Introduction
Ornamental plants are used primarily for aesthetic purposes, which is an important aspect of human life (Dewir 2016; Kravanja 2006). They can be observed in public areas including schools, shopping areas, workplaces, and along the streets, where they are used to decorate open and indoor spaces (Ciesla 2002). Various locations that require landscaping have diverse conditions in terms of lighting, temperature, relative humidity, and moisture (Edmond et al. 1978). The choice of ornamental plant for specific environments is a key decision for interior designers, landscapers, as well as homeowners (Baldwin 2013). Succulents, among ornamental plants, have recently gained popularity as a landscape and pot plant (Baldwin 2007; Graham 1987).
Often associated with semi-arid conditions (Graham 1987), succulents exhibit physiological tolerance to drought by storing water in fleshy assimilatory organs (Edwards and Ogburn 2013). Considered as Crassulacean Acid Metabolism (CAM) plants, succulents and cacti close their stomata and exhibit CO2 fixation during the day and under hot temperatures, and open their stomata during the cool night, and exhibit CO2 assimilation (Erwin 2009; Herrera 2009; Lee 2010).
Presently, succulents are commercially produced and have increased in popularity among plant collectors, landscapers, and interior designers (Altman 2011; Walters et al. 2011; Weber 2003). Succulents have been in high demand because of specific characteristics, and are both practical and efficient as they are drought resistant and can survive even under minimal light making them suitable for indoor use (Alaspa 2016; Bell 2001; Nefzaoui 2007). Coupled with their architectural, sculptural, and geometric shapes, succulents provide clear and simple lines that can be made into bizarre, eye-catching, and collectible arrangements (Baldwin 2013).
According to Nyffeler and Eggli (2010), 12,500 species of succulents have been recognized from 690 genera. These species are commonly identified within the families. However, they can also be classified as stem or leaf succulents depending on which plant organ stores moisture (Rowley 1978). These organs are identified as they are slightly thicker and are the rounded structures of the plant (Kelaidis 2008). Succulents are considered for ornamental use, especially those belonging to the Crassulaceae and Mesembryanthemaceae (Graham 1987; Rowley 1978).
Grown and globally distributed in all habitat types, succulents are utilized for more than just for aesthetic purposes; they are also cultivated for their environmental benefits (IUCN 1997). More studies have been conducted investigating the utilization of succulents for green-roofing systems among Sedum species (S. acre, S. oryzifolium, S. kamtschaticum, S. reflexum, S. rupestre, and S. spurium ‘Green Mental,’) (Kim et al. 2010). Studies have also been performed on the use of succulents for ground cover, green roofs, and walls for extreme conditions, highlighting their tolerance to cold and drought (Van Woert et al. 2005) These green roofing systems aim to provide vegetation with added environmental benefits in an urban environment by enriching biodiversity using efficient and high-surviving plants (Li and Yeung 2014).
These plants have also gained attention in medical fields as potential sources of raw materials for medication. Studies of three Echeveria species from Mexico (E. craigiana, E. kimnachii, and E. subrigida) are being analyzed for antioxidant α-glycosidase inhibitors as well as anti-bacterial properties. Antioxidant compounds such as β-carotene, ascorbic acid, α-tocopherol, total phenolic, and flavonoids, and in vitro antioxidants (DPPH, ABTS, ORAC, and β -carotene bleaching [β-CBM]) have been found in the succulent extracts (Angulo et al. 2014). Other succulents have been further studied to play a potential role as a treatment for obesity (Habeck 2002).
Because of their influence on human health and the environment, succulents are desired to be propagated rapidly and in large quantities. Plant propagation is performed to perpetuate species, multiply species, and maintain their youthfulness, and can be achieved via sexual and asexual reproduction (McNeilan and Gorman 2013; Sorensen 2015).
