo. This represents the existence of a structural polarity.
The cells at the apical pole of the embryo are destined to form a meristematic zone and the basal cells are designed to house the symbiont upon seed germination. Since the small and large cells have distinct cell fate upon seed germination, a physiological polarity must also be present within the embryo prior to seed germination.
It is interesting to note that for easy-to-germinate species such as Phalaenopsis amabilis , a marked gradient of cell size exists within the embryo Lee et al. This is in contrast to the difficult-to-germinate species such as C. Another indication of histodifferentiation is the formation a protoderm. Judging from the uniform cell profile of the surface layer, the embryo proper has acquired protoderm characteristics. A cuticle is found to envelop the embryo proper such as in C.
The ability to form a cuticle is an indication that the surface cells have acquired epidermal cell characteristics. The orchid embryo has to establish a protocorm body plan during early embryogeny, in order to germinate precociously.
In orchids, the embryo is small with a reduced number of cells. Mitotic activities are arrested early. One can encounter mitotic figures readily within the embryo proper mainly soon after fertilization. A recent study on the expression patterns of the cell-cycle genes in Phalaenopsis aphrodite indicates that genes are coordinately regulated from ovule development to embryogenesis Lin et al.
It would be interesting to determine the intrinsic factors within the embryo which regulate the cell cycle program and mitotic activity as embryo develops. The absence of a cotyledon is a common characteristic of orchid species. Not more than 10 species have been reported to have a small protrusion that appears to be a rudimentary cotyledon see Nishimura In a majority of cases, the protrusions fail to develop further in a protocorm and appear to have no contribution to protocorm development Nishimura Bletilla striata has the most distinct protrusion at the apical end of the embryo; can this protrusion be considered a cotyledon?
It is important to note that B. Judging from scanning electron micrographs, the expanding rudimentary cotyledon of B. Unlike other orchid protocorms, an expanded meristematic zone is not present prior to SAM formation. The close physical proximity between the protrusion and the SAM suggests that it might be a cotyledon. As shown in Arabidopsis embryo development, the SAM and cotyledons are formed at the time during histodifferentiation Laux et al. Future molecular genetics studies on embryo development will provide new insight into the identity of B.
In flowering plants, storage product deposition begins soon after the completion of the histodifferentiation phase.
In dicots such as canola and Arabidopsi s, the embryo proper begins to expand and fills the endosperm cavity and mitotic activities soon subside. This is followed by the appearance of starch deposits.
Starch granules are subsequently replaced by the formation of storage protein and lipid bodies. In orchids, a similar pattern is observed. The large vacuoles are gradually replaced by small ones and storage protein and lipid bodies begin to appear. In a mature orchid embryo, there are abundant storage protein and lipid deposits; starch granules are rare. Besides histological studies, little is known about the physiology of storage product deposition and how storage products influence germination behavior and long term seed storability Schwallier et al.
In canola microspore-derived embryos, abscisic acid ABA has been shown to induce lipid and oleosin biosynthesis Zou et al. ABA levels begin to increase in Cypripedium formosanum Lee et al. Are there sufficient storage reserves to sustain seed germination? Orchid embryos though small, have cells packed with storage products. In their natural habitat, it has been reported that the storage products, especially lipid reserves are not utilized unless mycorrhizal establishment has initiated.
In Arabidopsis , ABA levels can influence availability of energy and nutrients in seeds during germination Garciarrubio et al. A high level of ABA can prevent the breakdown of storage proteins Garciarrubio et al.
The final stage of embryo development is considered as the maturation phase. Seeds need to be desiccation resistant in order to survive the natural environment. Recent evidence indicates that a desiccation period is necessary, preparing seeds for subsequent germination Holdsworth et al. The presence of high levels of ABA in mature orchid seeds may be essential for the acquisition of desiccation tolerance, as demonstrated in the study of alfalfa seeds Xu and Bewley In order to respond to potential abiotic stresses, a common biochemical marker, the late embryogenesis abundant LEA proteins, are shown to accumulate at the time of seed maturation in Dendrobium officinale Ling et al.
The presence of long lived mRNAs in mature seeds is a well-documented phenomenon in many flowering plants Dure , ; Sano et al. Recent proteomic analyses in rice indicated the upregulation of 20 proteins even in the presence of a transcription inhibitor, actinomycin D, indicating that long lived mRNAs must be present in mature seeds Sano et al.
In Spathoglotis plicata , Raghavan and Goh used the 3 H-poly- U in situ hybridization method to demonstrate that poly A -RNAs are present and uniformly distributed throughout the mature embryo.
