Abstract
Hoya imperialis (H. imperialis) and H. coronaria (Apocynaceae) are known to have ornamental value due to their beautiful flowers; however, the feasibility of propagating these plants have not been reported despite the wild populations in Brunei Darussalam being highly threatened due to habitat loss and overcollection. Thus, the present study aimed to conduct a preliminary study of the feasibility of two alternative propagation methods, stem cutting and micropropagation, as a potential approach for their ex situ conservation. Hoya stem cuttings were treated with either indole-3-butyric acid (IBA) or 1-naphthaleneacetic acid (NAA) (0–2000 mg/L), and then propagated onto a mixture of peat moss and perlite. For micropropagation, Hoya leaf explants were cultured onto Murashige and Skoog (MS) agar media that were supplemented with IBA and/or kinetin (KN) (0–10.0 mg/L). This present study shows that both Hoya species were successfully propagated by stem cutting even without hormone treatment. However, interestingly, in H. imperialis, when compared with control, the mean number of new leaves (6.3 ± 1.0) and the mean relative growth rate (RGR) based on stem diameter (0.004 ± 0.0007 cm cm−1 day−1) significantly increased when treated with 500 mg/L NAA and 2000 mg/L IBA, respectively. Meanwhile, in H. coronaria, significantly higher mean number of roots was achieved by treating with 1000 mg/L NAA (16.6 ± 1.4) or 2000 mg/L IBA (17.5 ± 2.7) compared with control. For micropropagation, callus induction was not promising and could only be observed at specific concentrations of both IBA and KN, with H. imperialis appearing to be more responsive towards these hormones in comparison to H. coronaria. The present study showed that stem cutting appeared more feasible in propagating both Hoya species.
Highlights
The genus Hoya from the Apocynaceae family is a species-rich genus comprising mainly epiphytes and are geographically distributed in the Indomalesian-Australian region (Kleijn & van Donkelaar 2001)
Two-nodal leafy stem cuttings (10 cm–20 cm long and 3 cm–6 cm in diameter for H. imperialis; 13 cm–17 cm long and 2.0 cm–4.5 cm in diameter for H. coronaria) were prepared with the leaves trimmed to half of their length and with the flowers and buds excised from the cuttings
High survivability (100%) of the cuttings was observed in the first 20 weeks for all treatments in both H. imperialis and H. coronaria. Measurements such as the number of new leaves, leaf area, number of roots, root length and relative growth rate (RGR) values based on stem length and stem diameter were recorded for both Hoya species as described below
Summary
The genus Hoya from the Apocynaceae family is a species-rich genus comprising mainly epiphytes and are geographically distributed in the Indomalesian-Australian region (Kleijn & van Donkelaar 2001). It is one of the most diverse plant genera in the tropical and subtropical regions, such as in South Africa, South America, Southeast Asia and New Guinea (Kloppenburg & Wayman 1992; Rodda et al 2013). Hoya plants have been introduced as exotic ornamental plants in Europe and the U.S since 1970s (Bermuli et al 2019) and their popularity has spread to Australia (Rahayu 2012) This can be seen by the increasing number of webpages, associations and societies that are dedicated to the horticulture, international trades and plant exchanges of Hoya in the recent years (Hodgkiss 1997; Rahayu 2018). H. lacunosa and H. latifolia were reported to have the potential to be used as bio-insecticide that could control the growth of pre-adult mosquitoes, Aedes aegypti and Culex quinquefasciatus which are known vectors of several viruses (Cahyadi 2005; Kusumawati 2005; Mukharam 2005; Rustandi 2005)
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