The chosen method must be able to perpetuate plants in the shortest amount of time while maintaining the quality of the plants. The technique must also take into consideration not to harm the mother plant as much as possible. Various plants, even those that belong to the same family or genus may require a different technique. Likewise, mastering the traditional propagation tools used may open to the possibility of using a more advanced propagation technique that are available for similar crops. Thus, the proper knowledge and appropriate plant propagation technique is essential to produce new plants.
Previous studies have shown that plant propagation of succulents can be performed in numerous ways. This review aims to identify and summarize available and published research on the multiplication process of ornamental succulents through both sexual and vegetative propagation methods. Finally, the management and care of plant propagules to maximize the growth and development of newly propagated ornamental succulents including appropriate conditions are likewise discussed in this paper.
Propagation Techniques
Because of the high market demand for succulents, the need to use appropriate propagation techniques for particular families or species is deemed necessary. Generally, propagation techniques for succulents can be divided into two main categories, sexual and asexual propagation. The latter does not involve the sexual process. Asexually propagated plants remain pure or genetically identical from one generation to the next. Conversely, the sexual process involves meiosis, which is the process that creates natural genetic variability (Acquaah 2009; Beyl and Trigiano 2014). The sexual method involves the production of new plants from seeds; whereas asexual or vegetative propagation involves techniques that make use of stems and leaf cuttings, and the separation of suckers and rhizomes. A more rapid method, which is still categorized as vegetative propagation, is in vitro culture or micropropagation.
These methods are discussed in this review, including their care and maintenance as well as their advantages and disadvantages regarding a particular technique in the propagation of ornamental succulents.
Sexual Propagation
To produce food, fiber, and other raw products, majority of the agricultural plant propagation relies on seed and seedling methods. Thus, seed propagation is considered to be a cornerstone for the production of valuable agronomic and horticultural plants (Hartmann et al. 2011). This method increases genetic variation, forming hybrids with superior qualities. Plants from seeds also exhibit superior quality compared with vegetatively produced plants, which may exhibit a prolonged lifespan and be disease or virus free (Adams and Early 2004).
Seed Propagation of Succulent Plants
The use of seeds to propagate succulents is not common. However, most recent articles published using seeds are limited to the Cactaceae and Kalanchoe families (Mihalte et al. 2011; Rethy et al. 1987; Rojas-Arechiga and Vazquez-Yanes 2000). The key characteristic associated with the propagation of succulents through seeds is their minute size. Numerous studies have reported varying seed sizes, including those of E. gibbiflora at 0.75 mm (Parra et al. 1993), E. yalmanantlanensis at 0.39–0.90 mm (Vazquez-Garcia et al. 2013), Bossfeldi, and Strobocactus at ≤0.5 mm, and Nyctocereus and Opuntia at ≥ 0.5 mm diameter (Rojas-Arechiga and Vazquez-Yanes 2000). Sedum succulents propagated through seeds include S. acrem, S. album, S. kamtschatticum, and S. reflexum (Monterusso et al. 2005).
The seeds are harvested when the flowers have been successfully pollinated and have matured (Hartmann et al. 2011). However, the majority of succulent and cacti are self-sterile, and many propagators are able to produce new hybrids using natural pollination (using male pollen from one species to fertilize a flower from a different species) (Harland and Harland 1981). Flowers are left to dry and ripen before tan-colored seeds are removed (Bregman and Bouman 1983).
Asexual Propagation
Also known as vegetative propagation, asexual propagation is based on the principle of totipotency (Adams and Early 2004), which is defined as “the properties of an individual cell (not a group of cells), with the two meanings of this term roughly corresponding to the progressive restriction in potential cells exhibited during normal development” (Condic 2014). However, Hartmann et al. (2011) simplified this statement to state that any plant cell has the capability to develop into a completely new plant. Considering this, several plant parts, such as the leaves, buds, nodes and internodes, buds, scion, cuttings, layer, bulb corm, tuber, and explants, may be used for vegetative propagation, and vary between plants (Li et al. 2013; Poethig 2013).
