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Germination and seedling establishment in orchids: A complex of requirements

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Background Seedling recruitment is essential to the sustainability of any plant population. Due to the minute nature of seeds and early-stage seedlings, orchid germination in situ was for a long time practically impossible to observe, creating an obstacle towards understanding seedling site requirements and fluctuations in orchid populations. The introduction of seed packet techniques for sowing and retrieval in natural sites has brought with it important insights, but many aspects of orchid seed and germination biology remain largely unexplored. Key Considerations The germination niche for orchids is extremely complex, because it is defined by requirements not only for seed lodging and germination, but also for presence of a fungal host and its substrate. A mycobiont that the seedling can parasitize is considered an essential element, and a great diversity of Basidiomycota and Ascomycota have now been identified for their role in orchid seed germination, with fungi identifiable as imperfect Rhizoctonia species predominating. Specificity patterns vary from orchid species employing a single fungal lineage to species associating individually with a limited selection of distantly related fungi. A suitable organic carbon source for the mycobiont constitutes another key requirement. Orchid germination also relies on factors that generally influence the success of plant seeds, both abiotic, such as light/shade, moisture, substrate chemistry and texture, and biotic, such as competitors and antagonists. Complexity is furthermore increased when these factors influence seeds/seedling, fungi and fungal substrate differentially. Conclusions A better understanding of germination and seedling establishment is needed for conservation of orchid populations. Due to the obligate association with a mycobiont, the germination niches in orchid species are extremely complex and varied. Microsites suitable for germination can be small and transient, and direct observation is difficult. An experimental approach using several levels of environmental manipulation/control is recommended.
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... Finding suitable medium ingredients and their optimized concentrations could not only improve the efficiency and success of orchid in vitro cultures, but also might introduce low-cost and economically suitable formulations. However, some mineral salt-rich substrates are commonly used for tropical epiphytic orchids; their high concentration of inorganic nitrate and ammonium is toxic for most terrestrial orchids [23]. In this connection, Malmgren [24] developed a basal medium containing simple organic compounds, including PJ, coconut water (CW), boiled potato, and other similar ingredients, Diversity 2022, 14, 1137 3 of 23 but each terrestrial species needs a unique combination of organic constituents in which finding the best formulation requires more study and attention. ...
... It is generally accepted that germination and PLB formation are more challenging and complex in terrestrial orchids than in epiphytic species because of different factors such as physical and physiological barriers and dependency on the mycorrhiza fungi [23]. The last factor is needed to provide essential nutrients, including macro and micro-elements, hydrocarbons, and amino acids, and also to adjust osmotic potential [48,49]. ...
... In the present study, the single implementation of organic nitrogen compounds was generally more efficient than their combinational use in enhancing seed germination and plantlet growth attributes. The simultaneous use of nitrogenous organic constituents may negatively affect water potential and thus disrupt water and nutrient uptake by the embryo and PLBs [23,61]. Nevertheless, both individual and combinational applications of CH, AV, and Pep eliminated dependency on inorganic nitrogen forms or mycorrhizal fungi, where CH and AV were more effective in seed time to germination and plantlet growth, respectively. ...
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... While the loss of older root tissue and the production of new roots could provide chances to drop the relationship with initial fungal colonizers, a set of biological conditions may be acting to maintain specific fungi across life stages. In the same research site as the present study, Cook et al. (2022) found that symbionts remain in the substrate over years, which could facilitate the colonization of fresh root tissue (McCormick et al. 2009;Rasmussen et al. 2015), while adult orchid plants may function as OMF storehouses that provide new sources of the same mycobionts to the younger individuals (Jacquemyn et al. 2007. Whether there is one or various underlying mechanisms that prompt the relationship between an orchid and the same mycobiont across the plant's life cycle remains to be determined. ...
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... The flasks are sealed and placed in high humidity and warm environment (20-28°C). It normally takes 4-6 months to grow for transplantation and 4-10 years for the seedlings to bloom Rasmussen et al. (2015) Cymbidium sp. Flasking and ...