This study indicates the potential presence of long-lived mRNAs in mature embryo cells. Future proteomic analysis, similar to that performed in rice, may enable the identification of proteins essential to early stages of germination.
From the available information, preparations during seed maturation in orchids appear to be similar to other flowering plants. As seeds mature, additional changes have taken place in preparation for developmental arrest and subsequent germination events. Therefore, one should consider that the pattern of embryo development in orchids is similar to that of other flowering plants; not simple, nor primitive.
Yam et al. Although some degrees of endosperm development, i. The failure to form a functional endosperm is one of the distinctive features in orchid seed development. Why does endosperm formation fail? The formation of the central cell in an embryo sac is complex Lopez-Villalobos et al. The competition for nutrients among numerous ovules, may have a negative effect on central cell development. The lack of a functional cytoskeletal network may be partially responsible for the absence of mitotic activities within the endosperm cell.
Can the male gametes contribute to endosperm failure? In the nun orchid, the gamete nucleus that fuses with the polar nuclei of the central cell also has a lower staining intensity, prior to fertilization Ye et al.
Gamete dimorphism has been reported in sperm cells of a number of species such as tobacco late in sperm development Tian et al. Preferential fertilization of one of the sperm cells with the egg has been noted.
In Plumbago zeylanica , the plastid-rich, mitochondrion-poor sperm cell tends to fuse with the egg cell Russell and a preferential transmission of supernumerary B chromosomes to the egg cells during sexual reproduction in maize has been reported see Weterings and Russell Although gamete dimorphism is not a universal phenomenon in flowering plants, if there is defect in the male gamete destined for the central cell, endosperm development could fail.
In orchids, since a massive number of pollen tubes is present within a capsule prior to fertilization, this is an excellent experimental system to study sperm cell structure and function and the results can provide clues as to whether the male gametes can contribute to endosperm failure in orchids.
A varied number of nuclei is present within the primary endosperm cell after fertilization. The origin of these nuclei has generated a lot of discussion. The nuclei within the endosperm cell can represent unfused nuclei from the central cell together with one of the gamete nuclei, or as mitotic products of the primary endosperm nucleus after fertilization.
It is generally agreed that there is no fusion of nuclei within the central cell and the nuclei soon degenerate within the primary endosperm cell as the embryo begins to develop. In Vanilla planifolia , Swamy reported the formation of a primary endosperm nucleus and having several rounds of division prior to degeneration; however, a recent study by Kodahl et al.
Even with improved optical instrumentation, the origin of various nuclei is difficult to determine. To confirm that the nuclei observed are indeed originated from the primary endosperm nucleus, one needs to clearly demonstrate the presence of mitotic figures within the primary endosperm cell. In the majority of flowering plants, after fertilization, endosperm develops rapidly aiding in seed enlargement. The endosperm also serves as a nutrient depot and is also a rich source of phytohormones such as gibberellins and cytokinins.
All these factors can promote and regulate embryo development Lopes and Larkins In the absence of an endosperm, the orchid embryo cannot expand and hence remains small. In general, when the endosperm fails to develop, the embryo also aborts see Vijayaraghavan and Prabhakar The fact that the orchid embryo continues to develop and survive without the presence of an endosperm indicates modifications to the embryo developmental program.
The ability of the suspensor to acquire nutrients and the presence of a cuticle to prevent rapid desiccation, are possible strategies that enable continual embryo development in the absence of an endosperm. The ability of orchid embryos to develop without an endosperm, removes one of the potential barriers for hybrid failure van der Pilj and Dodson One can envisage that orchids have devised a new strategy in seed development during their evolution.
Since the success of plantlet formation rests on the success of protocorms in establishing proper mycorrhizal interactions, endosperm formation is deemed unnecessary. Phytohormones play important roles in all aspects of plant growth and development. Similar to other flowering plants, phytohormones play important roles in orchids. At present, studies on phytohormones focusing on orchid reproductive development are few. More efforts are needed in the future. The hybridizing potential found in orchids, both at the inter-specific and inter-generic level, has been utilized by amateur and commercial orchid growers to produce thousands of artificial hybrids.
The desire to observe the resulting hybrids quickly leads to research and advances in in vitro culture techniques. The availability of large numbers of orchid seeds, with little or no food reserve and relatively uniform cultural and genetic characteristics, has prompted researchers to use orchid seeds for in vitro nutritional and developmental studies. These studies have led to many innovations in in vitro culture techniques. Refinements in aseptic technique and equipment Sauleda, have lowered the rate of contamination during seed sowing and replating.