Vegetative propagation is a suitable option for the propagator to maintain the genetic integrity of the plant, and assures true-to-type propagated plants (Hartman et al. 1990; Hogberg 2003), which means that they will produce new plants that are always genetically identical to the parent or clone (Toogood 1999). For succulents, the methods available for vegetative propagation include stem and leaf cuttings, suckers, and micropropagation.
Propagation by Cuttings
Generally, the use of cuttings for propagation is the most frequently used method for succulents, and is easy to perform (Baldwin 2013). Likewise, when there is an immediate condition under which succulents are affected by pest and diseases, cuttings may be used for rescue and are the last resort for the retention of succulent plants (Kapitany and Schultz 2004). Two kinds of cuttings are practiced for succulent plants, stem cuttings and leaf cutting.
These propagation methods are made possible by the existence of meristematic tissues in certain parts of the plants (Ichihashi and Tsukaya 2015; Machida et al. 2013); specifically, the intercalary meristems, which are actively dividing cells located in the internodes or bases of the leaves. These are usually found in monocotyledonous plants (Evans and Perez 2004). These meristems can produce new plants, providing shoots and roots. Thus, stem and leaf cuttings can be used in propagation techniques.
Stem Cuttings
A study conducted by Mihaela et al. (2011) reported the rooting of stem cuttings of various succulent species (Aeonium domesticum, Aeonium tortuosum, Kalanchoe rhombopilosa, Kalanchoetubiflorum, Senecio articulatum, and Senecio jacobsenii) including valuable ornamental and medicinal cactuses, such as those belonging to the Optunia species (Stintzing and Carle 2005). Respective genera exhibit differences in rooting time namely with Senecio (21 days), Aeonium and Crassula (28 days), Sedum (39 days), and Kalanchoe (53 days).
Each stem-cutting propagule is taken by making a cut on the lower portion of the mother plant or the sucker, approximately 3 – 6 cm long. This allows the mother plant to develop side shoots. The stem-cutting propagule is separated from the mother plant using a sharp tool, forming a slanted sharp surface. The stem must be cut in a way that generates about 4 – 5 nodes or leaves. Unlike other ornamental crops, succulents do not require the removal of leaves during stem-cutting propagation.
Based on above mentioned study, stem cuttings take longer to produce a proper root system. Therefore, plant growth regulators are used to enhance the rooting process. The use of exogenous auxins, such as indole-3-butyric acid (IBA), indole-3-acetic acid (IAA), and naphthalene-3-acetic acid (NAA) are used to promote rooting among hardy stem cuttings.
Conversely, according to the results of Saniewski et al. (2014) regarding the use of auxin polar transport inhibitors among Crassulaceae species (Bryophyllum daigremontianum, Bryophyllum calycinum, Kalanchoeblossfeldiana, and Kalanchoetubiflora), the effect of the control or water alone was comparable to the effect of 0.2% IAA, IBA, and NAA, and of 0.2% 1-N-naphthylphthalamic acid (NPA). Successful total inhibition of rooting was only observed in cuttings treated with 0.2% 2,3,5-triiodobenzoic acid (TIBA) and methyl 2-chloro-9-hydroxyfluorene-9-carboxylate (Morphactin).
Saniewski et al. (2014) recommended that stem cuttings are pre-soaked in compete Hoagland solution, which would increase the percent rooting by 20% compared with the control. Likewise, the study suggested that succulent where it is rather hardy or at the lower portions of the mother plant where mature stems are found, increases the chances of survival as well as the rooting rate by 50%.
Leaf Cuttings
Defying the growth of simple leaves, which are not capable of developing new roots, the shoots, succulents, semi succulents, and cacti are able to propagate through their leaves by differentiating leaf parenchyma or through the presence of intercalary meristematic activity (Donnelly et al. 1999; Evert 2006; Gorelick 2015). Because of this, succulents can propagate through leaf cuttings especially those of the Crassulaceae family (Hagemann 1932; Kerner and Oliver 1902). This method seems to be more popular than seed propagation due to the lengthy germination period; thus, cuttings and offsets are rooted first in nursery flats filled with potting mixes before being placed in gardens or replication areas (Baldwin 2013). The success of propagating succulents is unpredictable, and some growers and enthusiasts term the multiplication of plants using leaf cuttings a game of chance (Artichoker 2016; Tuttle 2012).