... varieties are inter-planted or placed in adjoining areas and hybrid seeds are collected for further processing and cultivationRasmussen et al. (2015) -Chance seedlings The seeds are harvested from orchid plants, presumed to be cross- ...
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One of the most exotic and ubiquitous plant species, the cultivation of orchids continues to gain momentum worldwide. The growing popularity of orchids and their cultivation worldwide is attributed to substantial progress in approaches like classical breeding, plant tissue culture and biotechnological interventions. In addition to ornamental value, orchids are extensively studied for their pharmacological properties and are widely used as food supplements, flavours, and medicine in different regions of the world. The unprecedented advances in whole-genome sequencing and omics technologies have significantly improved our knowledge of orchid biology, leading to translational success in the production of exotic varieties. With the emerging investigations into the cultivation and global trade popularity of orchids in the present era, this thematic article provides comprehensive insights into the existing and emerging trends in orchid cultivation, diverse ethnomedicinal uses, and multi-faceted applications, and the need for the legalisation of orchids for conservation and trade in the present era. In this way, advances in breeding and molecular approaches aim to significantly improve orchid cultivation and its socio-economic attributes. However, a detailed understanding of challenges in orchid conservation and implementation of domestic/global legislative guidelines are necessary for the protection of endangered species and their legalised trade across the globe.
... Ideally, conservation efforts for threatened orchid populations would include the restoring of extirpated population or establishment of new populations. Orchids can be slow growing and a new population can take decades to establish itself, and for some species it has been observed to take upwards of 80 years to occur (Pedersen et al., 2012;Rasmussen et al., 2015). However, in cultivation, T. undulatum has been shown to reach an adult size with the capability of flowering at 4 years of age (personal communications, Fairchild Tropical Botanic Gardens, Dr. Jason Downing). ...
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... Penicillium is a cosmopolitan genus of fungi, present in a diverse array of habitats including in vegetation, soil, the atmosphere, and the indoor environment and is a common contaminant of food and other products [45]. Sistotrema is a common component of the fungal communities associated with orchids such as Erythrorchis altissima [46], Limodorum [47], and others. Bjerkandera is a cosmopolitan genus of polypore fungi which grow primary on rotting angiosperm wood [48]. ...
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... Although orchid species usually require an ecological network of pollinators for reproduction and mycorrhizal fungi for seed germination (Micheneau et al., 2009;Phillips et al., 2011;Zotz, 2013;Pereira et al., 2014;Rasmussen et al., 2015), epiphytic species require an additional association with phorophytes (Rasmussen & Rasmussen, 2018;Wraith & Pickering, 2018;Zarate-García et al., 2020). The re-establishment of epiphytic populations can be impaired after a disturbance (Gradstein, 2008), because their recovery is directly associated to tree regeneration, which is a slow process (Adhikari et al., 2012;Reid et al., 2016). ...
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... Orchids can be difficult to cultivate via seeds because natural seed germination requires mycorrhizal associates (Rasmussen et al., 2015). However, the development of asymbiotic germination and tissue culture Table 3 List of characteristics hypothesized to be important to population-level harvest decisions. ...
... Rasmussen, 1995). Terrestrial orchid species seeds have a greater dependency on a particular fungal strain as the nutrient provider for germination, and generally have a lower success rate under asymbiotic germination test conditions whereas epiphytic orchids have a wide compatibility with general mycorrhiza and simple nutritional requirements has a higher success rate of asymbiotic germination (Arditti et al., 1982;Rasmussen et al., 2015). Different culture media formulations, such as KC (Knudson, 1946), MS (Murashige and Skoog, 1962), M (Mitra et al., 1976), VW (Vacin and Went, 1949) etc. have been successfully used for germination of several orchid species. ...
... Subsequently, ORM fungal hyphae enter the protocorm to form coiled complexes called pelotons, and orchids obtain nutrients by digesting the pelotons [5,8,9]. Additionally, the further development of orchids from protocorms to plants also requires the support of ORM fungi [5,10]. ...
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