The development of specialized culture media Withner, , both defined and undefined, for germination of orchid seeds and replating has increased the number of seeds germinating and accelerated seedling growth rates. These advances and developments have contributed to a substantial reduction in flowering time.
Phalaenopsis Blume, Oncidium Swartz and Dendrobium Swartz, usually requiring four to five years to flower from seed sowing, can now be regularly flowered in less than two years. Another advancement increasing the germination of orchid seeds and reducing flowering time was the development of a green pod culture process Tsuchiya-Itaru, In most laboratories this process has replaced, whenever possible, the dry seed culture process.
In the dry seed process, the seed capsule is removed from the plant at the first sign of dehiscence. The seeds are separated from the seed capsule, treated with a sterilizing agent and sown using aseptic procedure. A large number of seeds may be lost by contamination as a result of incomplete seed sterilization. Overexposure to the sterilizing agent may also cause excessive seed loss due to burning.
The time required by the sterilizing agent to decontaminate the seeds without burning the proto-embryo is sometimes difficult to estimate. The effect of the sterilizing agent may vary depending on the genus and the age of the seeds being sown. In the green pod culture process, the seed capsule is removed from the plant after fertilization, but prior to dehiscence. Long term storage of orchids requires cryopreservation at ultra-low temperature as a general rule because longevity under conventional seed banking conditions is not completely reliable Merritt et al.
They are found in several phytogeographical regions with unique climatic conditions. This global biodiversity hotspot is a perfect study area to test orchids for collecting methods and assessing viability. The habitats of the study comprised inselbergs, savannah, grasslands, montane rocky scrubland and gallery forests. This area has granitic rocks, marble and quartzite with the majority of orchids being lithophytes and the rest true epiphytes and terrestrials.
In the present paper we describe methods for collecting seeds at different maturity levels from the CHM habitats using IVC methods. IVC is the method of initiating in vitro cultures from plants in the wild. The method has been used in the past for a variety of functions ranging from horticulture to conservation Warren ; Alvarenga et al. Propagation of orchids from seeds has long been achieved through the use of capsules which are either fully matured or green, un-dehisced and near-mature.
Collected dry seeds are cleaned and dried before either storage or culturing for germination. Seed viability upon storage under standard seed bank conditions is still not completely reliable for orchids Merritt et al. In many cases, especially in temperate terrestrial orchids, higher germination rates have been achieved using green capsules than mature seeds. This method is dependent on the immediate culturing of seed after harvest from green capsules.
Seeds collected from plants in the wild, spread over large swathes of land mass, from remote locations usually perish due to moisture loss and microbial contamination unpublished results.
The critical aspect of working towards developing a successful IVC protocol is the control of contamination Pence along with maintaining the viability of the differentially matured seeds. In many cases the techniques were specifically developed for only a limited number of taxa as described in the present study. We discuss the importance of this new method to improve collecting and germination efficiency of differentially matured orchid seeds.
The implications of this approach to cryopreservation and living collection development for reintroduction and assisted colonisation are discussed in detail. Studies in the field were conducted in the CHM habitats where a large number of endemic orchids are found in discontinuous populations. The orchid seed capsules were collected from plants in the wild in inselberg, savannah, montane rocky grassland, and gallery forests Fig.
Orchids and their habitats in Itremo, Central Highlands, Madagascar. Note smoke visible in tile c as a result of extensive grass fires. Only a small number of capsules were allowed to be collected due to the fragmented nature of populations and few available capsules in individual plants. Orchids from these diverse habitats exist as epiphytes, lithophytes, and terrestrials. Different habitats and life forms contribute to the diversity of seed capsules which were available for this study.
Seed capsules were categorised into five groups based on their size. Seed capsules were considered mature when they were yielding to the touch and had yellow, red or brown colouration, and contained mature seeds. Seed capsules were collected in vitro if they contained near-mature or immature seeds. These capsules were green, firm and of similar dimensions to mature capsules. Mature capsules were cut from the plant, wrapped in paper in a sealed seed envelope and placed in a field desiccator.
This consisted of a sealable plastic box containing orange silica gel. Several collections were made per species for both IVC and mature capsules, and collection was performed in an opportunistic manner in accordance with guidelines laid down by the seed conservation department, RBG Kew Way Flower parts were removed by scalpel blade and the capsules were cleaned with moist sterile towels immediately after collection.
These seed capsules were then sterilised in 0. For sizes 1—3 plastic disposable containers ml volume with screw cap lids Sterilin, UK were used to culture the capsules following surface sterilisation; for size 4 and 5 Dilu vials 27 ml volume, Alpha Laboratories, UK with flip tops were used. Sterilised seed capsules were washed in one change of 0.