Studies by Cabahug et al. (2016a) showed that when producing leaf cuttings, the lower leaves or leaves found at the base of the mother plant should be taken. Likewise, leaves should be properly removed without any scaring in order to increase the chances of successful bud and root imitation. After removing leaves from the mother plant, cuttings are used to develop calli and are placed in a dry location with indirect sunlight, preferably at room temperature (20 - 25°C); it takes about 5 – 7 days. or until the green part of the point of turns to light or dark brown. This study also showed that the cuttings are planted upright in the medium at the point of leaf removal. With this method, successful rooting, root development, and normal plantlets were grown from Echeveria spp., including E. ‘A Grimm One’, E. ‘Momorato,’E. pulvinata, and E. pulidonis. However, other guidelines for planting succulent leaf cuttings recommend that leaf cuttings should only be planted on soil when the leaf-cuttings have already developed roots during the callusing process.
Conversely, in a study on ornamental Sansevieria trifasciata L. the use of different parts of the leaf cuttings (apical, middle, and bottom parts of the snake plant) for meristemoid production revealed no significant differences in the rooting, shooting, or proliferation of the plant. However, the use of dipping treatments such as 2000 mg･L-1 of IBA resulted in the highest rooting success (Sarmast et al. 2009).
In a study by Cabahug et al. (2016b), plant growth hormones, IBA and kinetin, were used at different concentrations to enhance both rooting and shoot development, respectively, to target the unpredictability of organ development in E. subsessilis and E. runyonii. Researchers concluded by recommending the use of combined 100 ppm IBA and 100 ppm Kinetin for proper root and shoot development. However, IBA may be preferable when only a single application is used. In addition, alternative forms of exogenous auxin hormones, including BAP and NAA, were also used for Pachyeria pachytoides and Sedum morganianum leaf cuttings. The use of combined 4.0 mg･L-1 BAP and 0.1 mg･L-1 NAA resulted in a 100% regeneration success (Xu and Zheng 2017).
Division or Separation of Suckers
Limited studies have assessed the division or separation of suckers. This method of vegetative propagation involves the removal of new side shoots from the mother plant, which is a result of lateral shoot and rhizome growth. This has been commonly observed in aloe succulents (Gantait et al. 2014; Saggoo and Kaur 2010; Smith and Van Wyk 2009). Although this method is the most secure way of propagating vegetatively, it takes time, since the separation of side shoots may take months or years to reproduce through natural propagation (Hartmann et al. 1990).
In this method, suckers, slips, or water sprouts may be taken from the mother plant. Suckers are adventitious shoots that emerge from a root or from the runners that grows near the mother plant, thus its name, while slips are offshoots that are formed from the peduncle of the mother plant. Similar to suckers and slips, waterspouts are shoots that emerge from a latent bud on the mother plant. (Bhende and Kurien 2015; Hartmann et al. 2011).
Succulents usually develop suckers, slips, and water sprouts, but this kind of growth system is more commonly applied to Sempervivum species. Succulent plants belonging to this genus are popularly referred to as ‘hens and chicks’ or houseleeks due to the production of numerous suckers and slips (Alberti et al. 2007; Karaer et al. 2011), as well as for other Sedum species (Calie 1981; Sajeva et al. 2009). There is limited information regarding the hastening of side shoots for common genera of succulents using this propagation tool.
