Orchids produce large quantities of tiny seeds. These seeds are incredibly light and are easily transported by wind or in some cases by water to optimal germination sites in the wild. The seeds of most orchids do not possess an endosperm, the primary food reservoir to aid initial germination. These seeds require an external source of food to trigger germination and for the development of the protocorm. Most orchid seeds are associated with micorhizal fungi which attack the testae of the seeds but then provide the sugars that are necessary for development. Few orchids however have developed a primitive endosperm and are no longer limited to this symbiotic association.
As a result of the microrhizal association in successful germination, orchid seeds are notoriously difficult to germinate at home. Although it is possible to a limited degree to germinate some seeds at the base of the mother plant which may harbour the micorhizal fungi, most orchid seeds today are cultivated in vitro using aseptic asymbiotic techniques. The fungi are replaced by a growing medium that supplies the nutrients required for germination and growth. However, this process must be done under sterile conditions to avoid contamination by other opportunistic fungi and bacteria which can also use the nutrients, competing directly with the orchid seeds and in most cases killing them directly.
Much work over the last few decades has seen the development of refined protocols for improved yield in support of mass propagation of orchids. These include cloning using meristematic tissue where selected plants are mass produced and remain identical to the original plant. Media for germination and enhanced growth are numerous and often specific for various growth habits or they may be designed for specific groups of orchids.
Seeds can be sown as dry seed which is ripe, dehisced seed from naturally opened pods, or as immature embryos which are harvested from unripe pods. We find that both methods pose advantages and disadvantages. Dry seed is considered non-sterile and is often contaminated with fungal spores and bacteria and required sterilization. Green pod embryos are considered sterile since they have not been exposed to the external environment and have been protected within the seed pod. Dry seeds are mature seeds but sometimes orchids produce mature seeds which begin life with a period or dormancy which is often not easy to break. Green pod embryos often yield a better successful overall germination but sometimes take longer to develop. The sterilization of dry seeds can often kill off many viable seeds but may also aid in germination by scarifying the testae and therefore enhancing water uptake to the embryo. Many flaskers shy away from sowing dry seed because of the “extra” steps required for proper sterilization but with carefully refined protocols, dry seed culture can often produce results mirrored by green pod embryo culture. However, we do agree that contamination rates remain slightly higher with dry seed culture but conversely, if green pods are harvested too early, an entire potential hybrid could be lost or at least set back several months until fresh flowers can be pollinated again. Green pod harvesting is not an exact science either and although there are many tables of suggested harvest times available on the internet and in publications, these depend strongly on the combination of variables including lighting intensity and duration, temperature and temperature differential, parentage and other factors.
In our lab both dry seeds and green pod embryos are cultured depending on the availability and the requirements of the customer. Seeds and embryos initially begin to swell as they take up water. The embryos get larger and larger until they break free of the testae (dry seeds) and become visibly larger to the naked eye. Orchid seeds take their time to germinate. Many orchids germinate in the presence of light after about 3-4 weeks of constant incubation. Some prefer dark conditions to stimulate germination.
We allow our first stage protocorms to develop until either the mother flask becomes too crowded which inhibits growth, or the first leaves are produced. Then these protocorms are transferred to new media to give them more space to grow and are moved to different types of media to enhance leaf and root growth. Once leaves and roots have been produced the seedlings remain in flasks until large enough to pot out in the greenhouse or are large enough to sell to customers. Often, excess seedlings are potted out and established seedlings may be offered from time to time as a result.
All flasking (germination and transfers) is done under sterile conditions using a sterile transfer hood or a laminar flow hood. The laminar flow hood incorporates a Hepa filter. These filters are specifically designed to filter out microorganisms and spores from the air which may pose a significant contamination risk. Effectively, the working space inside the laminar flow hood is kept at a positive pressure by the flow of pre-filtered sterile air over and through the work space. Ultraviolet germicidal lamps are also incorporated in some instances to enhance sterile conditions within the working area.
Seedlings once removed from flasks are often delicate and require a period of “hardening off.” This should always be kept in mind when purchasing flasks of seedlings. Once removed from flasks the seedlings are suddenly placed into a non-sterile environment and one which is also less humid so drying out can be a significant threat to the young seedlings. It is advisable to cover the seedlings with a transparent plastic container or bag to maintain a high level of humidity while hardening off and seedlings should be kept away from direct sunlight or strong indirect light.