Seed capsules were observed under a stereo field microscope for contamination within 48 h and rescued if necessary using the following procedure. Capsules were carefully removed from the vial, wiped with a sterile antiseptic wipe and gently agitated in 0.
Re-sterilised capsules were placed in vials containing IVC medium as described above. Once the seed capsules arrived at RBG Kew, seeds were cultured directly into the culture medium containing MS nutrients, Phytamax Sigma Chemicals, UK or other appropriate media under sterile conditions depending on the species.
This study was conducted to see the effect of seed coats on germination. Approximately — Angraecum protensum seeds each were suspended in water in 10 ml screw cap centrifuge tubes and sonicated for 0, 1 and 3 min in a Decon Fb ultrasonic bath operating at a frequency of 40 kHz Decon laboratories, Sussex UK. Staining to observe lignin on the carapace covering of the embryo was done by embedding seeds into JB-4 resin and sectioning using an ultra—microtome Sullivan-Brown et al.
The stain was washed off with deionised water and the sample mounted with 0. Number of seed capsules contaminated at stage 1, stage 2 and stage 3 were recorded. For both IVC and mature seed, numbers of total full seeds within randomly selected fields under microscope approximately seeds in each field were used to calculate the percentage of full seeds.
Germination was defined as emergence of the protocorm from the testa. Germination was recorded based on the number of full seeds, discounting seeds with poorly formed or no embryos and four replicates were used.
Percent of clean cultures and percent of seed germination data were analysed against the seed capsule size and geographical location of the plant. Kruskal—Wallis test was employed as a non-parametric method to look for the difference of germination rate between species because it does not assume a normal distribution of the residuals, and is applicable to comparing groups with different sample sizes Theodorsson-Norheim Small numbers of capsules were collected from fragmented populations, single plants in some cases, from the CHM following the guidelines stipulated in the collecting permit from Madagascar Conservation authorities.
The majority of seed capsules collected by the IVC method yielded seeds in a good state of freshness that stayed sterile inside the capsules.
There was some discoloration in a small percentage of small seed capsules. Based on the size, colour and texture of the seed capsules the sterilisation regime was followed as in Table 1 , which shows that almost all the seed capsules collected in Madagascar by IVC gave rise to sterile seed cultures.
In total Both NaDCC and PPM were essential to suppress microbial growth at various steps of the process from wild collecting to culture of seeds under laboratory conditions.
We have selected this genus to study in length about the seed behaviour of both mature and near mature seeds. IVC seed appeared to germinate much more quickly than mature seed. Germination of mature seeds from mature capsules versus seeds of in vitro-collected near-mature capsules of Angraecum magdalenae , recorded over a 4-week period. Mature seeds of many Angraecum species had a good proportion of full seeds but when compared against IVC seeds there are significant differences in germination Fig.
Up to 2—3 times increase in germination was recorded when Angraecum magdalenae IVC seeds were germinated compared to mature seeds under in vitro culture conditions Fig. The results of the germination data of Angraecum rutenbergianum show the importance of collecting seed capsules at the right maturity.
None of the seeds from IVC seed capsule 1 Fig. Seeds from seed capsules 2 and 3 germinated at a reasonable percentage which is rather better than the corresponding values for mature seeds Fig. Further collections of seed capsules of Angraecum protensum were made and we have found wide differences in germination between seed capsules of different maturity all seeds have coloured testa and opaque embryo but capsule colour varied from green to brown Fig.
We have observed very low germination from this species as described earlier. The histogram shows a clear difference in germination between seed capsules although most capsules had very similar percentage of full seed.
Germination percentage of mature seeds of Angraecum protensum originating from different seed capsules collected from Itremo, Central Highlands, Madagascar. Each numbered bar represents an individual capsule. Immature and near-mature seeds of Angraecum spp. The lower row a1 — d1 , scale bar 1 mm shows protocorms of in vitro collected seeds. There was an increased viability and germination, compared to the control, when seeds were sonicated for 3 min.
Different sonication times 0. However, the carapace was damaged by the process of physical scarification through sonication treatments. There were some variations in full seed percentage in both Angraecum calceolus and Angraecum rutenbergianum Fig. Germination in mature seeds was nil Fig. Seed germination was erratic in Angraecum protensum and for that reason we have conducted the above pilot study to understand the nature of the covering of the embryo and its effect on seed germination.
The carapace was found to be a significant layer of lignin surrounding the embryo of this species.
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