Micropropagation or In-Vitro Propagation
Micropropagation, or in vitro propagation, is used to rapidly propagate cultivars, and shows promise for the production of pathogen-free and virus-tested propagation sources (Hartmann et al. 2011). This modern method usually involves growing plant pieces (explants) in glass under sterile environmental conditions (Toogood 1999). Several ornamental crops have been grown using this method, especially those that are difficult to propagate (Ruchala 2002), in order to hasten production. This method has been successful for orchids (Chugh et al. 2009) and anthuriums (Matsumoto and Kuehnle 1997).
A few studies have used this current fast-paced method in the propagation of succulents, with a particular focus on the factors affecting micropropagation, including the use of explants, media, callus induction, shoot regeneration and proliferation, growth conditions, and acclimatization. Famous families of succulents have been successfully micropropagated: Aizoaceae, Asphodelaceae, Asclepiadaceae, Crassulacea, and Euphorbiaceae (De Langhe et al. 1974; Fay et al. 1995; Gratton and Fay 1992; Karthik Prabu et al. 2013; Kim et al. 2017; Starling and Dodds 1983).
a. Explants and Planting Materials
In studies by Gratton and Fay (1992), a shoot with two dormant buds was chosen as an explant for Caralluma micropropagation. Explants were excised by carefully removing the areole using a vertical incision, with at least 3–5 mm of the surrounding tissue. However, the use of different tissue types may vary depending upon the species. Shoot-tips are used for Kalanchoe (Smith and Nightingale 1979) and Lithops (Rogers and Lineberger 1981), inflorescences for Hwarthia and Mammillaria (Kaul and Sabharwal 1972; Wyka et al. 2009), seed fragments for Agave (Groenewald et al. 1977) and Aloe (Groenewald et al. 1975), and leaves for Sansevieria (Adaci et al. 1976) and Echeveria (Raju and Mann 1970).
These tissues are then treated and sterilized to minimize contamination. Lee et al. (2009), propagated Sedum sarmentosum using 70% (v/v) ethanol for 30 seconds, dipping in 0.5% sodium hypochlorite solution for 15 minutes, and rinsing four times with sterile water. Tissues were then left to be air-dried inside a laminar-flow cabinet for 30 min.
A different method of explant sterilization was used for Caralluma difusa. First, tissues were washed with running tap water for 20 – 30 min, treated with 1% (v/v) Tween-20 for 10 min, and then rinsed twice with distilled water. These was further sterilized by placing tissues inside a laminar-flow hood and surface-sterilized with 60% (v/v) ethanol for 1 min and 0.01% (v/v) HgCl2 for 5 min. Then, tissues were washed thoroughly 6 – 8 times with sterile double-distilled water (Karthik Prabhu et al. 2013). Of note, tap water should be used sparingly, if at all, to rinse tissues or explants, as it may increase the possibility of contamination (Hartmann et al. 2011).
b. Growing Media and Callus Induction
Common to all published studies on the micropropagation of succulents is the use of the Murashige and Skoog’s medium (MS). This is prepared by mixing a premix powder or agar with 3% w/v of sucrose, and maintaining at pH 5.8 before autoclaving (Fay et al. 1995; Oh et al. 2017; Starling and Dodds 1983). A comparative study by Lee et al. (2009) on plant regeneration medium for S. sarmentosum revealed that the use of MS was more intensive compare with B5 medium (Gamborg et al. 1968).
MS or basal medium is then supplemented with growth regulators or high concentrations of cytokinin and auxin. Different cytokinins (adenine sulfate, BA, 2iP, Ki, and zeatin) and auxins (IAA, IBA, NAA, and 4-amino-3,5,6-trichloropicolinic acid/ Picloram) have been used (Balch et al. 2015; Goldammer et al. 2003; Han et al. 2009; Lee et al. 2009). Studies on E. laui revealed that the use of 0.1 mg･L-1 NAA + 3.0 mg･L-1 was the most effective mixture for callus formation.
c. Multiplication and Proliferation
Over time, auxiliary buds will grow from the explants following preliminary planting in the medium. During this stage (explants with 2 – 3 asclepiads), samples were cut into sections with one bud per piece. Each piece is placed in a new separated container. These sections continue to produce lateral shoots and proliferate (Gratton and Fay 1992). Subculturing continues using this process at intervals of 1 – 2 months until sufficient shoots are observed. However, to hasten the multiplication process, the culture media is supplemented with plant growth regulators for subsequent subcultures in order to sustain nutritional needs and promote shoot and root formation (Hartman et al. 2011; Toogood 1999).
