True aloe (Aloe vera) and krantz aloe (Aloe arborescens) are currently being used for the extraction of cosmetic and nutraceutical active ingredients (Cardarelli et al., 2017; Espinosa-Leal and Garcia-Lara, 2019). Krantz aloe has a wide geographical distribution in the African continent, with populations in South Africa, Botswana, Swaziland, Lesotho, Mozambique, Zimbabwe, Mapaura and Timberlake, and Malawi. Although krantz aloe is common within its range, human activity has caused a negative impact on populations (Smith et al., 2008). Wild populations of aloe species are currently threatened as a result of their continuous collection for transplantation to private gardens or the extraction of active ingredients (Maundu et al., 2006). Conventional propagation using lateral shoots and rhizome cuttings is not able to fulfill the increasing market demand for aloe (Cristiano et al., 2016).
In vitro tissue culture represents a promising alternative to wild collections of krantz aloe plants and conventional propagation by allowing the production of multiple plants for their reintroduction to the wild and fields, and production of active metabolites to be optimized (Espinosa-Leal et al., 2018). Although some reports exist on the in vitro tissue culture of krantz aloe (Bedini et al., 2009; Cardarelli et al., 2017; Kawai et al., 1993), the establishment of cultures from plant explants has proved difficult because of the long response times of the explants and their release of polyphenols, resulting in the need for constant subcultures (Bedini et al., 2009).
Seeds are an alternative explant for in vitro culture establishment. True aloe seeds are not commonly used as a means of propagation (traditional and in vitro) because of their scarcity in nature and low GPs of 0% to 25% (traditional) and 60% to 70% (in vitro) (Cristiano et al., 2016). Studies have investigated the optimization of germination conditions for bitter aloe (Aloe ferox) and krantz aloe in petri dishes without substrate and with filter paper, which have shown that the use of karrikinolide (KAR1)-rich SSW either as a priming treatment or a watering solution promotes seed germination (Bairu et al., 2009; Kulkarni et al., 2013). Studies have demonstrated that SSW can also promote seedling vigor, including leaf and root length and number (Demir et al., 2018). Hydration of seeds before sowing is another method for improving germination (Demir et al., 2018; Khan, 1992). However, the effect of SSW and SI, separately and combined, on the in vitro germination of krantz aloe under aseptic conditions using Murashige and Skoog (MS) culture media (Murashige and Skoog, 1962) as a standard substrate has not been investigated previously. Therefore, the objective of this work was to evaluate the effect of SI with water and the addition of SSW to the culture media on the in vitro germination and initial seedling development of krantz aloe.
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