Fortification of the MS medium during shoot multiplication and root proliferation was suggested by Kailmuthu et al. (2014) with the use of BAP for Caralluma diffusa. However, Gratton and Fay (1992) reported that combinations of NAA between 5 and 20 mg･L-1 are generally the most suitable for ornamental succulents.
Similarly, a different mixture was used for leaf explants of Gasteria croucheri, which included a combination of 2.0 and 5 mg･L-1 NAA and was later supplemented with Ki to produce roots at the last subculture, and was later left to root in a hormone-free medium.
The differences in the literature suggest that the use of hormones may differ for each species; therefore, the concentration and type of cytokinin and auxin should be evaluated for other high-valued ornamental succulent plants.
Care and Maintenance
Care of Seeds and Seedlings
Boarder (1969) suggested that the best time to sow seeds is in April or when the weather is sufficiently warm and the location provides sufficient light. Conversely, Flores et al. (2016) found out that all succulent species in Mexico are considered as neutral photoblastic or neutral to light. However, some species (Mammillaria compressa, Agave salmiana, and other varieties of Derocactus latispinus) also exhibit higher germination under high light conditions.
Fenner and Thompson (2005) emphasized that although they may be photoblastic, temperature requirements may be a key factor in germination, which is commonly associated with light. A temperature of 25°C is considered to be suitable to germinate seeds in a growth chamber as recommended for cacti, agave, and related succulent species (Nobel 1988). Other studies have reported that succulents within various families and species are capable of germinating under temperatures of 15 – 33°C in their natural habitat (Flores and Briones 2001; Pritchard and Miller 1995).
Additionally, Zimmer (1971) and Nobel (1988) provided guidelines on the germination of succulent seeds. They noted that extremes of temperature (below 12°C and above 35°C) will decrease the germination rate. Following the planting of seeds of succulents, numerous recommendations have been made in terms of soil mixtures (Bach 1998; Kohlschreiber 1998).
The use of compost mixed with enough sandy soil to protect the succulents from rot, facilitate proper drainage, and provide proper aeration, has been emphasized in previous studies (Graham 1987; Monterusso et al. 2005). When planting succulent seeds, it is advised not to cover the seeds with compost, as it should remain on the surface, with the exception of large-sized seeds. Relative to each species, seeds are able to germinate from 2 – 4 days to 3 – 5 weeks (Graham 1987; Harland and Harland 1981).
When initiating seed growth, various techniques have been used to hasten germination. Gibberellic acid (GA3) was applied to Kalanchoe seeds to lessen the light requirement. However, traditionally, Kalanchoe seeds are incubated in KNO3 solution to prevent secondary dormancy (De Petter et al. 1985). In line with plant growth regulators, Mihalte et al. (2011) suggested treatment with 1.8% sodium nitrophenolate for 8 h for various Cactaceae species. Likewise, the use treatment with 9% naphthalene acetic acid (NAA) for 8 h in an aqueous solution was suggested for maximum germination. Pre-treatments are usually recommended only to increase the germination rate.
Care for Succulent Cuttings and Suckers
Propagation techniques for the potting medium of succulents were similar to those for the seed media. The use of 50% sand and 50% peat or compost is needed to provide cuttings with sufficient moisture and the ability to drain excess water (Ellern 1972).
A more recent study by Oh et al. (2015) suggested the use of a 5 : 3 : 2 soil medium mixture of coco peat: peat moss: vermiculite for E. agavoides ‘romeo’, a 2.5 : 3 : 2 : 2.5 soil media mixture of coco peat: peat moss: vermiculite: sand for E. agavoides var. ‘cristata’ and E. ‘leibeulli’, which provided 87 – 90% rooting success for their respective species. It was also emphasized that whatever mixture was used to propagate stem cuttings should facilitate proper drainage in order to reduce the rotting of leaf and stem cuttings (Sorensen 2015).
After planting, plants should not be exposed to direct sunlight, and if necessary the use of shading materials such as nets, burlap, or polyethylene shelters are used to protect from sudden and drastic changes in conditions (Fowler and Chaffee 2010; Semchenko et al. 2012)
Recommended environmental conditions suggested by the results of previous experiment include temperatures of 15 – 30°C and relative humidity of 60 – 75% (Cabahug et al. 2016a; 2016b). Furthermore, succulent cuttings should not be drenched in water; a misting technique should be used as to provide sufficient moisture while reducing the probability of stem or leaf rotting, and may be watered every 4 – 5 days (Wiesner and Johnson 1977).
Nam et al. (2016) recommended the use of supplemental lighting with an intensity of 120 μmol･m-2 ･s-1 for 6 h, in addition to natural light during the day. Higher quality of succulents (Sedeveria ‘Letizia,’ Sedum ‘Sun Red,’ Crassula rupestris, Echeveria ‘Momotaro,’ and Graptoveria opalina) based on height, diameter, and color were taken from high intensity and long duration exposure to LED light. Likewise, Cabahug et al. (2017) also recommended a high intensity level (150 μmol･m-2 ･s-1) to enhance the quality of E. agavoides and E. marcus. Succulents exposed to this level of light had higher visual quality, normal plant development, and intensified color, which corresponds to increased anthocyanin content.
Studies on some areas of succulent care and management are lacking, such as plant nutrition and pest and disease control. Research in these areas is required, which may provide more information for maintaining the quality and proper environmental management for succulent species.
Care for Micropropagated Succulents
During the course of propagation, the use of sterile tools, medium, planting materials, and containers is necessary to reduce the contamination percentage (Balch et al. 2015). Upon completion of micropropagation in the lab, the cultures are grown in a separate lab under controlled conditions (Hartmann et al. 2011).
It is recommended that micropropagated plants are incubated for at least 16 h under light and 8 h under dark conditions with fluorescent plants and about 15 μmol･m-2 ･s-1 for Echynocactus and Mammillaria species. However, a much higher light intensity was used in a study on the organogenesis of Ariocapus species with 50 μmol･m-2·s-1 at temperatures of 25°C (Balch et al. 2015; Goldammer et al. 2003)
The success of micropropagation depends on the percent survival rate during the acclimatization stage. For aloes, Gupta et al. (2014) reported a 70% survival rate, for E. subsessilis a 50 – 70% survival rate (Oh et al. 2017), and for ornamental cacti an 85% survival rate (Balch et al. 2015).
The plantlets are rinsed or teased from the medium, gently rinsed under running or distilled water to remove attached agar, and then planted in regular growing medium. Plantlets are then subjected to a hardening process wherein they are slowly introduced to light until they are able to withstand field or outdoor conditions (Hartmann et al. 2011). Notably, limited studies have investigated the incubation as well as the acclimatization of ornamental succulents.
Conclusion
The increased popularity of ornamental succulents indicates that there is a high demand for increased varieties, and for a higher supply of this unique and water-efficient ornamental crop. Several propagation techniques are available, which may be used by plant propagators and farm owners. Each species or genera may require different methods of care and management. However, based on this mini-review, several areas were identified that require further research, including: i) pre-treatment and planting of seeds on appropriate media and under environmental conditions; ii) cutting and soaking the leaf and stem using appropriate media mixtures; iii) division and separation of suckers, scar and growth treatments; iv) micropropagation trials for other common ornamental succulents and growth hormone mixed with the base media; and v) incubation and acclimatization protocols.
